gastrulation

what is the primitive streak

groove in the midline of the epiblast (appears during 3rd week)

• within the primitive streak lies the primitive node at cranial end

• within the primitive node lies the primitive pit

what cells undergo gastrulation

cells of the bilaminar disk

how are 3 germ layers formed

cells of the epiblast break off and migrate towards the primitive pit.

they migrate and penetrate through epiblast layer to form new germ layers

endoderm

• inner layer

• formed by epiblast cells that migrate through primitive pit and displace hypoblast cells

mesoderm

• middle layer

• formed by epiblast cells that migrate through primitive pit and lies between epiblast and newly created endoderm

ectoderm

• outer layer

• formed by epiblast cells that remain in position

what is the step by step formation of the primitive streak

• cells in the middle of the epiblast converge to the middle of the disk

• epiblast cells ingress individually rather than pile on top (they edge towards the middle then ingress)

• they form the 3 layers

• as the PS elongates, migrating epiblast cells join the streak at the cranial end forming a mass of cells around the primitive pit

• this forms a node of a knot of cells

• the epiblast cells that ingress through the node migrate anteriorly to form parts of the head

• head endoderm formed first then mesoderm

• the node regresses caudally eventually dissapearing

• this process deposits cells forming the notocord

tailbud

• node runs out of stem cells at the hindlimb

• tailbud is a knot of stem cells at the posterior of the streak which moves posteriorly moving cells behind

• this only forms the mesoderm cells of posterior notocord and somites