Organization of the Body
Check Your Understanding
What are the three main steps of tissue repair?
Why does a deep injury to the skin result in abundant scar
For answers, see Appendix H.
Developmental Aspects
of Tissues
Indicate the embryonic origin of each tissue class.
Brieﬂy describe tissue changes that occur with age.
One of the ﬁrst events of embryonic development is the forma-
tion of the three
germ layers
, which lie one atop the
next like a three-layered cellular pancake. From superﬁcial to
deep, these layers are the
(Figure 4.13)
. Tese primary germ layers then
specialize to form the four primary tissues—epithelium, ner-
vous tissue, muscle, and connective tissues—that make up all
body organs.
By the end of the second month of development, the primary
tissues have appeared, and all major organs are in place. In gen-
eral, tissue cells remain mitotic and produce the rapid growth
that occurs before birth. Te division of nerve cells, however,
stops or nearly stops during the fetal period. A±er birth, the
cells of most other tissues continue to divide until adult body
size is achieved. Cellular division then slows greatly, although
many tissues retain some ability to regenerate.
In adults, only epithelia and blood-forming tissues are highly
mitotic. Some tissues that regenerate through life, such as the
glandular cells of the liver, do so through division of their mature
(specialized) cells. Others, like the epidermis of the skin and cells
lining the intestine, have abundant
stem cells
, relatively undiﬀer-
entiated cells that divide as necessary to produce new cells.
Regenerative Capacity of Different Tissues
²issues vary widely in their capacity for regeneration. Epithelial
tissues, bone, areolar connective tissue, dense irregular connec-
tive tissue, and blood-forming tissue regenerate extremely well.
Smooth muscle and dense regular connective tissue have a mod-
erate capacity for regeneration, but skeletal muscle and cartilage
have a weak regenerative capacity. Cardiac muscle and the nervous
tissue in the brain and spinal cord have virtually no
generative capacity, and they are routinely replaced by scar tissue.
However, recent studies show that some unexpected (and highly
selective) cellular division occurs in both these tissues a±er dam-
age, and eﬀorts are under way to coax them to regenerate better.
In nonregenerating tissues and in exceptionally severe wounds,
ﬁbrosis totally replaces the lost tissue. Over a period of months, the
ﬁbrous mass shrinks and becomes more and more compact. Te
resulting scar appears as a pale, o±en shiny area composed mostly
of collagen ﬁbers. Scar tissue is strong, but it lacks the ﬂexibility
and elasticity of most normal tissues. Also, it cannot perform the
normal functions of the tissue it has replaced.
Homeostatic Imbalance
Scar tissue that forms in the wall of the urinary bladder, heart,
or other muscular organ may severely hamper the organ’s func-
tion. Te normal shrinking of the scar reduces the internal
volume and may hinder or even block substances from mov-
ing through a hollow organ. Scar tissue hampers muscle’s abil-
ity to contract and may interfere with its normal excitation by
the nervous system. In the heart, these problems may lead to
progressive heart failure. In irritated visceral organs, particu-
larly following abdominal surgery,
may form as the
newly forming scar tissue connects adjacent organs together.
Such adhesions can prevent the normal shi±ing about (churn-
ing) of loops of the intestine, dangerously obstructing the ﬂow
of foodstuﬀs. Adhesions can also restrict heart movements and
immobilize joints.
16-day-old embryo
(dorsal surface view)
(from all three
germ layers)
Nervous tissue
(from ectoderm)
Muscle and connective tissue
(mostly from mesoderm)
Inner lining of
digestive system
(from endoderm)
Figure 4.13
Embryonic germ layers and the primary tissue types they produce.
The three
embryonic layers collectively form the very early embryonic body.
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