Organogenesis Part 2

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Organogenesis Part 2

• V. Lateral Plate Mesoderm
• VI. Endoderm
• VII. Development of the Tetrapod Limb
• VIII. Sex Determination

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V. Lateral Plate Mesoderm
paraxial mesoderm
chordamesoderm
intermediate mesoderm
lateral plate mesoderm
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Lateral Plate Mesoderm
Terminology - Somatopleure somatic
mesoderm plus ectoderm - Splanchnopleure
splanchnic mesoderm plus endoderm - Coelom
body cavity forms between them
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Lateral Plate Mesoderm

• The Coelom
• eventually left and right cavities fuse into one
• runs from neck to anus in vertebrates
• portioned off by folds of somatic mesoderm
• pleural cavity surrounds the thorax and lungs
• pericardial cavity surrounds the heart
• peritoneal cavity surrounds the abdominal organs

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Figure 12.1 Mesodermal development in frog and
chick embryos (Part 3)
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Heart Development

• The heart is the first organ to function in the
  embryo and the circulatory system is the first
  functional system.
• heart?arteries?capillaries?veins?heart
• Before the embryo can get very big it must switch
  from nutrient diffusion to active nutrient
  transport

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Heart Development
Anatomical Stages

1. Tube Formation
2. Looping
3. Chamber Formation

outflow
inflow
human timeline
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Heart Development Tube Formation
presumptive heart cells are specified but not
determined in the epiblast
outflow forming cells (red) migrate in first,
inflow second
migrate through together near node
the heart field
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Heart Development Tube Formation
The cardiogenic mesoderm migrates out of the
mesodermal layer towards the endoderm to form
endocardial tubes on either side.
At the same time the endoderm is folding inward
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Heart Development Tube Formation
The endoderm continues folding inward until it
forms its own tube, which drags the two
endocardial primordia close to each other.
The endocardial tubes are surrounded by
myocardial progenitors
When the endocardial tubes get close enough, they
fuse together
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Heart Development Tube Formation
If you mess with endoderm migration or signaling,
you end up with two hearts
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Heart Development Tube Formation

• Heart Tube Cell Biology
• Splanchnic mesoderm cells express cadherins and
  form an epithelial sheet for their inward
  migration - MET
• The presumptive endocardial cells undergo EMT to
  migrate away from the sheet and another MET to
  form tubes
• The cells in the original mesodermal sheet form
  the myocardium
• The myocardial epithelium fuses first and the two
  endocardial tubes exist together inside for a
  while before fusing
• Both the rostral end (outflow) and caudal end
  (inflow) remain as unfused double tubes
• The heart beat starts spontaneously as myocardial
  cells express the sodium-calcium pump - before
  fusion is even complete

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Heart Development Looping and Chamber Formation
left-right asymmetry is due to Nodal and Pitx2
anterior
posterior
left
right
Looping requires cytoskeletal rearrangement
extracellular matrix
remodeling
asymmetric cell division
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Heart Development Looping and Chamber Formation
valve formation
heart valves keep the blood from flowing back
into the chamber it was just ejected from
The septa separate the two atria and the two
ventricles
septation
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Heart Development Looping and Chamber Formation
The truncus arteriosis, or outflow tract, also
becomes septated allowing one great artery to
flow from right ventricle to lungs and the other
from left ventricle to the body.
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Heart Development Looping and Chamber Formation

• The tricuspid valve is between the right atrium
  and right ventricle.
• The pulmonary or pulmonic valve is between the
  right ventricle and the pulmonary artery.
• The mitral valve is between the left atrium and
  left ventricle.
• The aortic valve is between the left ventricle
  and the aorta.

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Heart Development Looping and Chamber Formation
Steps 1. Endocardial cushions form and fuse
2. Septa grow towards cushion
3. Valves form from myocardium
In utero, the foramen ovale allows right left
shunting of blood
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Embryonic circulatory systems
All of the blood must circulate outside of
the embryo for oxygenation
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Redirection of human blood flow at birth
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Blood Vessel Development

• The vessels form independently of the heart
• They form for embryonic needs as much as adult
• Must get nutrition before there is a GI tract
• Must circulate oxygen before there are lungs
• Must excrete waste before there are kidneys
• They do these through links to extraembryonic
  membranes

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Blood Vessel Development

• The vessels are constrained by evolution
• Mammals still extend vessels to empty yolk sac
• Birds and mammals also build six aortic arches as
  if we had gills, eventually settling on a single
  arch
• The vessels adapt to the laws of fluid dynamics
• Large vessels move fluid with low resistance
• Diffusion requires small volumes and slow flow
• Highly organized size variance controls volume
• And superbranching smaller vessels control speed

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Embryonic circulatory systems
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Blood Vessel Development
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Blood Vessel Development
Vasculogenesis is the de novo differentiation of
mesoderm into endothelium
It is followed by the endothelium recruiting
smooth muscle cell coat
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Blood Vessel Development
Starts in the extraembryonic mesoderm as well as
in the large embryonic blood vessels
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Blood Vessel Development
Angiogenesis is the growth and remodeling of the
1st vessels in response to blood flow
and tissue-derived recruitment signals
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Blood Vessel Development
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Blood Vessel Development
Secondary Vasculogenesis
1. PEO forms from splanchnic mesoderm overlying
the liver
2. PEO contacts the ventricle and migrates as
epicardium
3. Subset of epicardial cells delaminate towards
myocardium
4. These undergo MET to form coronary endothelium
5. Coronary arteries then plug into the aorta
where nerves are
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Blood Vessel Development
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Blood Vessel Development
It is a common phenomenon for arteries and
nerves to form together
Less so for veins....
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Blood Vessel Development

• Lymphatic drainage forms from jugular vein
• Sprouts as lymphatic sacs by angiogenesis
• Continues to form secondary drainage system
• Major conduit for immune cells

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Where do the hematopoietic stem cells of the
adult bone marrow come from?
Splanchnic mesoderm of aorta-gonad-mesonephros
(AGM) region in embryo
Hemogenic endothelium from sclerotome
Hemogenic endothelium from many sites
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Wherever they come from....
WOW!
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Development of the Endoderm

• The Digestive Tube
• Anterior endoderm forms anterior intestinal
  portal
• Posterior endoderm forms posterior intestinal
  portal
• Midgut goes through expansion and contraction to
  yolk
• Each end has ectodermal cap, then forms an
  entrance
• The Derivatives
• 4 pharyngeal pouches form head and neck
  structures
• Floor between 4th pair buds out to form
  respiratory tube
• Gut tube forms esophagus, stomach, SI, LI, rectum
• Gut tube buds out to form liver, gall bladder,
  pancreas

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Development of the Endoderm
Human Timeline
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Development of the Endoderm
The cranial neural crest cells migrate through
this endoderm and contribute component structures
around them
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Development of the Endoderm
Localized Wnt/B-Catenin and retinoic acid cause
budding
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Development of the Endoderm
Normal-time birth is signaled from the lungs
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Development of the Endoderm
Anterior-Posterior specification of
the gastrointestinal tract
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Development of the Endoderm
Reciprocal Induction Simultaneous Anterior-Posteri
or specification of both endoderm and mesoderm
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Development of the Endoderm
Mesoderm also induces liver bud
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Development of the Endoderm
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Development of the Endoderm
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The Extraembryonic Membranes

• Adaptation for development on dry land
• As the body starts to develop epithelium expands
  to isolate embryo within them
• Four sets of extraembryonic membranes
• Somatopleure forms amnion and chorion
• Splanchnopleure forms yolk sac and allantois

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The Extraembryonic Membranes
Somatopleure forms amnion and chorion Splanchnople
ure forms yolk sac and allantois
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The Extraembryonic Membranes
The amnion folds up to cover the embryo and keep
it from drying out
The cells of the amnion secrete water
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The Extraembryonic Membranes
The chorion surrounds the entire embryo and
controls gas exchange
In birds and reptiles it lines shell In mammals
it forms the placenta
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The Extraembryonic Membranes
The yolk sac expands to surround yolk (even if
you dont have any)
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The Extraembryonic Membranes
The allantoic membrane creates a space for waste
storage
Bird and reptile eggs gotta have it
We dont use it for waste but it contributes to
our umbilical cord