BIOL1040 Module 2 L6: Gastrulation and Organogenesis

What is Gastrulation

It is the dramatic reorganisation of the hollow blastula into a two-layered or three-layered embryo called a gastrula

What are the three germ layers formed during gastrulation?

Ectoderm (outside), Mesoderm (middle, in bilaterians), Endoderm (inside, lining digestive tract).

What happens during frog gastrulation?

• Dorsal cells invaginate at grey crescent.

• Dorsal lip forms above crease.

• Cells migrate inward toward animal pole.

• Archenteron forms inside.

• Neural plate develops.

What structures do each germ layer form in adults?

• Ectoderm → nervous system, outer body layer (skin).

• Mesoderm → muscles, skeleton.

• Endoderm → digestive tract lining, organs, ducts.

How does chick gastrulation occur?

• All embryonic cells come from epiblast.

• Cells migrate inward at primitive streak.

• Neural groove and notochord form.

• Heart tube and foregut form.

flat disk of. cells

What initiates human embryo implantation?

The trophoblast (outer blastocyst epithelium) secretes enzymes that break down the endometrium, allowing blastocyst invasion.

What happens after implantation in mammals?

The trophoblast expands into the endometrium, forming four extraembryonic membranes.

What structures allowed vertebrates to reproduce on land?

Evolution of the shelled egg (birds, reptiles, monotremes) and the uterus (marsupials and eutherians).

What is organogenesis?

Formation of organs from germ layers, beginning with neurulation and notochord development.

What triggers neurulation?

Dorsal mesoderm forms the notochord, which releases signals to induce overlying ectoderm into neural plate.

What is cell fate?

The developmental outcome of a cell.

What is the difference between determination and differentiation?

Determination = commitment to a fate; Differentiation = development of specialised structure and function.

What are the main embryonic axes in frogs?

• Left/right → symmetrical.

• Anterior/posterior → asymmetrical (animal pole vs vegetal pole).

• Dorsal/ventral → established by cortical rotation after sperm entry.

What is the ZPA (Zone of Polarising Activity)?

A mesodermal region that dictates anterior–posterior limb axis; secretes sonic hedgehog (Shh) to form digits (posterior) and thumb-like (anterior) structures.

How is fin-to-limb evolution explained genetically?

Early gene expression is similar in fins and limbs, but later gene expression in distal limb buds produces digits.

Explain Limb formation process

Cells of limb buds secrete specific proteins to provide positional info. The tip of the bud secretes a growth protein. The ZPA dictates anterior/posterios axis and gives rise to posterior (digits) and anterior (thumb-like) structures - it secretes sonic hedgehod protein to help.