organogenesis part 2 - california state university, · pdf file ·...
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Organogenesis Part 2
V. Lateral Plate Mesoderm VI. EndodermVII. Development of the Tetrapod LimbVIII. Sex Determination
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V. Lateral Plate Mesoderm
chordamesodermparaxial mesoderm
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
1. Tube Formation2. Looping3. Chamber Formation
human timeline
Anatomical Stages:
inflow
outflow
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Heart Development: Tube Formation
presumptive heart cells are specifiedbut not determined in the epiblast
migrate through together near node
outflow forming cells (red)migrate in first, inflow second
“the heart field”
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Heart Development: Tube FormationThe cardiogenic mesoderm migrates out of the mesodermal layertowards the endoderm to form endocardial tubes on either side.
At the same timethe endoderm isfolding inward
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Heart Development: Tube FormationThe endoderm continues folding inward until it forms its own tube,which drags the two endocardial primordia close to each other.
The endocardial tubesare surrounded by myocardial progenitors
When the endocardialtubes get close enough,they fuse together
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Heart Development: Tube Formation
If you mess withendoderm migrationor signaling, you endup 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
anterior
posterior leftright
left-right asymmetry is due to Nodal and Pitx2
Looping requires: cytoskeletal rearrangementextracellular matrix remodelingasymmetric cell division
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Heart Development: Looping and Chamber Formation
valve formation
septation
heart valves keepthe blood from flowing back intothe chamber it wasjust ejected from
The septa separatethe two atria and the two ventricles
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Heart Development: Looping and Chamber Formation
The truncus arteriosis, or outflow tract, also becomes septated allowing one greatartery to flow from right ventricle to lungsand 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 fuse2. Septa grow towards cushion3. 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 theembryo 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
Vasculogenesisis the de novodifferentiationof mesoderminto endothelium
It is followed bythe endotheliumrecruiting smoothmuscle cell coat
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Blood Vessel Development
Starts in theextraembryonicmesoderm aswell as in thelarge embryonicblood vessels
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Blood Vessel Development
Angiogenesisis the growthand remodelingof the 1st vesselsin response toblood flow andtissue-derivedrecruitment signals
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Blood Vessel Development
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Blood Vessel Development
1. PEO forms from splanchnicmesoderm overlying the liver
2. PEO contacts the ventricleand migrates as epicardium
3. Subset of epicardial cellsdelaminate towards myocardium
4. These undergo MET toform coronary endothelium
Secondary Vasculogenesis
5. Coronary arteries then pluginto the aorta where nerves are
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Blood Vessel Development
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Blood Vessel Development
It is a commonphenomenon forarteries and nervesto 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 endotheliumfrom sclerotome
Hemogenicendotheliumfrom 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 migratethrough this endoderm and contributecomponent 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 EndodermNormal-time birth issignaled from the lungs
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Development of the Endoderm
Anterior-Posteriorspecification of thegastrointestinal tract
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Development of the Endoderm
Reciprocal InductionSimultaneous
Anterior-Posteriorspecification of both endoderm and mesoderm
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Development of the Endoderm
Mesoderm alsoinduces 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 chorionSplanchnopleure forms yolk sac and allantois
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The Extraembryonic MembranesThe amnion folds up to cover the embryo and keep it from drying out
The cells of theamnion 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 shellIn mammals it forms the placenta
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The Extraembryonic Membranes
The yolk sac expands to surround yolk (even if you don’t have any)
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The Extraembryonic Membranes
The allantoic membrane creates a space for waste storage
Bird and reptileeggs gotta’ have it
We don’t use it forwaste but it contributesto our umbilical cord
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