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
Vasculogenesis
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