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Histology_Ross. Important multiadhesive glycoproteins found in the ECM of connective tissue are summarized in Table 6.5. Connective tissue cells can be resident or wandering. The cells that make up the resident cell population are relatively stable; they typically exhibit little movement and can be regarded as permanent residents of the tissue. These resident cells include  fbroblasts and a closely related cell type, the myof broblast, macrophages, adipocytes, mast cells,and  adult stem cells. The wandering cell population or transient cell population consists primarily of cells that have migrated into the tissue from the blood in response to specific stimuli. These include  lymphocytes, plasma cells, neutrophils, eosinophils, basophils,and  monocytes. The fibroblast is the principal cell of connective tissue.


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Histology_Ross. The fibroblast is the principal cell of connective tissue. Fibroblasts are responsible for the synthesis of collagen, elastic and reticular fibers, and the complex carbohydrates of the ground substance. Research suggests that a single fibro blast is capable of producing all of the ECM components.


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Histology_Ross. Fibroblasts reside in close proximity to collagen fibers. In routine H&E preparations, however, often only the nucleus is visible. It appears as an elongated or disclike structure, sometimes with a nucleolus evident. The thin, pale-staining, flattened processes that form the bulk of the cytoplasm are usually not visible, largely because they blend with the collagen fibers. In some specially prepared specimens, it is possible to distinguish the cytoplasm of the cell from the fibrous components (Fig. 6.19a). When ECM material is produced during active growth or in wound repair (in activated fbroblasts), the cytoplasm of the fibroblast is more extensive and may display basophilia as a result of increased amounts of rER associated with protein synthesis (Fig. 6.19b). When examined with the TEM, the fibroblast cytoplasm exhibits profiles of rER and a prominent Golgi apparatus (Fig. 6.20).


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Histology_Ross. TABLE Proteoglycans 6.4 kDa, kilodaltons. Molecular Molecular Name Weight (kDa) Composition Localization Function Aggrecan 250 Linear molecule; binds via a Cartilage, Responsible for hydration of link protein to hyaluronan; Chondrocytes extracellular matrix of contains 100 to 150 molecules cartilage of keratan sulfate and chondroitin sulfate chains Decorin 38 Small protein that contains Connective tissue, Functions in collagen only one chondroitin sulfate fibroblasts, cartilage, fibrillogenesis; by attaching or dermatan sulfate chain and bone to neighboring collagen molecules, helps to orient fibers. Regulates the thickness of the fibril and interacts with transforming growth factor (TGF-) Versican 260 Associated with a link protein; Fibroblasts, skin, Possesses EGF-like domains contains main and 12–15 smooth muscle, on the core protein; chains of chondroitin sulfate brain, and mesangial participates in cell-to-cell and attached to core protein cells of the kidney


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Histology_Ross. domains contains main and 12–15 smooth muscle, on the core protein; chains of chondroitin sulfate brain, and mesangial participates in cell-to-cell and attached to core protein cells of the kidney cell-to-extracellular matrix interactions; binds to fibulin-1 Syndecan 33 Family of at least four different Embryonic epithelia, The extracellular domain types of transmembrane mesenchymal cells, binds collagens, heparin, proteoglycans, containing developing lymphatic tenascin, and fibronectin, varying amounts of both tissue cells, intracellular domain binds heparan sulfate and lymphocytes, and to cytoskeleton via actin chondroitin sulfate molecules plasma cells


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Histology_Ross. The myofibroblast displays properties of both fibrmyosinoblasts and smooth muscle cells.


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Histology_Ross. The myofbroblast is an elongated, spindly connective tissue cell not readily identifiable in routine H&E preparations. It is characterized by the presence of bundles of actin filament with associated actin motor proteins such as nonmuscle myosin (page 61). Expression of the -smooth muscle actin ( -SMA; actin isoform found in the vascular smooth muscles) in myo fibroblasts is regulated by TGF-1. The actin bundles transverse the cell cytoplasm originating and terminating on the opposite sites of the plasma membrane. The site of actin fibers attachment to the plasma membrane also serves as a cell-to-ECM anchoring junction and is called fbronexus. It resembles focal adhesion found in the epithelial cells (page 144). This arrangement is of the basis of a mechanotransduction system in which force that is generated by the contraction of intracellular actin bundles is transmitted to the ECM. With the TEM, the myofibroblast displays typical characteristics of the fibroblast along with


Histology_Ross

Histology_Ross. force that is generated by the contraction of intracellular actin bundles is transmitted to the ECM. With the TEM, the myofibroblast displays typical characteristics of the fibroblast along with characteristics of smooth muscle cells. In addition to rER and Golgi profiles, the myofibroblast contains bundles of longitudinally disposed actin filaments and dense bodies similar to those observed in smooth muscle cells (Fig. 6.21). As in the smooth muscle cell, the nucleus often shows an undulating surface profile, a phenomenon associated with cell contraction. The myofibroblast differs from the smooth muscle cell in that it lacks a surrounding basal lamina (smooth muscle cells are surrounded by a basal or external lamina). Also, it usually exists as an isolated cell, although its processes may contact the processes of other myofibroblasts. Such points of contact exhibit gap junctions, indicating intercellular communication.


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Histology_Ross. FIGURE 6.18 • Common multiadhesive glycoproteins. These proteins reside in the extracellular matrix and are important in stabilizing the matrix and linking it to the cell surface. They are multifunctional molecules of different shapes and possess multiple binding sites for a variety of extracellular matrix proteins such as collagens, proteoglycans, and GAGs. Note that multiadhesive proteins interact with basal membrane receptors such as integrin and laminin receptors.


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Histology_Ross. TABLE Multiadhesive Glycoproteins6.5 CAM, cell adhesion molecule; ECM, extracellular matrix; EGF, epithelial growth factor; kDa, kilodaltons. Molecular Name Weight (kDa) Molecular Composition Localization Function Fibronectin 250–280 Dimer molecule formed Present in the ECM of Responsible for cell adhesion from two similar peptides many tissues and mediate migration; possesses linked by a disulfide bond binding sites for integrins, type IV collagen, heparin, and fibrin Laminin 140–400 Cross-shaped molecule Present in basal laminae Anchors cell surfaces to the basal formed from three of all epithelial cells and lamina. It possesses binding polypeptides ( chain and external laminae of muscle sites for collagen type IV, heparan two chains) cells, adipocytes, and sulfate, heparin, entactin, laminin, Schwann cells and integrin receptors on the cell surface Tenascin 1,680 Giant protein formed Embryonic mesenchyme, Modulates cell attachments to the from six chains connected perichondrium,


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Histology_Ross. laminin, Schwann cells and integrin receptors on the cell surface Tenascin 1,680 Giant protein formed Embryonic mesenchyme, Modulates cell attachments to the from six chains connected perichondrium, perio-ECM; possesses binding sites for by disulfide bonds steum, musculotendi-fibronectin, heparin, EGF-like nous junctions, wounds, growth factors, integrins, and CAMs tumors Osteopontin 44 Single-chain glycosylated Bone Binds to osteoclasts; possesses polypeptide binding sites for calcium, hydroxyapatite, and integrin receptor on the osteoclast membrane Entactin/ 150 Single-chain rodlike Basal lamina–specific Links laminin and type IV collagen; Nidogen sulfated glycoprotein protein has binding sites for perlecan and fibronectin


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Histology_Ross. FIGURE 6.19 • Fibroblasts in connective tissue. a. Photomicrograph of a connective tissue specimen in a routine H&E-stained, paraffin-embedded preparation shows nuclei of fibroblasts (F ). 600. b. During the repair process of a wound, the activated fibroblasts (F ) exhibit more basophilic cytoplasm, which is readily observed with the light microscope. 500. Macrophages are phagocytotic cells derived from monocytes. Connective tissue macrophages, also known as tissue histiocytes, are derived from blood cells called monocytes. Monocytes migrate from the bloodstream into the connective tissue, where they differentiate into macrophages.


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Histology_Ross. In the light microscope and with conventional stains, tissue macrophages are difficult to identify unless they display obvious evidence of phagocytotic activity—for example, visible ingested material within their cytoplasm. Another feature that assists in identifying macrophages is an indented or kidney-shaped nucleus (Fig. 6.22a). Lysosomes are abundant in the cytoplasm and can be revealed by staining for acid phosphatase activity (both in the light microscope and with the TEM); a positive reaction is a further aid in identification of the macrophage. With the TEM, the surface of the macrophage exhibits numerous folds and fingerlike projections (Fig. 6.22b). The surface folds engulf the substances to be phagocytosed.


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Histology_Ross. FIGURE 6.20 • Electron micrograph of fbroblasts. The processes of several fibroblasts are shown. The nucleus of one fibroblast is in the upper right of the micrograph. The cytoplasm contains conspicuous profiles of rER. The cisternae of the reticulum are distended, indicating active synthesis. The membranes of the Golgi apparatus (G) are seen in proximity to the rER. Surrounding the cells are collagen fibrils (CF), almost all of which have been cut in cross section and thus appear as small dots at this magnification. 11,000. The macrophage contains a large Golgi apparatus, rER and sER, mitochondria, secretory vesicles, and lysosomes. The lysosomes of the macrophage, along with the surface cytoplasmic projections, are the structures most indicative of the specialized phagocytotic capability of the cell. The


Histology_Ross

Histology_Ross. The lysosomes of the macrophage, along with the surface cytoplasmic projections, are the structures most indicative of the specialized phagocytotic capability of the cell. The FIGURE 6.21 • Electron micrograph of a myofbroblast. The cell exhibits some features of a fibroblast, such as areas with a moderate amount of rER. Compare with Figure 6.20. Other areas, however, contain aggregates of thin filaments and cytoplasmic densities (arrows), features that are characteristic of smooth muscle cells. The arrowheads indicate longitudinal profiles of collagen fibrils. 11,000.


Histology_Ross

Histology_Ross. macrophage may also contain endocytotic vesicles, phagolysosomes, and other evidence of phagocytosis (i.e., residual bodies). The rER, sER, and Golgi apparatus support the synthesis of proteins involved in the cell’s phagocytotic and digestive functions, as well as in its secretory functions. The secretory products leave the cell by both the constitutive and regulated exocytotic pathways. Regulated secretion can be activated by phagocytosis, immune complexes, complement, and signals from lymphocytes (including the release of lymphokines, biologically active molecules that influence the activity of other cells). The secretory products released by the macrophage include a wide variety of substances related to the immune response, anaphylaxis, and inflammation. The release of neutral proteases and GAGases (enzymes that break down GAGs) facilitates the migration of the macrophages through the connective tissue.


Histology_Ross

Histology_Ross. Although the main function of the macrophage is phagocytosis, either as a defense activity (e.g., phagocytosis of bacteria) or as a cleanup operation (e.g., phagocytosis of cell debris), it also plays an important role in immune response reactions. Macrophages have specific proteins on their surfaces known as major histocompatibility complex II (MHC II) molecules that allow them to interact with helper CD4 T lymphocytes. When macrophages engulf a foreign cell, antigens—short polypeptides, 7 to 10 amino acids long, from the foreign cell—are displayed on the surface of MHC II molecules. If a CD4 T lymphocyte recognizes the displayed antigen, it becomes activated, triggering an immune response (see Chapter 14). Because macrophages “present” antigen to helper CD4 T lymphocytes, they are called antigen-presenting cells (APCs).


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Histology_Ross. When macrophages encounter large foreign bodies, they may fuse to form a large cell with as many as 100 nuclei that engulfs the foreign body. These multinucleated cells are called foreign body giant cells (Langhans cells). Mast cells develop in bone marrow and differentiate in connective tissue. Mast cells are large, ovoid, connective tissue cells (20 to 30 m in diameter) with a spherical nucleus and cytoplasm filled with large, intensely basophilic granules. They are not easily identified in human tissue sections unless special fixatives are used to preserve the granules. After glutaraldehyde fixation, mast cell granules can be displayed with basic dyes such as toluidine blue. It stains the granules intensely and metachromatically because they contain heparin, a highly sulfated proteoglycan (Fig. 6.23a). The cytoplasm displays small amounts of rER, mitochondria, and a Golgi apparatus. The cell surface contains numerous microvilli and folds.


Histology_Ross

Histology_Ross. The mast cell is related, but not identical, to the basophil, a white blood cell that contains similar granules (Table 6.6). They both arise from a pluripotential hemopoietic stem cell (HSC) in the bone marrow. Mast cells initially circulate in the peripheral blood as agranular cells of monocytic appearance. After migrating into the connective tissue, immature mast cells differentiate and produce their characteristic granules (Fig. 6.23b). In contrast, basophils differentiate and remain within the circulatory system. The surface of mature mast cells express a large number of high-affinity Fc receptors (Fc RI) to which immunoglobulin E (IgE) antibodies are attached. Binding of specific antigen to exposed IgE antibody molecules on the mast cell surface leads to an aggregation of Fc receptors.


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Histology_Ross.  FOLDER 6.3 Clinical Correlations: Role of Myofibroblasts in Wound Repair cess of wound healing. A clean surgical skin incision begins the healing process when a blood clot containing fibrin and blood cells fill the narrow space between the edges of the incision. The infammatory process, which begins as early as 24 hours after initial injury, contains the damage to a small area, aids in the removal of injured and dead tissues, and initiates deposition of new ECM proteins. During the initial phases of inflammation, neutrophils and monocytes infiltrate the injury (maximum infiltration by neutrophils occurs in the first 1 to 2 days after injury). Monocytes transform into macrophages (they usually replace neutrophils by day 3 after injury) (page 181). At the same time, in response to local growth factors, fibroblasts and vascular endothelial cells begin to proliferate and migrate into the delicate fibrin matrix of the blood clot, forming the granulation tissue, a specialized type of


Histology_Ross

Histology_Ross. growth factors, fibroblasts and vascular endothelial cells begin to proliferate and migrate into the delicate fibrin matrix of the blood clot, forming the granulation tissue, a specialized type of tissue characteristic of the repair process. Usually by day 5 after injury, the fully developed granulation tissue bridges the incision gap. It is composed mainly of large numbers of small vessels, fibroblasts, and myofibroblasts, and variable numbers of other inflammatory cells. Migra ting fibroblasts exert tractional forces on the ECM, reorganizing it along lines of stress. Under the influence of growth factors such as TGF-1 and mecha nical forces, fibro blasts undergo differentiation into myofbroblasts.This -SMA. This type of actin is not present in the cytoplasm of fibroblasts (Fig. 6.3.1). The myofibroblasts generate and maintain steady contractile force (similar to that of smooth muscle cells) that cause shortening of the connective tissue fibers and wound closure. At the same time,


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Histology_Ross. The myofibroblasts generate and maintain steady contractile force (similar to that of smooth muscle cells) that cause shortening of the connective tissue fibers and wound closure. At the same time, myofibroblasts synthesize and lay down collagen fibers and other ECM components that are responsible for tissue remodeling. During the second week of wound healing, the amount of cells in tissue undergoing repair decreases; most of the myofibroblasts undergo apoptosis and disappear, resulting in a connective scar that has very few cellular elements. In some pathologic conditions, myofibroblasts persist and continue the process of remodeling. This continued remodeling causes hypertrophic scar formation, resulting in excessive connective tissue contracture. Extensive numbers of myofibroblasts are found in most contractive diseases of connective tissue (fibromatoses). For example, palmar fbromatosis (Dupuytren’s disease) is characterized by the thickening of palmar aponeurosis, which leads to


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Histology_Ross. found in most contractive diseases of connective tissue (fibromatoses). For example, palmar fbromatosis (Dupuytren’s disease) is characterized by the thickening of palmar aponeurosis, which leads to progressive flex-ion contracture of the fourth and fifth digits of the hand (Fig. F 6.3.2) If scar tissue grows beyond boundaries of the original wound and does not regress, it is called a keloid. Its formation is more common among African Americans than other ethnic groups.


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Histology_Ross. FIGURE F6.3.1 • Fibroblasts and myofbroblasts in the culture. This immunofluorescence image shows wild-type 3T3 fibroblasts cultured on the collagen lattice. Under the stimulation of certain growth factors such as TGF-1, some fibroblast differentiate to myofibroblasts, expressing -SMA, the marker of myofibroblasts differentiation. Cells were stained with fluorescein-labeled phalloidin to visualize F-actin filaments (green), and -SMA were labeled with primary antibodies against -SMA and visualized with secondary goat anti-mouse antibodies conjugated with FITC (red). Co-localization of -SMA with F-actin is indicated by yellow color. Note that some cells have completed their differentiation, and others are in the early stages. 1,000. (Courtesy of Dr. Boris Hinz.)


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Histology_Ross. FIGURE 6.22 • Photomicrograph and electron micrograph of a macrophage. a. This photomicrograph shows several macrophages (M) in the connective tissue from the area of wound healing. They can be distinguished from other cells by the presence of an indented or kidney-shaped nucleus. Note several mature neutrophils (N) with segmented nuclei located in the connective tissue that surround blood vessel filled with red and white blood cells in the center of the image. 480. b. The most distinctive EM feature of the macrophage is its population of endocytotic vesicles, early and late endosomes, lysosomes, and phagolysosomes. The surface of the cell reveals a number of fingerlike projections, some of which may be sections of surface folds. 10,000.  FOLDER 6.4 Functional Considerations: The Mononuclear Phagocytotic System


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Histology_Ross. The cells that are included in the mononuclear phagocy-totic system (MPS) are derived from monocytes and denote a population of antigen-presenting cells involved in the processing of foreign substances. These cells are able to phagocytose avidly vital dyes such as trypan blue and India ink, which makes them visible and easy to identify in the light microscope. The common origin of MPS cells from monocytes serves as the major distinguishing feature of the system as it is currently perceived and is the basis for the system’s name. In addition, cells of the MPS display receptors for complement and Fc fragments of im-munoglobulins. The various cells of the MPS are listed in the following table. Most cells of the MPS become fixed in specific tissues and may adopt a variety of morphologic appearances as they differentiate. The main functions of MPS cells are phagocytosis, secretion (lymphokines), antigen process-ing, and antigen presentation to other cells of the immune system. Some


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Histology_Ross. appearances as they differentiate. The main functions of MPS cells are phagocytosis, secretion (lymphokines), antigen process-ing, and antigen presentation to other cells of the immune system. Some functionally important phagocytotic cells are not derived from monocytes. For example, microglia are small, stellate cells located primarily along capillaries of the central nervous system that function as phagocy-totic cells. They are generally thought to arise from the me-sectoderm of the neural crest and not from monocytes; nevertheless, they are included in the MPS. Similarly, fibroblasts of the subepithelial sheath of the lamina propria of the intestine and uterine endometrium have been shown to differentiate into cells with morphologic, enzy-matic, and functional characteristics of connective tissue macrophages. Cells of the Mononuclear Phagocytotic System Name of Cell Location Macrophage (histiocyte) Connective tissue Perisinusoidal macrophage (Kupffer cell) Liver Alveolar macrophage


Histology_Ross

Histology_Ross. tissue macrophages. Cells of the Mononuclear Phagocytotic System Name of Cell Location Macrophage (histiocyte) Connective tissue Perisinusoidal macrophage (Kupffer cell) Liver Alveolar macrophage Lungs Fetal placental antigen-presenting cell (Hofbauer cell) Placenta Macrophage Spleen, lymph nodes, bone marrow, and thymus Pleural and peritoneal macrophage Serous cavities Osteoclast Bone Microglia Central nervous system Langerhans’ cell Epidermis Fibroblast-derived macrophage Lamina propria of intestine, endometrium of uterus Dendritic cell Lymph nodes, spleen


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Histology_Ross. This triggers mast cell activation, which results in granule exocytosis (degranulation) and the release of granule content into the ECM. Mast cells can also be activated by the IgEindependent mechanism during complement protein activation. Two types of human mast cells have been identified based on morphologic and biochemical properties. Most mast cells in the connective tissue of the skin and intestinal submucosa and breast and axillary lymph nodes contain cytoplasmic granules with a latticelike internal structure. These cells contain granule-associated tryptase and chymase and are referred to as MCTC mast cells. In contrast, mast cells in the lungs and intestinal mucosa have granules with a scroll-like internal structure. These cells produce only tryptase and are termed MCT mast cells. Nearly equivalent concentrations of each type are found in nasal mucosa.


Histology_Ross

Histology_Ross. Mast cells are especially numerous in the connective tissues of skin and mucous membranes, but are not present in the brain and spinal cord. Mast cells are distributed chiefly in the connective tissue of skin (MCTC mast cells) in the vicinity of small blood vessels, hair follicles, sebaceous glands, and sweat glands. Mast cells are also present in the capsules of organs and the connective tissue that surrounds the blood vessels of internal organs. A notable exception is the central nervous system. Although the meninges (sheets of connective tissue that surround the FIGURE 6.23 • The mast cell. a. Photomicrograph of a mast cell stained with H&E. The granules stain intensely and, because of their numbers, tend to appear as a solid mass in some areas. The nucleus of the cell is represented by the pale-staining area.


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Histology_Ross. 1,250. b. This electron micrograph shows the cytoplasm of a mast cell that is virtually filled with granules. Note a small lymphocyte present in the upper left of the figure. 6,000. brain and spinal cord) contain mast cells, the connective tissue around the small blood vessels within the brain and spinal cord is devoid of mast cells. The absence of mast cells protects the brain and spinal cord from the potentially disruptive effects of the edema of allergic reactions. Mast cells are also numerous in the thymus and, to a lesser degree, in other lymphatic organs, but they are not present in the spleen. Most mast cell secretory products (mediators of inflammation) are stored in granules and are released at the time of mast cell activation. chemical substances known as mediators of infammation.


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Histology_Ross. chemical substances known as mediators of infammation. Mediators produced by mast cells are divided into two cate gories: Preformed mediators that are stored in secretory thesized mediators (mostly lipids and cytokines) that are often absent in the resting cells, although they are produced and secreted by activated mast cells. Preformed mediators found inside mast cell granules are the following:  Histamine is a biogenic amine that increases the permeability of small blood vessels, causing edema in the surrounding tissue and a skin reaction demonstrated by an itching sensation. In addition, it increases mucus production in the bronchial tree and prompts contraction of smooth muscle in the pulmonary airways. Histamine’s effects can be blocked by antihistaminic agents. These competitive inhibitors have a similar chemical structure and bind to histamine receptors without initiating histamine’s effects.


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Histology_Ross.  Heparin is a sulfated GAG that is an anticoagulant. Its expression is limited essentially to the granules of mast cells and basophils. When heparin unites with antithrombin III and platelet factor IV, it can block numerous coagulation factors. On the basis of its anticoagulant properties, heparin is useful for treatment of thrombosis. It also interacts with FGF and its receptor to induce signal transduction in the cells.  Serine proteases (tryptase and chymase). Tryptase is selectively concentrated in the secretory granules of human mast cells (but not basophils). It is released by mast cells together with histamine and serves as a marker of mast cell activation. Chymase plays an important role in generating angiotensin II in response to vascular tissue injury. Mast cell chymase also induces apoptosis of vascular smooth muscle cells, particularly in the area of atherosclerotic lesions.


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Histology_Ross.  Eosinophil chemotactic factor (ECF) and neutrophil chemotactic factor (NCF), which attract eosinophils and neutrophils, respectively, to the site of inflammation. The secretions of eosinophils counteract the effects of the histamine and leukotriens. Newly synthesized mediators include the following:  Leukotriene C (LTC4) is released from the must cell and then cleaved in the ECM, yelding two active leukotrienes— D (LTD4) and E (LTE4). They represent a family of modified lipids conjugated to glutathione (LTC4) or cysteine (LTD4 and LTE4). Leukotrienes are released by mast cells during anaphylaxis (see Folder 6.5 for a description of anaphylaxis). Similar to histamine, leukotrienes trigger prolonged constriction of smooth muscle in the pulmonary airways, causing bronchospasm. This constriction,


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Histology_Ross. TABLE Comparison of Features Characteristic of Mast Cells and Basophils 6.6 Characteristic Features Mast Cells Basophils Origin Hemopoietic stem cell Hemopoietic stem cell Site of differentiation Connective tissue Bone marrow Cell divisions Yes (occasionally) No Cells in circulation No Yes Life span Weeks to months Days Size 20–30 m 7–10 m Shape of nucleus Round Segmented (usually bilobar) Granules Many, large, metachromatic Few, small, basophilic High-affinity surface receptors Present Present for IgE antibodies (Fc RI) Marker of cellular activity Tryptase Not yet established however, cannot be reversed by treatment with antihistaminic agents.  Tumor necrosis factor (TNF-) is a major cytokine produced by mast cells. It increases expression of adhesion molecules in endothelial cells and has antitumor effects.


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Histology_Ross.  Tumor necrosis factor (TNF-) is a major cytokine produced by mast cells. It increases expression of adhesion molecules in endothelial cells and has antitumor effects.  Several interleukins (IL-4,-3 -5, -6, -8 and -16), growth factors (GM-CSF), and prostaglandin D2 (PGD2) are also released during mast cell activation. These mediators are not stored in granules but are synthesized by the cell and released immediately into the ECM. Mediators released during mast cell activation, as a result of interactions with allergens, are responsible for variety of symptoms and signs that are characteristic for allergic reactions. Basophils that develop and differentiate in bone marrow share many features with mast cells.


Histology_Ross

Histology_Ross. Basophils are granulocytes that circulate in the bloodstream and represent less than 1% of peripheral white blood cells (leukocytes). Developmentally, they represent a separate lineage from mast cells, despite sharing a common precursor cell in the bone marrow. Basophils develop and mature in the bone marrow and are released to the circulation as mature cells. They have also many other common features with mast cells such as basophilic secretory granules, an ability to secrete similar mediators, and an abundance of high-affinity Fc receptors for IgE antibodies on their cell membrane. They participate in allergic reactions (see Folder 6.5) and together with mast cells release histamine, heparin, heparan sulfate, ECF, NCF, and other mediators of inflammation. In contrast to mast cells, basophils do not produce prostaglandin D2 (PGD2) and interleukin-5 (IL-5). Basophils and their features are discussed in more details in Chapter 10.


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Histology_Ross. The adipocyte is a connective tissue cell specialized to store neutral fat and produce a variety of hormones. Adipocytes differentiate from mesenchymal stem cells and gradually accumulate fat in their cytoplasm. They are located throughout loose connective tissue as individual cells and groups of cells. When they accumulate in large numbers, they are called adipose tissue. Adipocytes are also involved in the synthesis of a variety of hormones, inflammatory mediators, and growth factors. This specialized connective tissue is discussed in Chapter 9. Niches of adult stem cells are located in various tissues and organs.


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Histology_Ross. Many tissues in mature individuals contain reservoirs of steam cells called adult stem cells. Compared with embryonic stem cells, adult stem cells cannot differentiate into multiple lineages. They usually are capable of differentiating only into lineage-specific cells. Adult stem cells are found in many tissues and organs, residing in specific sites referred to as niches. Niches of stem cells residing in tissues and organs (excluding bone marrow) are called tissue stem cells. They been identified in the gastrointestinal tract—for instance, in the stomach (isthmus of the gastric gland), small and large intestines (base of the intestinal gland), and many other areas. Bone marrow represents a unique reservoir of stem cells. In addition to containing HSCs (see Chapter 10), bone marrow also contains at least two other populations of stem cells: A heterogenous population of multipotent adult progenitor cells (MAPCs) that appear to have broad developmental capabilities and


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Histology_Ross. FOLDER 6.5 Clinical Correlation: The Role of Mast Cells and Basophils in Allergic Reactions


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Histology_Ross. When an individual is exposed to a specific antigen (aller-gen) that reacts with IgE antibodies bound to the surface of mast cells or basophils via their high-affinity receptors (FcεRI), it initiates mast cell activation. This type of IgE-dependent activation initiates a cascade of events, result-ing in allergic reactions. These reactions can occur as immediate hypersensitiviy reactions, (usually within sec-onds to minutes after exposure to an allegen), late-phase reactions, or chronic allergic inflammations. The immediate hypersensitivity reaction involves IgE-mediated release of histamine and other mediators from mast cells and also from basophils. The clinical symp-toms caused by the above mediators vary, depending on which organ system is affected. The release of mediators in the superficial layers of the skin can manifest as erythema (redness), swelling and itch-ing, or pain sensations. Respiratory system symptoms in-clude sneezing, rhinorrhea (runny nose), increased production


Histology_Ross

Histology_Ross. layers of the skin can manifest as erythema (redness), swelling and itch-ing, or pain sensations. Respiratory system symptoms in-clude sneezing, rhinorrhea (runny nose), increased production of mucus, coughing, bronchospasm (constric-tion of bronchi), and pulmonary edema. Individuals with these symptoms often complain of tightness in the chest, shortness of breath, and wheezing. The gastrointestinal tract can also be affected with symptoms of nausea, vomit-ing, diarrhea, and abdominal cramping. In highly sensitive individuals, the antigen injected by an insect can trigger a massive discharge of mast cells and ba-sophil granules that affect more than one system. This con-dition is known as anaphylaxis. Dilation and increased permeability of systemic blood vessels can cause anaphy-lactic shock. This often-explosive, life-threatening reaction is characterized by significant hypotension (decreased blood pressure), decreased circulating blood volume (leaky vessels), and smooth muscle cells


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Histology_Ross. This often-explosive, life-threatening reaction is characterized by significant hypotension (decreased blood pressure), decreased circulating blood volume (leaky vessels), and smooth muscle cells constriction in the bronchial tree. The individual has difficulty breathing and may exhibit a rash, as well as have nausea and vomiting. Symptoms of anaphylactic shock usually develop within 1 to 3 minutes, and immediate treatment with vasoconstric-tors such as epinephrine is required. The assessment of the activation of basophils in systemic anaphylactic reac-tions is still problematic because an assay for a specific cellular marker released by basophils (and not by other cells such as mast cells) has not yet been developed. After the signs or symptoms of the immediate hyper-sensitiviy reaction have been resolved, an affected indi-vidual may develop late-phase allergic reactions 6 to 24 hours later. The symptoms of these reactions may in-clude redness, persistent swelling of the skin, nasal


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Histology_Ross. have been resolved, an affected indi-vidual may develop late-phase allergic reactions 6 to 24 hours later. The symptoms of these reactions may in-clude redness, persistent swelling of the skin, nasal dis-charge, sneezing, and coughing, usually accompanied by an elevated white blood cell count. These symptoms usu-ally last a few hours and then disappear within 1 to 2 days of the initial allergen exposure. In the respiratory system, the late-phase reaction is believed to be responsible for the development of persistent asthma. If the exposure to an allergen is persistent (for instance by a dog-owning patient who is allergic to dogs), it can result in chronic allergic infammation. Tissues in such individuals accumulate variety of immune cells, such as eosinophils and T lymphocytes that cause more tissue dam-age and prolong inflammation. This can lead to permanent structural and functional changes in the affected tissue.


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Histology_Ross. bone marrow stromal cells (BMSCs) that can generate chondrocytes, osteoblasts, adipocytes, muscle cells, and endothelial cells. The MAPCs are adult counterparts of embryonic stem cells. Niches of adult stem cells called mesenchymal stem cells are found in loose connective tissue of the adult. These cells give rise to differentiated cells that function in the repair and formation of new tissue such as in wound healing and in the development of new blood vessels (neovascularization). The vascular pericytes found around capillaries and venules are mesenchymal stem cells.


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Histology_Ross. The vascular pericytes found around capillaries and venules are mesenchymal stem cells. Pericytes, also called adventitial cells or perivascular cells, are found around capillaries and venules (Fig. 6.24). Several observations support the interpretation that vascular pericytes are indeed mesenchymal stem cells. Experimental studies show that in response to external stimuli, pericytes express a cohort of proteins similar to those of stem cells in the bone marrow. Pericytes are surrounded by basal lamina material that is continuous with the basal lamina of the capillary endothelium; thus, they are not truly located in the connective tissue compartment. The pericyte is typically wrapped, at least partially, around the capillary, and its nucleus takes on a shape similar to that of endothelial cells (i.e., flattened but curved to conform to the tubular shape of the vessel).


Histology_Ross

Histology_Ross. TEM studies demonstrate that pericytes surrounding the smallest venules have cytoplasmic characteristics almost identical with those of the endothelial cells of the same vessel. Peri cytes associated with larger venules have characteristics of smooth muscle cells of the tunica media of small veins. In fortuitous sections cut parallel to the long axis of venules, the distal portion and proximal portion of the same pericyte exhibit characteristics of endothelial cells and smooth muscle cells, respectively. These studies suggest that during the development of new vessels, cells with characteristics of pericytes may differentiate into smooth muscle of the vessel wall. The role of pericytes as mesenchymal stem cells was confirmed experimentally in studies in which cultured pericytes from retinal capillaries were able to differentiate into a variety of cells, including osteoblasts, adipocytes, chondrocytes, and fibroblasts. FIGURE 6.24 • Electron micrograph of a small blood vessel.


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Histology_Ross. FIGURE 6.24 • Electron micrograph of a small blood vessel. The nucleus at the upper left belongs to the endothelial cell that forms the wall of the vessel. At the right is another cell, a pericyte, that is in intimate relation to the endothelium. Note that the basal lamina (BL) covering the endothelial cell divides (arrows) to surround the pericyte. 11,000. The fibroblasts and blood vessels within healing wounds develop from mesenchymal stem cells associated with the tunica adventitia of venules. Autoradiographic studies of wound healing using parabiotic (crossed-circulation) pairs of animals have established that mesenchymal stem cells located in the tunica adventitia of venules and small veins are the primary source of new cells in healing wounds. In addition, fibroblasts, pericytes, and endothelial cells in portions of the connective tissue adjacent to the wound divide and give rise to additional cells that form new connective tissue and blood vessels.


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Histology_Ross. Lymphocytes, Plasma Cells, and Other Cells of the Immune System Lymphocytes are principally involved in immune responses. Connective tissue lymphocytes are the smallest of the wandering cells in the connective tissue (see Fig. 6.23b). They have a thin rim of cytoplasm surrounding a deeply staining, heterochromatic nucleus. Often, the cytoplasm of connective tissue lymphocytes may not be visible. Normally, small numbers of lymphocytes are found in the connective tissue throughout the body. The number increases dramatically, however, at sites of tissue infammation caused by pathogenic agents. Lymphocytes are most numerous in the lamina propria of the respiratory and gastrointestinal tracts, where they are involved in immunosurveillance against pathogens and foreign substances that enter the body by crossing the epithelial lining of these systems. Lymphocytes are a heterogeneous population of at least three major functional cell types: T cells, B cells, and natural killer (NK) cells.


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Histology_Ross. Lymphocytes are a heterogeneous population of at least three major functional cell types: T cells, B cells, and natural killer (NK) cells. At the molecular level, lymphocytes are characterized by the expression of specific molecules on the plasma membrane known as cluster of differentiation (CD) proteins. CD proteins recognize specific ligands on target cells. Because some CD proteins are present only on specific types of lymphocytes, they are considered specific marker proteins. On the basis of these specific markers, lymphocytes can be classified into three functional cell types.  T lymphocytes are characterized by the presence of the CD2, CD3, CD5, and CD7 marker proteins and the T-cell receptors (TCRs). These cells have a long life span and are effectors in cell-mediated immunity.


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Histology_Ross.  B lymphocytes are characterized by the presence of CD9, CD19, and CD20 proteins and attached immunoglobulins IgM and IgD. These cells recognize antigen, have a variable life span, and are effectors in antibody-mediated (humoral) immunity.  NK lymphocytes are non-T, non-B lymphocytes that express the CD16, CD56, and CD94 proteins, not found on other lymphocytes. These cells neither produce immunoglobulins nor express TCR on their surface. Thus, NK lymphocytes are not antigen specific. Similar in action to T lymphocytes, however, they destroy virus-infected cells and some tumor cells by a cytotoxic mechanism. In response to the presence of antigens, lymphocytes become activated and may divide several times, producing clones of themselves. In addition, clones of B lymphocytes mature into plasma cells. A description of B and T lymphocytes and their functions during immune response reactions is presented in Chapter 14.


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Histology_Ross. Plasma cells are antibody-producing cells derived from B lymphocytes. Plasma cells are a prominent constituent of loose connective tissue where antigens tend to enter the body (e.g., the gastrointestinal and respiratory tracts). They are also a normal component of salivary glands, lymph nodes, and hematopoietic tissue. Once derived from its precursor, the B lymphocyte, a plasma cell has only limited migratory ability and a somewhat short life span of 10 to 30 days. The plasma cell is a relatively large, ovoid cell (20 m) with a considerable amount of cytoplasm. The cytoplasm displays strong basophilia because of an extensive rER (Fig. 6.25a). The Golgi apparatus is usually prominent because of its relatively large size and lack of staining. It appears in light microscope preparations as a clear area in contrast to the basophilic cytoplasm.


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Histology_Ross. The nucleus is spherical and typically offset or eccentrically positioned. It is small, not much larger than the nucleus of the lymphocyte. It exhibits large clumps of peripheral heterochromatin alternating with clear areas of euchromatin. This arrangement has traditionally been described as resembling a cartwheel or analog clock face, with the heterochromatin resembling the spokes of the wheel or the numbers on a clock (Fig. 6.25b). The heterochromatic nucleus of the plasma cell is somewhat surprising given the cell’s function in synthesizing large amounts of protein. However, because the cells produce large amounts of only one type of protein—a specific antibody—only a small segment of the genome is exposed for transcription. Eosinophils, monocytes, and neutrophils are also observed in connective tissue.


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Histology_Ross. Eosinophils, monocytes, and neutrophils are also observed in connective tissue. As a result of immune responses and tissue injury, certain cells rapidly migrate from the blood to enter the connective tissue, particularly neutrophils and monocytes. Their presence generally indicates an acute inflammatory reaction. In these reactions, neutrophils migrate into the connective tissue in substantial numbers, followed by large numbers of monocytes. As noted, the monocytes then differentiate into macrophages. A description of these cells and their roles is found in Chapter 10. The eosinophil, which functions in allergic reactions and parasitic infections, is also presented in that chapter. Eosinophils may be observed in normal connective tissue, particularly the lamina propria of the intestine, as a result of chronic immunologic responses that occur in these tissues.


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Histology_Ross. FIGURE 6.25 • The plasma cell. a. This photomicrograph shows the typical features of a plasma cell as seen in a routine H&E preparation. Note clumps of peripheral heterochromatin alternating with clear areas of euchromatin in the nucleus. Also note the negative Golgi (arrows) and basophilic cytoplasm. 5,000. b. Electron micrograph shows that an extensive rER occupies most of the cytoplasm of the plasma cell. The Golgi apparatus (G) is also relatively large, a further reflection of the cell’s secretory activity. 15,000. This page intentionally left blank.


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Histology_Ross. This page intentionally left blank. Loose and dense connective tissue represents one of the several types of connective tissue. The others are namely cartilage, bone, blood, adipose tissue, and reticular tissue. Loose connective tissue is characterized by a relatively high proportion of cells within a matrix of thin and sparse collagen fibers. In contrast, dense irregular connective tissue contains few cells, almost all of which are fibroblasts that are re-sponsible for the formation and maintenance of the abundant collagen fibers that form the matrix of this tissue. The cells that are typically as-sociated with loose connective tissue are fibroblasts, the collagen-forming cells, and those cells that function in the immune system and those of the body’s general defense system. Thus, in loose connective tissue, there are, to varying degrees, lymphocytes, macrophages, eosinophils, plasma cells, and mast cells.


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Histology_Ross. Loose and dense irregular connective tissue, mammary gland, human, H&E, ×175; insets ×350.


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Histology_Ross. This micrograph shows at low magnification both loose connective tissue (LCT ) and dense irregular connective tissue (DICT ) for comparative purpose. The loose connective tissue surrounds the glandular epithelium (GE). The dense ir192 regular connective tissue consists mainly of thick bundles of collagen fibers with few cells present, whereas the loose connective tissue has a rela tive paucity of fibers and a considerable number of cells. The upper inset is a higher magnification of the dense connective tissue. Note that only a few cell nuclei are present relative to the larger expanse of collagen fibers. The lower inset, revealing the glandular epithelium and surrounding loose connective tissue, shows very few fibers, but large numbers of cells. Typically, the cellular component of loose connective tissue contains a relatively small proportion of fibroblasts, but large numbers of lymphocytes, plasma cells, and other connective tissue cell types.


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Histology_Ross. Loose connective tissue, colon, monkey, Mallory, ×250. This micrograph reveals an extremely cellular loose connective tissue (LCT), also called lamina propria, which is located between the intestinal glands of the colon. The simple columnar mucous secreting epithelial cells seen here represent the glandular tissue. The Mallory stain colors cell nuclei red and collagen blue. Note how the cells are surrounded by a framework of the blue stained collagen fibers. Also shown in this micrograph is a band of smooth muscle, the muscularis mucosa (MM) of the colon and below that, seen in part, is dense irregular connective tissue (DICT) that forms the submucosa of the colon. Typically, the collagen fibers (C) that lie just below the epithelial cells (Ep) at the luminal surface are more concentrated and thus appear prominently in the micrograph.


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Histology_Ross. Loose connective tissue, colon, monkey, threads that form a stroma surrounding the cells. The majority of cells that Mallory, ×700. are present here consists of lymphocytes and plasma cells (P). Other cells that are present within the stromal framework consist of fibroblasts, Shown at higher magnification is the boxed area in the adjasmooth muscle cells, macrophages, and occasional mast cells. cent figure. The base of the epithelial cells is seen on each side of the micrograph. The collagen fibers (C) appear as thin KEY CF, collagen fibers DICT, dense irregular connective tissue Ep, epithelial cells GE, glandular epithelium LCT, loose connective tissue MM, muscularis mucosa P, plasma cells


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Histology_Ross. PLATE 5 Dense Regular Connective Tissue, Tendons,and Ligaments 194 Dense regular connective tissue is distinctive in that its fibers are very densely packed and are organized in parallel array into fascicles. The collagen fibrils that make up the fibers are also arranged in an ordered parallel array. Tendons, which attach muscle to bone, and ligaments, which attach bone to bone, are examples of this type of tissue. Ligaments are similar to tendons in most respects, but their fibers and the or-ganization of the fascicles tend to be less ordered. In tendons as well as ligaments, the fascicles are separated from one another by dense irregular connective tissue, the endotendineum, through which travel vessels and nerves. Also, a fascicle may be partially divided by connective tissue septa that extend from the endo-tendineum and contain the smallest vessels and nerves. Some of the fascicles may be grouped into larger functional units by a thicker, sur-rounding connective tissue, the


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Histology_Ross. that extend from the endo-tendineum and contain the smallest vessels and nerves. Some of the fascicles may be grouped into larger functional units by a thicker, sur-rounding connective tissue, the peritendineum. Finally, the fascicles and groups of fascicles are surrounded by dense irregular connective tissue, the epitendineum. The fibroblasts, also called tendinocytes in tendons, are elongated cells that possess exceedingly thin, sheet-like cytoplasmic processes that reside between and embrace adjacent fibers. The margins of the cytoplasmic processes contact those of neighboring tendon cells, thus forming a syncytium-like cytoplasmic network. The most regular dense connective tissue is that of the stroma of the cornea of the eye (see Chapter 24). In this tissue, the collagen fibrils are arranged in parallel in lamellae that are separated by large, flattened fibroblasts. Adjacent lamellae are arranged at approximately right an-gles to one another, thus forming an orthogonal array. The


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Histology_Ross. in parallel in lamellae that are separated by large, flattened fibroblasts. Adjacent lamellae are arranged at approximately right an-gles to one another, thus forming an orthogonal array. The extreme regularity of fibril size and fibril spacing in each lamella, in conjunction with the orthogonal array of the lamellae, is believed to be the basis of corneal transparency.


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Histology_Ross. PLATE 5 • DENSE REGULAR CONNECTIVE TISSUE, TENDONS, AND LIGAMENTS Dense regular connective tissue, tendon, longitudinal section, human, H&E ×100. This specimen includes the surrounding dense irregular connective tissue of the tendon, the epitendineum (Ept). The tendon fascicles (TF) that make up the tendon are surrounded by a less dense connective tissue than that associated with the epitendineum. In longitudinal sections such as this, the connective tissue that surrounds the individual fascicles, the endotendineum (Ent), Dense regular connective tissue, tendon, longitudinal section, human, H&E ×400. This higher magnification micrograph shows the ordered single-file array of the tendinocyte nuclei (TC) along with the intervening collagen. The latter has a homogeneous appearance.


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Histology_Ross. This higher magnification micrograph shows the ordered single-file array of the tendinocyte nuclei (TC) along with the intervening collagen. The latter has a homogeneous appearance. seems to disappear at certain points, with the result that one fascicle appears to merge with a neighboring fascicle. This is due to an obliqueness in the plane of section rather than an actual merging of fascicles. The collagen that makes up the bulk of the tendon fascicle has a homogeneous appearance as a result of the orderly packing of the individual collagen fibrils. The nuclei of the tendinocytes appear as elongate profiles arranged in linear rows. The cytoplasm of these cells blends in with the collagen, leaving only the nuclei as the representative feature of the cell.


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Histology_Ross. The cytoplasm of the cells is indistinguishable from the collagen, as is typical in H&E paraffin specimens. The variation in nuclear appearance is due to the plane of section and the position of the nuclei within the thickness of the section. A small blood vessel (BV) coursing within the endotendineum is also present in the specimen. Dense regular connective tissue, tendon, cross section, human, H&E ×400.


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Histology_Ross. Dense regular connective tissue, tendon, cross section, human, H&E ×400. This specimen is well preserved, and the densely packed collagenous fibers appear as a homogeneous field, even though the fibers are viewed on their cut ends. The nuclei appear irregularly scattered, as opposed to their more uniform pattern in the longitudinal plane. This is explained by examining the dashed line in lower left figure which is meant to represent an arbitrary cross-sectional cut of the tendon. Note the irregular spacing of the nuclei that are in the plane of the cut. Lastly, several small blood vessels (BV) are present within the endotendineum (Ent) within a fascicle. KEY BV, blood vessel Ent, endotendineum Ept, epitendineum TC, tendinocytes nuclei TF, fascicle of tendon dashed line, arbitrary cross-sectional cut of tendon PLATE 5 • DENSE REGULAR CONNECTIVE TISSUE, TENDONS, AND LIGAMENTS


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Histology_Ross. Elastic fibers are present in loose and dense connective tissue throughout the body, but in lesser amounts than collagenous fibers. Elastic fibers are not conspicuous in routine H&E sections but are visualized readily with special staining methods. (The following selectively color elastic material: Weigert’s elastic tissue stain, purple-violet; Gomori’s aldehyde fuchsin stain, blue-black; Verhoeff’s hematoxylin elastic tissue stain, black; and modified Taenzer-Unna orcein stain, red-brown.) By using a combination of the special elastic stains and counterstains, such as H&E, not only the elastic fibers but also the other tissue components may be revealed, thus allowing study of the relationships between the elastic material and other connective tissue components. Elastic material occurs in both fibrous and lamellar forms. In loose and dense connective tissue and in elastic cartilage (see Plate 9, page 221), the elastic material is in fibrous form. Similarly, the elastic ligaments that


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Histology_Ross. both fibrous and lamellar forms. In loose and dense connective tissue and in elastic cartilage (see Plate 9, page 221), the elastic material is in fibrous form. Similarly, the elastic ligaments that connect the cervical vertebrae and that are particularly prominent in grazing animals have a mixture of elastic and collagenous fibers in a tightly packed array. In the major, largest diameter arteries (e.g., aorta, pulmonary, common carotid, and other primary branches of the aorta), the tunica media consists of fenestrated layers of elastic tissue alternating with layers containing smooth muscle cells and collagenous tissue. This allows stretching and elastic rebound to assist in the propulsion of the blood. All arteries and most large arterioles have an internal elastic membrane that supports the delicate endothelium and its immediately subjacent connective tissue. It should be noted that both the collagen and elastic components of the tunica media are produced by the smooth muscle cells


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Histology_Ross. the delicate endothelium and its immediately subjacent connective tissue. It should be noted that both the collagen and elastic components of the tunica media are produced by the smooth muscle cells of this layer.


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Histology_Ross. Elastic fbers, dermis, monkey, Welgert’s ×160. This shows the connective tissue of the skin, referred to as the dermis, stained to show the nature and distribution of the elastic fibers (E), which appear purple. The collagen fibers (C) have been stained by eosin, and the two fiber types are easily differentiated. The connective tissue at the top of the figure, close to the epithelium (the papillary layer of the dermis), contains thin elastic fibers (see upper left of figure) as well as less coarse collagen fibers. The KEY BV, blood vessel C, collagen fibers D, duct of sweat gland E, elastic fibers PLATE 6 • E LASTIC FIBERS AND E LASTIC LAM E LLAE Elastic fbers, mesentery, rat, Weigert’s ×160. beginnings or endings and with a somewhat irregular course. Again, the collagen fibers (C) are contrasted by their eosin staining and appear as long, This is a whole mount specimen of mesentery prepared to straight profiles that are considerably thicker than the elastic fibers.


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Histology_Ross. This is a whole mount specimen of mesentery prepared to straight profiles that are considerably thicker than the elastic fibers. show the connective tissue elements and differentially stained to reveal elastic fibers. The elastic fibers (E) appear as thin, long, crisscrossing and branching threads without discernible Elastic lamellae, elastic artery, monkey, Weigert’s ×80. Elastic material also occurs in sheets or lamellae rather than string-like fibers. This figure shows the wall of an elastic artery (pulmonary artery) that was stained to show the elastic material. Each of the wavy lines is a lamella of elastic material that is organized in the form of a fenestrated sheet or membrane. The plane of section is such that the elastic membranes are seen on edge. This specimen was not subsequently stained with H&E.


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Histology_Ross. lower portion of the figure shows considerably heavier elastic and collagen fibers. Also note that many of the elastic fibers appear as short rectangular profiles. These profiles simply represent fibers traveling through the thickness of the section at an oblique angle to the path of the knife. Careful examination will also reveal a few fibers that appear as dot-like profiles. They represent cross-sectioned elastic fibers. Overall, the elastic fibers of the dermis have a three-dimensional interlacing configuration, thus the variety of forms. The empty-appearing spaces between elastic layers contain collagen fibers and smooth muscle cells, but they remain essentially unstained. In the muscular layer of blood vessel, both elastin and collagen are secreted by the smooth muscle cells. Tissues of the body containing large amounts of elastic material are limited in distribution to the walls of elastic arteries and some ligaments that are associated with the spinal column.


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Histology_Ross. Tissues of the body containing large amounts of elastic material are limited in distribution to the walls of elastic arteries and some ligaments that are associated with the spinal column. OVERVIEW OF CARTILAGE / 198 HYALINE CARTILAGE / 199 ELASTIC CARTILAGE / 204 FIBROCARTILAGE / 204 CHONDROGENESIS AND CARTILAGE GROWTH / 206 REPAIR OF HYALINE CARTILAGE / 207 Folder 7.1 Clinical Correlation: Osteoarthritis / 199 Folder 7.2 Clinical Correlation: Malignant Tumors of the Cartilage; Chondrosarcomas / 208 Cartilage is a form of connective tissue composed of cells called chondrocytes and a highly specialized extracellular matrix. Cartilage is an avascular tissue that consists of chondrocytes and an extensive extracellular matrix. More than 95% of cartilage volume consists of extracellular matrix, which is a functional element of this tissue. The chondrocytes are sparse but essential participants in producing and maintaining the matrix (Fig. 7.1).


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Histology_Ross. The extracellular matrix in cartilage is solid and firm but also somewhat pliable, which accounts for its resilience. Because there is no vascular network within cartilage, the composition of the extracellular matrix is crucial to the survival of the chondrocytes. The large ratio of glycosaminoglycans (GAGs) to type II collagen fibers in the cartilage matrix permits diffusion of substances between blood vessels in the surrounding connective tissue and the chondrocytes dispersed within the matrix, thus maintaining the viability of the tissue. Close interactions are seen between two classes of structural molecules that possess contrasting biophysical characteristics: the meshwork of tension-resisting collagen fibrils and the large amounts of heavily hydrated proteoglycan aggregates, the latter being extremely weak in shear, make the cartilage well adapted to bear weight, especially at points of movement, as in synovial joints. Because it maintains this property even while growing,


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Histology_Ross. latter being extremely weak in shear, make the cartilage well adapted to bear weight, especially at points of movement, as in synovial joints. Because it maintains this property even while growing, cartilage is a key tissue in the development of the fetal skeleton and in most growing bones.


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Histology_Ross. FIGURE 7.1 • General structure of hyaline cartilage. This photomicrograph of a routine H&E preparation of hyaline cartilage shows its general features. Note the extensive extracellular matrix that separates a sparse population of chondrocytes. 450. Three types of cartilage that differ in appearance and mechanical properties are distinguished on the basis of characteristics of their matrix:  Hyaline cartilage is characterized by matrix containing type II collagen fibers, GAGs, proteoglycans, and multiad hesive glycoproteins.  Elastic cartilage is characterized by elastic fibers and elas tic lamellae in addition to the matrix material of hyaline cartilage.  Fibrocartilage is characterized by abundant type I colla gen fibers as well as the matrix material of hyaline cartilage. Table 7.1 lists the locations, functions, and features of each type of cartilage. Hyaline cartilage is distinguished by a homogeneous, amorphous matrix.


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Histology_Ross. The matrix of hyaline cartilage appears glassy in the living state: hence, the name hyaline [Gr. hyalos, glassy]. Throughout the cartilage matrix are spaces called lacunae. Located within these lacunae are the chondrocytes. Hyaline cartilage is not a simple, inert, homogeneous substance but a complex living tissue. It provides a low-friction surface, participates in lubricating synovial joints, and distributes applied forces to the underlying bone. Although its capacity for repair is limited, under normal circumstances it shows no evidence of abrasive wear over a lifetime. An exception is articular cartilage, which, in many individuals, breaks down with age (Folder 7.1). The macromolecules of hyaline cartilage matrix consist of collagen (predominantly type II fibrils and other cartilage-specific collagen molecules), proteoglycan aggregates containing GAGs, and multiadhesive glycoproteins (noncollagenous proteins). Figure 7.2 illustrates the relative distribution of the various


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Histology_Ross. collagen molecules), proteoglycan aggregates containing GAGs, and multiadhesive glycoproteins (noncollagenous proteins). Figure 7.2 illustrates the relative distribution of the various components that constitute cartilage matrix.


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Histology_Ross. Hyaline cartilage matrix is produced by chondrocytes and contains three major classes of molecules. Three classes of molecules exist in hyaline cartilage matrix.  Collagen molecules. Collagen is the major matrix pro tein. Four types of collagen participate in the formation of three-dimensional meshwork of the relatively thin (20-nm diameter) and short matrix fibrils. Type II collagen cells 3–5% multiadhesive glycoproteins 5% proteoglycans (aggrecan) 9% collagens 15% intercellular water 60–80% 5% III, VI, X, XII, XIV 15% IX, XI 80% II FIGURE 7.2 • Molecular composition of hyaline cartilage. Cartilage contains 60–80% of the wet weight of intercellular water, which is bound by proteoglycan aggregates. About 15% of the total weight is attributed to collagen molecules, of which type II collagen is the most abundant. Chondrocytes occupy only 3–5% of the total cartilage mass.


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Histology_Ross. Osteoarthritis, a degenerative joint disease, is one of the most common types of joint diseases. The pathogenesis of osteoarthritis is unknown, but it is related to aging and injury of articular cartilage. Most individuals show some evidence of this disease by age 65. The disease is charac-terized by chronic joint pain with various degrees of joint deformity and destruction of the articular cartilage. Os-teoarthritis commonly affects weight-bearing joints: hips, knees, lower lumbar vertebra, and joints of the hand and foot. There is a decrease in proteoglycan content, which re-sults in reduction in intercellular water content in the carti-lage matrix. Chondrocytes also play an important role in the pathogenesis of osteoarthritis. By producing interleukin-1 (IL-1) and tumor necrosis factor (TNF-), the production of metalloproteinases is stimulated, whereas synthesis of type II collagen and proteoglycans by the chondrocyte is in-hibited. In the early stages of the disease, the


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Histology_Ross. factor (TNF-), the production of metalloproteinases is stimulated, whereas synthesis of type II collagen and proteoglycans by the chondrocyte is in-hibited. In the early stages of the disease, the superficial layer of the articular cartilage is disrupted. Eventually, de-struction of the cartilage extends to the bone, where the ex-posed subchondral bone becomes a new articular surface. These changes result in progressive reduction of mobility and increased pain with joint movement. Osteoarthritis has no cure, and treatment focuses on relieving pain and stiff-ness to allow a greater range of joint movement. Os-teoarthritis may stabilize with age, but more often it slowly progresses with eventual long-term disability.


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FOLDER 7.1

Histology_Ross.  FOLDER 7.1


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Histology_Ross. Clinical Correlation: Osteoarthritis constitutes the bulk of the fibrils (see Fig. 7.2); type IX collagen facilitates fibril interaction with the matrix proteoglycan molecules; type XI collagen regulates the fibril size; and type X collagen organizes the collagen fibrils into a threedimensional hexagonal lattice that is crucial to its successful mechanical function. In addition, type VI collagen is also found in the matrix, mainly at the periphery of the chondrocytes where it helps to attach these cells to the matrix framework. Because types II, VI, IX, X, and XI are found in significant amounts only in the cartilage matrix, they are referred to as cartilage-specific collagen molecules. (Review the types of collagen in Table 6.2.)  Proteoglycans. The ground substance of hyaline cartilage contains three kinds of glycosaminoglycans: hyaluronan, chondroitin sulfate, and keratan sulfate. As in loose connective tissue matrix, the chondroitin and keratan sulfate of the cartilage matrix


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Histology_Ross. contains three kinds of glycosaminoglycans: hyaluronan, chondroitin sulfate, and keratan sulfate. As in loose connective tissue matrix, the chondroitin and keratan sulfate of the cartilage matrix are joined to a core protein to form a proteoglycan monomer. The most important proteoglycan monomer in hyaline cartilage is aggrecan. It has a molecular weight of 250 kilodaltons. Each molecule contains about 100 chondroitin sulfate chains and as many as 60 keratan sulfate molecules. Because of the presence of the sulfate groups, aggrecan molecules have a large negative charge with an affinity for water molecules. Each linear hyaluronan molecule is associated with a large number of aggrecan molecules (more than 300), which are bound to the hyaluronan by link proteins at the N terminus of the molecule to form large proteoglycan aggregates. These highly charged proteoglycan aggregates are bound to the collagen matrix fibrils by electrostatic interactions and multiadhesive glycoproteins (Fig.


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Histology_Ross. to form large proteoglycan aggregates. These highly charged proteoglycan aggregates are bound to the collagen matrix fibrils by electrostatic interactions and multiadhesive glycoproteins (Fig. 7.3). The entrapment of these aggregates within the intricate matrix of collagen fibrils is responsible for the unique biomechanical properties of hyaline cartilage.


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Histology_Ross. Cartilage matrix also contains other proteoglycans (i.e., decorin, biglycan, and fibromodulin). These proteoglycans do not form aggregates but bind to other molecules and help stabilize the matrix.  Multiadhesive glycoproteins, also referred to as noncollagenous and nonproteoglycan-linked glycoproteins, influence interactions between the chondrocytes and the matrix molecules. Multiadhesive glycoproteins have clinical value as markers of cartilage turnover and degeneration. Examples of such proteins are anchorin CII (cartilage annexin V), a small 34-kilodalton molecule that functions as a collagen receptor on chondrocytes, tenascin, and fibronectin (see Table 6.5, page 180), which also help anchor chondrocytes to the matrix.


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Histology_Ross. FIGURE 7.3 • Molecular structure of hyaline cartilage matrix. This schematic diagram shows the relationship of proteoglycan aggregates to type II collagen fibrils and chondrocytes in the matrix of hyaline cartilage. A hyaluronan molecule forming a linear aggregate with many proteoglycan monomers is interwoven with a network of collagen fibrils. The proteoglycan monomer (such as aggrecan) consists of approximately 180 glycosaminoglycans joined to a core protein. The end of the core protein contains a hyaluronan-binding region that is joined to the hyaluronan by a link protein. Hyaline cartilage matrix is highly hydrated to provide resilience and diffusion of small metabolites.


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Histology_Ross. Hyaline cartilage matrix is highly hydrated to provide resilience and diffusion of small metabolites. Like other connective tissue matrices, cartilage matrix is highly hydrated. From 60% to 80% of the net weight of hyaline cartilage is intercellular water (see Fig. 7.2). Much of this water is bound tightly to the aggrecan– hyaluronan aggregates, imparting resilience to the cartilage. Some of the water is bound loosely enough, however, to allow diffusion of small metabolites to and from the chondrocytes.


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Histology_Ross. In articular cartilage, both transient and regional changes occur in water content during joint movement and when the joint is subjected to pressure. The high degree of hydration and the movement of water in the matrix allow the cartilage matrix to respond to varying pressure loads and contribute to cartilage’s weight-bearing capacity. Throughout life, cartilage undergoes continuous internal remodeling as the cells replace matrix molecules lost through degradation. Normal matrix turnover depends on the ability of the chondrocytes to detect changes in matrix composition. The chondrocytes then respond by synthesizing appropriate types of new molecules. In addition, the matrix acts as a signal transducer for the embedded chondrocytes. Thus, pressure loads applied to the cartilage, as in synovial joints, create mechanical, electrical, and chemical signals that help direct the synthetic activity of the chondrocytes. As the body ages, however, the composition of the matrix changes, and the


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Histology_Ross. joints, create mechanical, electrical, and chemical signals that help direct the synthetic activity of the chondrocytes. As the body ages, however, the composition of the matrix changes, and the chondrocytes lose their ability to respond to these stimuli.


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Histology_Ross. Chondrocytes are specialized cells that produce and maintain the extracellular matrix. In hyaline cartilage, chondrocytes are distributed either singularly or in clusters called isogenous groups (Fig. 7.4). When the chondrocytes are present in isogenous groups, they represent cells that have recently divided. As the newly divided chondrocytes produce the matrix material that surrounds them, they are dispersed. They also secrete metalloproteinases, enzymes that degrade cartilage matrix, allowing the cells to expand and reposition themselves within the growing isogenous group.


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Histology_Ross. The appearance of chondrocyte cytoplasm varies according to chondrocyte activity. Chondrocytes that are active in matrix production display areas of cytoplasmic basophilia, which are indicative of protein synthesis, and clear areas, which indicate their large Golgi apparatus (Fig. 7.5). Chondrocytes not only secrete the collagen present in the matrix but also all of the glycosaminoglycans and proteoglycans. In older, less active cells, the Golgi apparatus is smaller; clear areas of cytoplasm, when evident, usually indicate sites of extracted lipid droplets and glycogen stores. In such specimens, chondrocytes also display considerable distortion resulting from shrinkage after the glycogen and lipid are lost during preparation of the tissue. In the transmission electron microscope (TEM), the active chondrocyte displays numerous profiles of rough-surfaced endoplasmic reticulum (rER), a large Golgi apparatus, secretory granules, vesicles, intermediate filaments, microtubules, and actin


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Histology_Ross. the active chondrocyte displays numerous profiles of rough-surfaced endoplasmic reticulum (rER), a large Golgi apparatus, secretory granules, vesicles, intermediate filaments, microtubules, and actin microfilaments (Fig. 7.6).


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Histology_Ross. Components of the hyaline cartilage matrix are not uniformly distributed. Because the proteoglycans of hyaline cartilage contain a high concentration of bound sulfate groups, ground substance stains with basic dyes and hematoxylin (Plate 7, page 210). Thus, the basophilia and metachromasia seen in stained sections of cartilage provide information about the distribution and relative concentration of sulfated proteoglycans.


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Histology_Ross. FIGURE 7.4 • Photomicrograph of a typical hyaline cartilage specimen stained with H&E. The upper portion of the micrograph shows the dense connective tissue (DCT ) overlying the perichondrium (P), from which new cartilage cells are derived. A slightly basophilic layer of growing cartilage (GC) underlying the perichondrium contains chondroblasts and immature chondrocytes that display little more than the nucleus residing in an empty-appearing lacuna. This layer represents deposition of new cartilage (appositional growth) on the surface of the existing hyaline cartilage. Mature chondrocytes with clearly visible nuclei (N) reside in the lacunae and are well preserved in this specimen. They produce the cartilage matrix that shows the dark-staining capsule or territorial matrix (TM) immediately surrounding the lacunae. The interterritorial matrix (IM) is more removed from the immediate vicinity of the chondrocytes and is less intensely stained. Growth from within the cartilage


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Histology_Ross. immediately surrounding the lacunae. The interterritorial matrix (IM) is more removed from the immediate vicinity of the chondrocytes and is less intensely stained. Growth from within the cartilage (interstitial growth) is reflected by the chondrocyte pairs and clusters that are responsible for the formation of isogenous groups (rectangles). 480.


Histology_Ross

Histology_Ross. FIGURE 7.5 • Photomicrograph of young, growing cartilage. This specimen was preserved in glutaraldehyde, embedded in plastic, and stained with H&E. The chondrocytes, especially those in the upper part of the photomicrograph, are well preserved. The cytoplasm is deeply stained, exhibiting a distinct and relatively homogeneous basophilia. The clear areas (arrows) represent sites of the Golgi apparatus. 520. However, the matrix does not stain homogeneously. Rather, three different regions are described based on the staining property of the matrix (Fig. 7.7).


Histology_Ross

Histology_Ross. However, the matrix does not stain homogeneously. Rather, three different regions are described based on the staining property of the matrix (Fig. 7.7).  The capsular (pericellular) matrix is a ring of more densely staining matrix located immediately around the chondrocyte (see Fig. 7.4). It contains the highest concentration of sulfated proteoglycans, hyaluronan, bigly-cans, and several multiadhesive glycoproteins (e.g., fibronectin, decorin, and laminin). The capsular matrix contains almost exclusively type VI collagen fibrils that form a tightly woven enclosure around each chondrocyte. Type VI collagen binds to integrin receptors on the cell surface and anchors the chondrocytes to the matrix. A higher concentration of type IX collagen is also present in the capsular matrix.


Histology_Ross

Histology_Ross.  The territorial matrix is a region that is more removed from the immediate vicinity of the chondrocytes. It surrounds the isogenous group and contains a randomly arranged network of type II collagen fibrils with smaller quantities of type IX collagen. It also has a lower concentra-


Histology_Ross

Histology_Ross. FIGURE 7.6 • Electron micrograph of a young, active chondrocyte and surrounding matrix. The nucleus (N) of the chondrocyte is eccentrically located, like those in Figure 7.5, and the cytoplasm displays numerous and somewhat dilated profiles of rER, Golgi apparatus (G), and mitochondria (M). The large amount of rER and the extensive Golgi apparatus indicate that the cell is actively engaged in the production of cartilage matrix. The numerous dark particles in the matrix contain proteoglycans. The particularly large particles adjacent to the cell are located in the region of the matrix that is identified as the capsule or territorial matrix. 15,000. (Courtesy of Dr. H. Clarke Anderson.) tion of sulfated proteoglycans and stains less intensely than the capsular matrix. The interterritorial matrix is a region that surrounds the territorial matrix and occupies the space between groups of chondrocytes.


Histology_Ross

Histology_Ross. In addition to these regional differences in the concentration of sulfated proteoglycans and distribution of collagen fibrils, there is a decrease in proteoglycan content that occurs as cartilage ages, which is also reflected by staining differences. Hyaline cartilage provides a model for the developing skeleton of the fetus. FIGURE 7.7 • Diagram of cartilage matrices. Note the areas of capsular, territorial, and interterritorial matrices. The characteristics of each are described in the text above.


Histology_Ross

Histology_Ross. In early fetal development, hyaline cartilage is the precursor of bones that develop by the process of endochondral ossification (Fig. 7.8). Initially, most long bones are represented by cartilage models that resemble the shape of the mature bone (Plate 8, page 212). During the developmental process, in which much of the cartilage is replaced by bone, the remaining cartilage serves as a growth site called the epiphyseal growth plate (epiphyseal disc). This cartilage remains functional as long as the bone grows in length (Fig. 7.9). In a fully grown individual, the cartilage that remains from the developing skeleton is found on the articular surfaces of joints (articular cartilage) and within the rib cage (costal cartilages). Hyaline cartilage also exists in the adult as the skeletal unit in the trachea, bronchi, larynx, and nose. A firmly attached connective tissue, the perichondrium, surrounds hyaline cartilage.


Histology_Ross

Histology_Ross. A firmly attached connective tissue, the perichondrium, surrounds hyaline cartilage. The perichondrium is a dense connective tissue composed of cells that are indistinguishable from fibroblasts. In many respects, the perichondrium resembles the capsule that surrounds glands and many organs. It also serves as the source of new cartilage cells. When actively growing, the perichondrium appears divided into an inner cellular layer, which gives rise to new cartilage cells, and an outer fibrous layer. This division is not always evident, especially in perichondrium that is not actively producing new cartilage or in very slow-growing cartilage. The changes that occur during the differentiation of new chondrocytes in growing cartilage are illustrated in Figure 7.4. Hyaline cartilage of articular joint surfaces do not possess a perichondrium. Hyaline cartilage that covers the articular surfaces of movable joints is termed articular cartilage. In general, the


Histology_Ross

Histology_Ross. Hyaline cartilage of articular joint surfaces do not possess a perichondrium. Hyaline cartilage that covers the articular surfaces of movable joints is termed articular cartilage. In general, the FIGURE 7.8 • Photomicrograph of several cartilages that form the initial skeleton of the foot. The hyaline cartilage of developing tarsal bones will be replaced by bone as endochondral ossification proceeds. In this early stage of development, synovial joints are being formed between developing tarsal bones. Note that nonarticulating surfaces of the hyaline cartilage models of tarsal bones are covered by perichondrium, which also contributes to the development of joint capsules. Also, a developing tendon (T ) is evident in the indentation of the cartilage seen on the left side of the micrograph. 85.


Histology_Ross

Histology_Ross. structure of articular cartilage is similar to that of hyaline cartilage. However, the free, or articular, surface has no perichondrium. Also, on the opposite surface, the cartilage contacts the bone, and there is no perichondrium. Articular cartilage is a remnant of the original hyaline cartilage template of the developing bone, and it persists throughout adult life. In adults, the articular cartilage is 2 to 5 mm thick and is divided into four zones (Fig. 7.10).  The superficial (tangential) zone is a pressure-resistant region closest to the articular surface. It contains numerous


Histology_Ross

Histology_Ross.  The superficial (tangential) zone is a pressure-resistant region closest to the articular surface. It contains numerous FIGURE 7.9 • Photomicrograph of the proximal end of a growing long bone. A disc of hyaline cartilage−the epiphyseal plate−separates the more proximally located epiphysis from the funnel-shaped diaphysis located distal to the plate. The articular cartilage on the surface of the epiphysis contributes to the synovial joint and is also composed of hyaline cartilage. The cartilage of the epiphyseal plate disappears when lengthwise growth of the bone is completed, but the articular cartilage remains throughout life. The spaces within the bone are occupied by marrow. 85. condensation of type II collagen fibrils that are arranged in fascicles parallel to the free surface.


Histology_Ross

Histology_Ross. condensation of type II collagen fibrils that are arranged in fascicles parallel to the free surface.  The intermediate (transitional) zone lies below the superficial zone and contains round chondrocytes randomly distributed within the matrix. Collagen fibrils are less organized and are arranged in a somewhat oblique orientation to the surface.  The deep (radial) zone is characterized by small, round chondrocytes that are arranged in short columns perpendicular to the free surface of the cartilage. The collagen fibrils are positioned between columns parallel to the long axis of the bone.


Histology_Ross

Histology_Ross.  The calcified zone is characterized by a calcified matrix with the presence of small chondrocytes. This zone is separated from the deep (radial) zone by a smooth, undulating, heavily calcified line called the tidemark. Above this line, proliferation of chondrocytes within the cartilage lacunae provides the new cells for interstitial growth. In articular cartilage renewal, chondrocytes migrate from this region toward the joint surface. The renewal process of mature articular cartilage is very slow. This slow growth is a reflection of the highly stable type II collagen network and the long half-life of its proteoglycan molecules. Also, in healthy articular cartilage, metalloproteinase (MMP-1 and MMP-13) activity is low. Elastic cartilage is distinguished by the presence of elastin in the cartilage matrix.


Histology_Ross

Histology_Ross. Elastic cartilage is distinguished by the presence of elastin in the cartilage matrix. In addition to containing the normal components of hyaline cartilage matrix, elastic cartilage matrix also contains a dense network of branching and anastomosing elastic fibers and interconnecting sheets of elastic material (Fig. 7.11 and Plate 9, page 214). These fibers and lamellae are best demonstrated in paraffin sections with special stains such as resorcinfuchsin and orcein. The elastic material gives the cartilage elastic properties in addition to the resilience and pliability that are characteristic of hyaline cartilage.


Histology_Ross

Histology_Ross. Elastic cartilage is found in the external ear, the walls of the external acoustic meatus, the auditory (Eustachian) tube, and the epiglottis of the larynx. The cartilage in all of these locations is surrounded by a perichondrium similar to that found around most hyaline cartilage. Unlike hyaline cartilage, which calcifies with aging, the matrix of elastic cartilage does not calcify during the aging process. Fibrocartilage consists of chondrocytes and their matrix material in combination with dense connective tissue.


Histology_Ross

Histology_Ross. Fibrocartilage consists of chondrocytes and their matrix material in combination with dense connective tissue. Fibrocartilage is a combination of dense regular connective tissue and hyaline cartilage. The chondrocytes are dispersed among the collagen fibers singularly, in rows, and in isogenous groups (Fig. 7.12 and Plate 10, page 216). These chondrocytes appear similar to the chondrocytes of hyaline cartilage, but they have considerably less cartilage matrix material. There is also no surrounding perichondrium as in hyaline and elastic cartilage. In a section containing fibrocartilage, a population of cells with rounded nuclei and a small amount of surrounding amorphous matrix material can typically be seen. These nuclei belong to the chondrocytes. Within the fibrous areas are nuclei that are flattened or elongated. These are fibroblast nuclei.


Histology_Ross

Histology_Ross. Fibrocartilage is typically present in intervertebral discs, the symphysis pubis, articular discs of the sternoclavicular and temporomandibular joints, menisci of the knee joint, the triangular fibrocartilage complex of the wrist, and certain places where tendons attach to bones. The presence of fibrocartilage in these sites indicates that resistance to both compression and shearing forces is required of the tissue. The cartilage serves much like a shock absorber. The degree to which such forces occur is reflected in the amount of cartilage matrix material present. Extracellular matrix of fibrocartilage is characterized by the presence of both type I and type II collagen fibrils.


Histology_Ross

Histology_Ross. The cells in fibrocartilage synthesize a wide variety of extracellular matrix molecules not only during its development stage but also during its mature, fully differentiated state. This allows the fibrocartilage to respond to changes in the external environment (such as mechanical forces, nutritional changes, and changing levels of hormones and growth factors). The extracellular matrix of fibrocartilage contains significant quantities of both type I collagen (characteristic of connective tissue matrix) and type II collagen (characteristic of hyaline cartilage). The relative proportions of these collagens can vary. For example, menisci of the knee joint contain only a small quantity of the type II collagen, whereas the intervertebral disc contains equal amounts of type I and type II collagen fibers. The ratio between type I and type II collagen in fibrocartilage changes with age. In older individuals, there is more type II collagen because of the metabolic activity of chondrocytes,


Histology_Ross

Histology_Ross. fibers. The ratio between type I and type II collagen in fibrocartilage changes with age. In older individuals, there is more type II collagen because of the metabolic activity of chondrocytes, which constantly produce and discharge type II collagen fibrils into the surrounding matrix. In addition, the extracellular matrix of fibrocartilage contains larger amounts of versican (a proteoglycan monomer secreted by fibroblasts) than aggrecan (produced by chondrocytes). Versican can also bind hyaluronan to form highly hydrated proteoglycan aggregates (see Table 6.4, page 179).


Histology_Ross

Histology_Ross. (tangential) zone intermediate (transitional) zone deep (radial) zone tidemark calcified zone subchondral bone cancellous bone ababarticularcartilageSZIZDZCZbonearticularcartilageSZIZDZCZboneFIGURE 7.10 • Diagram and photomicrograph of articular cartilage. a. This diagram shows the organization of the collagen network and chondrocytes in the various zones of articular cartilage. b. Photomicrograph of normal articular cartilage from an adult. The superficial zone (SZ ) exhibits elongated and flattened chondrocytes. The intermediate zone (IZ ) contains round chondrocytes. The deep zone (DZ ) contains chondrocytes arranged in short columns. The calcified zone (CZ ), which borders the bone, exhibits a calcified matrix and lacks chondrocytes. Also, this zone is lighter staining than the matrix of the more superficial zones. The tidemark is indicated by the dashed line. 160.


Histology_Ross

Histology_Ross. FIGURE 7.11 • Photomicrograph of elastic cartilage from the epiglottis. This specimen was stained with orcein and reveals the elastic fibers, stained brown, within the cartilage matrix. The elastic fibers are of various sizes and constitute a significant part of the cartilage. Chondrocyte nuclei are evident in many of the lacunae. The perichondrium is visible at the top of the photomicrograph. 180.


Histology_Ross

Histology_Ross. TABLE Summary of Cartilage Features 7.1 Features Hyaline Cartilage Elastic Cartilage Fibrocartilage Location Fetal skeletal tissue, epiphyseal Pinna of external ear, external Intervertebral discs, symphysis plates, articular surface of acoustic meatus, auditory publis, articular discs synovial joints, costal cartilages (Eustachian) tube, cartilages of (sternoclavicular and of rib cage, cartilages of nasal larynx (epiglottis, corniculate, temporomandibular joints), cavity, larynx (thyroid, cricoid, and cuneiform cartilages) menisci (knee joint), triangular and arytenoids), rings of trachea fibrocartilage complex (wrist joint), and plates in bronchi insertion of tendons Function Resists compression Provides flexible support Resists deformation under stress Provides cushioning, smooth, and low-friction surface for joints Provides structural support in respiratory system (larynx, trachea, and bronchi) Forms foundation for development of fetal skeleton and further endochondral bone


Histology_Ross

Histology_Ross. low-friction surface for joints Provides structural support in respiratory system (larynx, trachea, and bronchi) Forms foundation for development of fetal skeleton and further endochondral bone formation and bone growth Presence of Yes (except articular cartilage Yes No perichondrium and epiphyseal plates) Undergoes Yes (i.e., during endochondral No Yes (i.e., calcification of calcification bone formation, during aging fibrocartilaginous callus during process) bone repair) Main cell Chondroblasts and Chondroblasts and Chondrocytes and fibroblasts types present chondrocytes chondrocytes Characteristic Type II collagen fibrils and Type II collagen fibrils, elastic Types I and II collagen fibers and features of aggrecan (the most important fibers, and aggrecan versican (a proteoglycan secreted extracellular proteoglycan) by fibroblasts) matrix Growth Interstitially and appositionally, very limited in adults Repair Very limited capability, commonly forms scar, resulting in


Histology_Ross

Histology_Ross. secreted extracellular proteoglycan) by fibroblasts) matrix Growth Interstitially and appositionally, very limited in adults Repair Very limited capability, commonly forms scar, resulting in fibrocartilage formation


Histology_Ross

Histology_Ross. Most cartilage arises from mesenchyme during chondrogenesis. Chondrogenesis, the process of cartilage development, begins with the aggregation of chondroprogenitor mesen chymal cells to form a mass of rounded, closely apposed cells. In the head, most of the cartilage arises from aggregates of ectomesenchyme derived from neural crest cells. The site of hyaline cartilage formation is recognized initially by an aggregate of mesenchymal or ectomesenchymal cells known as a chondrogenic nodule. Expression of transcription factor SOX-9 triggers differentiation of these cells into chondro blasts, which then secrete cartilage matrix (expression of SOX-9 coincides with secretion of type II collagen). The chondroblasts progressively move apart as they deposit matrix. When they are completely surrounded by matrix material, the cells are called chondrocytes. The mesenchymal tissue immediately surrounding the chondrogenic nodule gives rise to the perichondrium.


Histology_Ross

Histology_Ross. Chondrogenesis is regulated by many molecules, including extracellular ligands, nuclear receptors, transcription factors, adhesion molecules, and matrix proteins. Furthermore, FIGURE 7.12 • Photomicrograph of fibrocartilage from an intervertebral disc. The collagen fibers are stained green in this Gomori trichrome preparation. The tissue has a fibrous appearance and contains a relatively small number of fibroblasts with elongated nuclei (arrows) as well as more numerous chondrocytes with dark round nuclei. The chondrocytes exhibit close spatial groupings and are arranged either in rows among the collagen fibers or in isogenous groups. 60. Inset. Higher magnification of an isogenous group. Chondrocytes are contained within lacunae. Typically, there is little cartilage matrix surrounding the chondrocytes. 700.


Histology_Ross

Histology_Ross. the growth and development of the cartilage skeleton is influenced by biomechanical forces. These forces not only regulate the shape, regeneration, and aging of cartilage but also modify cell–to–extracellular matrix interactions within the cartilage. Cartilage is capable of two kinds of growth, appositional and interstitial. With the onset of matrix secretion, cartilage growth continues via a combination of two processes:  Appositional growth, the process that forms new carti lage at the surface of an existing cartilage; and  interstitial growth, the process that forms new cartilage


Histology_Ross

Histology_Ross. New cartilage cells produced during appositional growth are derived from the inner portion of the surrounding perichondrium. The cells resemble fibroblasts in form and function, producing the collagen component of the perichondrium (type I collagen). When cartilage growth is initiated, however, the cells undergo a differentiation process guided by an expression of the transcription factor SOX-9. The cytoplasmic processes disappear, the nucleus becomes rounded, and the cytoplasm increases in amount and prominence. These changes result in the cell becoming a chondro blast. Chondroblasts function in cartilage matrix production, including secretion of type II collagen. The new matrix increases the cartilage mass, while new fibroblasts are produced simultaneously to maintain the cell population of the perichondrium.


Histology_Ross

Histology_Ross. New cartilage cells produced during interstitial growth arise from the division of chondrocytes within their lacunae (see Fig. 7.4). This is possible only because the chondrocytes retain the ability to divide and the surrounding matrix is distensible, thus permitting further secretory activity. Initially, the daughter cells of the dividing chondrocytes occupy the same lacuna. As new matrix is secreted, a partition is formed between the daughter cells; at this point each cell occupies its own lacuna. With continued secretion of matrix, the cells move even farther apart from each other. The overall growth of cartilage thus results from the interstitial secretion of new matrix material by chondrocytes and by the appositional secretion of matrix material by newly differentiated chondro blasts (Folder 7.2). Cartilage has limited ability for repair.


Histology_Ross

Histology_Ross. Cartilage has limited ability for repair. Cartilage can tolerate considerable intense and repetitive stress. However, when damaged, cartilage manifests a striking inability to heal, even in the most minor injuries. This lack of response to injury is attributable to the avascularity of cartilage, the immobility of the chondrocytes, and the limited ability of mature chondrocytes to proliferate. Some repair can occur but only if the defect involves the perichondrium. In these injuries, repair results from activity of the pluripotential progenitor cells located in the perichondrium. Even then, however, few cartilage cells, if any, are produced. Repair mostly involves the production of dense connective tissue.