Chp#5 Third Week Of Development Flashcards
Most characteristic event occur in which week ?
3rd week of gestation
What is the 3rd week of gestation called ?
Gastrulation period
What happens in this week ?
3 germ layers are formed endo, meso and ecto
How does this week begins?
Begins with the formation of primitive streak on the surface of the epiblast
In what day narrow groove is visible?
In 15 to 16 day embryo narrow groove is visible with slightly bulging regions on either side
What is cephalic end of streak called ?
Primitive node, consists of a slightly elevated area surrounding the small primitive pit.
Where do the cells of epiblast migrate?
Towards the primitive streak
What shape forms upon the arrival of the cells in the region of streak?
They become flask shaped l, detach from epiblast and slip beneath it
Inward movement is called
Invagination
Cell migration and specification is controlled by
Fibroblast growth factor 8 (FCF8) synthesized by streak cells themselves
Function of FGF8
Controls cell movement by down regulating E-cadherin
Controls cell specification into mesoderm by regulating BRACHYURT (T) expression
What is E-cadherin
A protein that normally binds epiblast cells together
What do cells do after invaginatiom
Displace hypoblast creating embryonic endoderm, others come to lie between the epiblast and newly created the endoderm to form mesoderm
What do the cells remaining in the epiblast do
Form ectoderm
Source of all the germ layers?
Epiblast through the process of gastrulation
What happens to the cells when more of them move b/w epiblast and hypoblast layers ?
Spread laterally and cranially gradually migrating beyong the margin of the disc and establishing contact with extraembryonic mesoderm covering the yolk sac and amnion.
What happens in cephalic direction
They pass on each side of the prechordal plate
Where Prechordal plate forms
Between the tip of the notochord and oropharyngeal membrane
Prechordal plate derived from
Some of the first cells that migrate through the node in the midline and move in a cephalic direction
Prechordal plate imp for
Induction of the forebrain
Ororpharyngeal membrane consists of
A small region of tightly adherent ectoderm and endoderm cells that represents the future opening of the oral cavity.
Which cells invaginate in the primitive node?
Prenotochordal cells invaginating in primitive node move forward cranially in the midline until they reach the prechordal plate. these prenotochordal cells become intercalated in the hypoblast so that for a short time the midline of embryo consists of 2 cells layers
Midline of embryo consists of 2 cell layers that form
Notochordal plate
When notochordal plate proliferate and detach from the endoderm
When hypoblast is replaced by the endoderm cells moving in at the streak
Cells of notochordal plate form a solid cord of cells called
Definitive notochord
What is definitive notochord
Which Underlies the neural tube and is a signaling center for inducing the axial skeleton
Which process is elongation of notochord
Dynamic process teh cranial end forms first and caudal regions are added as the primitive streak assume a more caudal position
How notochord and prenotochordal cells extend
Cranially to prechordal plate (an area just caudal to the oropharyngeal membrane) and caudally to primitive pit.
