Flashcards in Early Embryonic development 2 Deck (33):
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What are the main events which occur in week 3 of development?
-Gastrulation
-Defining of the axes
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What is gastrulation?
-The process of forming all three germ layers from the epiblast
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What is the first event in gastrulation?
-Formation of the primitive streak
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Where does the primitive streak form?
-In any direction on the surface of the epiblast
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What is the structure of the primitive streak?
-A streak of cells which appears on the surface of the epiblast which has narrow bulging at the edges
-The primitive node is an elevated area located at the cranial end of the streak
-The primitive pit is a central depression in the node
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What is the first event that occurs involving the primitive streak?
-Invagination-> cells of the epiblast migrate towards the streak, detach from the epiblast and depress down into the streak
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What does the process of invagination into the primitive streak cause?
-Displacement of the hypoblast by the differentiated cells of the epiblast known as endoderm
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What happens to the primitive streak as invagination is occuring?
-Begins to regress towards the caudal end
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How are the mesoderm and ectoderm layers formed during gastrulation?
-Once the endoderm has formed, the epiblast cells begins to diferentiate into ectoderm, whilst invagination of epiblast cells which are differentiating to mesoderm fills the space inbetween the two layers
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What happens to the primitive yolk sac during gastrulation?
-The hypoblast regresses and endoderm now lines the primitive yolk sac
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What is the end result of gastrulation?
-Formation on the trilaminar disc containing the three germ layers
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Does mesoderm fill the entire space between the ectoderm and endoderm?
-Yes, except from 2 areas, one at the cranial end and one at the caudial end which will be the future mouth and anus
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How are the openings for the mouth and anus produced?
-These areas are not filled with mesoderm which gives rise to the CVS
-Without CVS these areas will die and rupture creating openings
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What is the notochord?
-A solid rod of cells which runs down the midline of the trilaminar disc
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How is the notochord formed?
-Cells migrate from the primitive node and form a line down the midline without spreading out
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What two events does the notochord permit?
-Defining of the axes
-Formation of the nervous system
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In what direction does the notochord form?
-Cranial to caudal
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Which event defines the formation of the cranial/caudal axes?
-Formation of the primitive streak
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Which axes does the notochord define?
-Dorso-ventral
-left-right
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How does the notochord define the left-right axes
-Signals for the beating action of cilia which produce a side-specific signalling cascade
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What is situs invertus and what causes it to occur?
-Complete mirror image of the organs and structures of the body-> often not associated with morbidity
-Immotile cilia
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What are the derivatives of ectoderm?
-Organs and structures which remain in contact with the outside world
-Epidermis
-Nervous system
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What are the derivatives of mesoderm?
-Supporting structures
-CVS, muscles, bone, cartilage
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What are the derivatives of the endoderm?
-Internal structures
-Epithelia of the GI tract and respiratory tract
-Parenchymal cells
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What are the three possible causes of monozygotic twins?
-Zygote separates at two cell stage
-Zygote separates at the inner cell mass
-Appearance of second primitive streak
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In twinning, why do two embryos have two placentas is separation occurs at the two cell stage?
-Embryo has split before any differentiation, meaning that each ambryo can then differentiate to produce a placenta of their own
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In twinning, if separation occurs at the inner cell mass, why are two embryos produced which share a placenta?
-The embryo has undergone differentiation into inner cell mass and outer cell mass
-Outer cell mass forms placenta, separation of the inner cell mass inside the outercell mass will produce two embryos sharing a placenta
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In twinning, why does the appearance of another primitive streak lead to embryos sharing an amniotic cavity?
-The primitive streak appears on the epiblast which lies inside of the amniotic cavity which has already developed
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In twinning, when can the appearance of a second primitive streak produce conjoined twins?
-If separation (of the bilaminar disc) is not complete
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What is teratogenesis?
-The disruption of embryonic development by whatever means
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Why does each organ system have a particularly sensitive window for teratogenesis?
-They all develop at different times
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Give an example of a chemical teratogenic agent
-Thalidomide
-Alchol
-Drugs
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