Gastrulation to Neurulation 1

Question 1

define gastrulation

Answer 1

• rearrangement of cells by migration and division resulting in the formation of 3 embryonic germ layers: ectoderm, mesoderm and endoderm

Question 2

what are triploblasts?

Answer 2

organisms whose body is derived from all 3 germ layers
- all triploblasts gastrulate

Question 3

what are the 3 mechanisms of internalisation?

Answer 3

invagination - a sheet of cells bends inwards
inregression - individual cells leave sheet and become freely mesenchyme
involution - epithelial sheet rolls inwards into deeper regions of embryo to form underlying layer (happens in frog and fish)

Question 4

what is internalisation in gastrulation and why is it important?

Answer 4

• involves inward movement/migration of cells from surface layer of an embryo towards its interior
• crucial for the formation of the 3 germs layers and subsequent rise to various tissues, organs, body structures in developing embryo

Question 5

describe gastrulation in xenopus

Answer 5

• gastrulation begins at blastopore/organizer (invagination of blastopore)
  cells involute through blastopore - endoderm rolls in, followed by mesoderm and ends up under ectoderm
• some mesoderm and endoderm moves through ventral lip

Question 6

what 2 mechanisms of internalisation (getting the mesoderm and endoderm inside) are involved in the xenopus

Answer 6

• involution and invagination
• initially invagination to form blastopore
• involution of mesoderm and endoderm which causes movement of sheet of cells to move inside embryo by rolling under itself (i.e. mesoderm and endoderm rolling under ectoderm)

Question 7

describe the blastopore formation in the xenopus

Answer 7

• via invagination
• cells constrict on apical side, epithelial sheet bends

Question 8

what is extension convergence?

Answer 8

• specific type of morphogenic movement
• cells from the dorsal lip of the blastopore move inward and extend along the embryo’s head-to-tail axis while maintaining their connections with neighboring cells
• As a result, the tissue becomes longer and narrower.

Question 9

describe the role of the blastopore in frog/xenopus gastrulation

Answer 9

• gastrulation movements start at the blastopore - the “Organiser”
• signalling centre directing development of whole embryo
• located on side of embryo where body is going to develop
• organises antero-posterior and dorso-ventral axis

Question 10

describe gastrulation in the zebrafish

Answer 10

EPIBOLY - cells on blastula surface divide and spread, covering yolk cell, results in thinning and expansion of blastula
INVOLUTION - organiser at one end of blastula invaginate, this initiates formation of the 3 germ layers
CONVERGENCE & EXTENSION - as involution occurs, cells from surrounding regions converge towards invaginating cells. Simultaneously, converging cells extend along anterior-posterior axis of embryo, elongating and narrowing overall shape
GERM LAYER FORMATION - invaginating cells give rise to mesoderm and endoderm layers, while outer layer forms ectoderm - they’ll all develop into different tissues and organs of zebrafish’s body

Question 11

what is epiboly?

Answer 11

morphogenetic movement of cells sliding or expanding over another layer of cells or substrate

Question 12

gastrulation begins with what in the chick embryo?

Answer 12

• formation of the primitive streak - a furrow through which cells migrate

Question 13

human embryos gastrulate like which embryos?

Answer 13

chick embryos

Question 14

describe gastrulation in a chick

Answer 14

FORMATION OF PRIMITIVE STREAK (PS) - epiblast forms disc-shaped layer on top of yolk-sac, PS appears along midline of epiblast
INVAGINATION & MIGRATION: cells from epiblast migrate towards PS by invagination folding into mesoderm. As cells continue to migrate they differentiate into various types of mesodermal cells
ESTABLISHMENT OF GERM LAYERS: as invagination proceeds, remaining cells of epiblast split into 2 layers. Upper layer - ectoderm - gives rise to nervous system and skin. Lower layer - endoderm - forms lining of digestive and respiratory systems
MESODERM FORMATION: as cells move through PS they give rise to mesoderm layer which will give rise to heart, blood vessel, kidneys and reproductive organs etc.
NEURAL TUBE FORMATION - at anterior end specialised ectoderm cells form neural plate. edges of neural plate fold and fuse forming neural tube which will develop into brain and spinal cord

Question 15

gastrulation starts in the primitive streak in what organisms?

Answer 15

chick
mouse
human

Question 16

in the sea urchin which cells invaginate first?

Answer 16

mesoderm cells invaginate first and form mesenchyme and migrate, then endoderm invaginate