Transcript Developmental Biology, 9e

Organogenesis Part 2
V. Lateral Plate Mesoderm
VI. Endoderm
VII. Development of the Tetrapod Limb
VIII. Sex Determination
V. Lateral Plate Mesoderm
paraxial mesoderm
chordamesoderm
intermediate mesoderm
lateral plate mesoderm
Lateral Plate Mesoderm
Terminology:
- Somatopleure: somatic mesoderm plus ectoderm
- Splanchnopleure: splanchnic mesoderm plus endoderm
- Coelom: body cavity forms between them
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
Figure 12.1 Mesodermal development in frog and chick embryos (Part 3)
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
Heart Development
Anatomical Stages:
1. Tube Formation
2. Looping
3. Chamber Formation
outflow
inflow
human timeline
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”
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
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
Heart Development: Tube Formation
If you mess with
endoderm migration
or signaling, you end
up with two hearts
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
Heart Development: Looping and Chamber Formation
left-right asymmetry is
due to Nodal and Pitx2
anterior
posterior
right
left
Looping requires: cytoskeletal rearrangement
extracellular matrix remodeling
asymmetric cell division
Heart Development: Looping and Chamber Formation
valve formation
septation
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
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.
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.
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
Embryonic circulatory systems
All of the blood must
circulate outside of the
embryo for oxygenation
Redirection of human blood flow at birth
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
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
Embryonic circulatory systems
Blood Vessel Development
Blood Vessel Development
Vasculogenesis
is the de novo
differentiation
of mesoderm
into endothelium
It is followed by
the endothelium
recruiting smooth
muscle cell coat
Blood Vessel Development
Starts in the
extraembryonic
mesoderm as
well as in the
large embryonic
blood vessels
Blood Vessel Development
Angiogenesis
is the growth
and remodeling
of the 1st vessels
in response to
blood flow and
tissue-derived
recruitment signals
Blood Vessel Development
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
Blood Vessel Development
Blood Vessel Development
It is a common
phenomenon for
arteries and nerves
to form together
Less so for veins....
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
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
Wherever they come from....
WOW!
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
Development of the Endoderm
Human
Timeline
Development of the Endoderm
The cranial neural crest cells migrate
through this endoderm and contribute
component structures around them
Development of the Endoderm
Localized Wnt/B-Catenin and retinoic acid cause budding
Development of the Endoderm
Normal-time birth is
signaled from the lungs
Development of the Endoderm
Anterior-Posterior
specification of the
gastrointestinal tract
Development of the Endoderm
Reciprocal Induction
Simultaneous
Anterior-Posterior
specification of
both endoderm
and mesoderm
Development of the Endoderm
Mesoderm also
induces liver bud
Development of the Endoderm
Development of the Endoderm
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
The Extraembryonic Membranes
Somatopleure forms amnion and chorion
Splanchnopleure forms yolk sac and allantois
The Extraembryonic Membranes
The amnion folds up to cover the embryo and keep it from drying out
The cells of the
amnion secrete water
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
The Extraembryonic Membranes
The yolk sac expands to surround yolk (even if you don’t have any)
The Extraembryonic Membranes
The allantoic membrane creates a space for waste storage
Bird and reptile
eggs gotta’ have it
We don’t use it for
waste but it contributes
to our umbilical cord