Lateral Plate Mesoderm and Endoderm


Lateral Plate Mesoderm

     Lateral plate mesoderm lies on either side of of intermediate mesoderm

     Each plate splits horizontally into

   Dorsal somatic (parietal) mesoderm - directly under the ectoderm

   Ventral splanchnic (visceral) mesoderm - overlies the endoderm

     Space between these layers becomes the coelom - left and right halves fuse later in development

Cardiogenic Mesoderm & Heart Formation

     Originate in the early primitive streak, just posterior to Hensen’s node and extending about halfway down its length

     Two groups of cells are formed

     At 18-20 hours - these presumptive heart cells move anteriorly toward the middle of the embryo remaining in close contact with endoderm

Cardiogenic Mesoderm & Heart Formation

     Cell differentiation occurs independently in the two cell populations that are migrating toward each other

     One population becomes the endocardium

  Produce many of the heart valves and regulate myocardial growth

     The other cells form the myocardium

Cardiogenic Cells of the Chick Embryo






Heart Formation

     As the gut forms the the two cardiac tubes are brought together into a single tube

     Fusion occurs in the chick at about 29 hours - at 3 months in humans

     The unfused posterior portions of the endocardium become the openings of the vitelline veins into the heart

   These veins carry nutrients from the yolk sac into the sinus venosus

     Blood then passes through a valvelike flap into the atrial region of the heart

Formation of the Chick Heart





Formation of the Chick Heart







Heart Formation

     Contractions of the truncus arteriosus speed blood into the aorta

     Pulsations of the heart begin while the paired primordia are still fusing

     Pacemaker is the sinus venosus - from here wavelike contractions spread into the tubular heart

   Pumps blood before the valves have been formed

Heart Formation - Looping & Formation of Chambers

     In a 3-day chick and 5-week human embryos - heart is two chambered

     Looping of the heart - converts the original anterior-posterior polarity of the heart tube - into right left polarity of the adult

     Looping is dependent upon the left-right patterning proteins (Nodal and Left 1)

Heart Formation - Looping & Formation of Chambers

     Separation of the atrium from the ventricle - accomplished when the myocardium produces a factor that causes cells from the endocardium to detach and enter the “cardiac jelly” between the two layers

     These cells form the endocardial cushion in humans - divides the tube into right & left atrioventricular channels

Heart Formation - Looping & Formation of Chambers

     Atrium becomes divided by two septa - still however there are holes between the two sides

  Holes close after the fetus begins to be breathe air

     By 7th week the heart of a human fetus has four chambers

Specification of Heart Chambers







Formation of Heart Chambers







Fetal to Newborn  Circulation

     Dustus arterious - diverts blood from the pulmonary artery into the aorta - eventually to the placenta

     Foramen ovale - hole in the septum between the two atria

  Blood can enter the right  atrium and pass through the foramen into the left atrium

Fetal to Newborn  Circulation

     On first breath - blood pressure in the left side of the heart increases - septa over the foramen ovale now closes - separate pulmonary & systemic circulation

   Dec prostagladins - cause muscle surrounding the ductus arteriosus to close that opening

   Thus the respiratory circulation is shunted from the placenta to the lungs

Fetal to Newborn Circulation






Formation of Blood Vessels

     Two majors processes involved

   Vasculogenesis - First step -involves condensing of visceral mesoderm cells to form blood islands -

   Hemangioblast (common precursor for blood cells and vessels)

   The outer cells of the islands become endothelial (blood vessel) cells

  Angioblasts - precursor of blood vessels
  Hematopoietic stem cells - precursors of all blood cells

   Angiogenesis -  Second step - involves remodeling the system into arteries, capillary beds and veins

   The smooth muscles comes from pericytes that are recruited to cover the endothelial cells

Fate of Hemangioblasts











Blood Cell Formation & Stem Cells

    Blood cells come from stem cells - pluripotential hematopoietic stem cell

Origin of Blood Cells






Origin of Blood Cells






Embryonic Endoderm

     Forms digestive tube

   Buds from this form the liver, gall bladder and pancreas

     Forms the respiratory tube

   Outgrowth of the digestive tube

   Bifurcates into the two lungs

     Pharynx - common chamber shared by both tubes

   Embryonic outgrowths - form the endodermal lining of the eustacian tube tonsils, thyroid gland, and parathyroid glands

Formation of Human Digestive System






Formation of Human Digestive System






Formation of Human Digestive System






Formation of the Pancreas






Respiratory Tube Development






Extraembryonic Membranes

    Yolk sac -  first to be formed - nutrition

    Allantois - sac for nitrogenous waste disposal

    Chorion - used in gas exchange - typically adheres to the shell in reptile and birds - placenta in mammals

    Amnion - secretes amnionic fluid to prevent desicccation - embryogenesis still in water

Extraembryonic Membranes - Chick