Gastrulation - origin of ToB

Question 1

What happens in embryonic period?

Answer 1

weeks 3-8, week 3 of 3’s
3 cavities
3 germ layers - rudimentary lineages (undeveloped) - all others arise from endoderm, ectoderm, mesoderm

Question 2

What happens in gastrulation?

Answer 2

establish 3 germ layers 'trilaminar disc' - 3 germ layers 'endo, meso, ecto'
cellular rearrangement (in epiblast of bilaminar disc): migration & invagination 
primitive streak appears: organisation centre (drives process), primitive node with pit
set axis: right/left, dorsal/ventral, anterior/posterior (head/feet)

Question 3

How does primitive streak appear? Where does it appear and what is it? primitive pit and node?

Answer 3

primitive streak appears on the dorsal (back) surface of epiblast in the 3rd week
it is a narrow groove with bulging edges
at cranial end is the primitive node and primitive pit at the centre of the node

Question 4

How are the 3 germ layers developed?

Answer 4

cranial (head) –> caudal (tail)

Question 5

Describe the process of migration & invagination

Answer 5

1. cells divide & differentiate as they migrate
2. pushing through epiblast layer as they reach the edge of the primitive streak
3. they move in every direction

Question 6

How is the trilaminar disk created?

Answer 6

through displacement of hypoblast & creation of 3rd layer: ectoderm, mesoderm & endoderm all from epiblast

Question 7

Where does mesoderm move to?

Answer 7

mesoderm spreads out between ectoderm & endoderm - apart from future mouth (oral cavity - develop head, neck, GI) and future anus opening

Question 8

What is the notochord?

Answer 8

solid rod of cells in middle of mesoderm layer - important signalling
drives formation of nervous system, basis for axial skeleton, centre of embryo, drives neurulation, directs conversion of ectoderm to neuroectoderm (producing neural tube), defines the phylum chordata, regresses in adults - vestigial remnant (slipped disc)

Question 9

What has happened by the end of week 2?

Answer 9

oocyte fertilised
zygote undergone cleavage (morula) & compaction (totipotent –> pluripotent), produce a blastocyst –> implanted
begun to differentiate - bilaminar disc formed within inner cell mass
extraembryonic spaces start to organise

Question 10

What is the function of the primitive streak?

Answer 10

anterior –> posterior, cranial –> caudal
molecular signals from primitive streak ensures correct dorsal-ventral & left-right development (orientation)

cells of epiblast invaginate through groove (primitive streak) allowing gastrulation