Embryology and Development Chapter 7

These vocabulary flashcards cover the early developmental stages of birds, including egg structure, cleavage patterns, axis formation, and the mechanisms of gastrulation and neurulation as described in the lecture notes.

Amniotes

Vertebrates, including reptiles, birds, and mammals, characterized by the development of an extraembryonic water sac called the amnion that surrounds the embryo.

Telolecithal

The type of egg found in birds which contains a very large amount of yolk, providing all necessary food for the chick until hatching.

Blastodisc

A small, yolk-free zone of cytoplasm, approximately $2-3\,mm$ in diameter in chicken eggs, where cleavage occurs.

Meroblastic Cleavage

A pattern of cell division where cleavage occurs only at the blastodisc rather than through the entire yolk.

Subgerminal Cavity

The space created between the blastoderm and the yolk as cells absorb water from the egg white and transport it underneath themselves.

Epiblast

The upper layer of the avian blastoderm formed after cells at the center of the blastoderm die; it eventually gives rise to the three germ layers.

Area Pellucida

The translucent, single layer of cells located at the center of the epiblast.

Area Opaca

The surrounding ring of cells in the avian embryo that remains in contact with the yolk.

Marginal Zone

The junction or boundary region between the area pellucida and the area opaca.

Hypoblast

A second layer of cells that forms below the epiblast in the area pellucida, consisting of a primary layer from delaminated cells and a secondary layer from the posterior marginal zone.

Koller’s Sickle

A local thickening of the posterior epiblast; its anterior portion contributes to Hensen’s node while the posterior portion contributes to the primitive streak.

Blastocoel

The space located between the epiblast and the hypoblast, which exists at about the time the hen lays the egg.

Gastrulation

The developmental process initiated by the formation of the primitive streak, involving the migration and ingression of epiblast cells to form germ layers.

Primitive Streak

A thickening of the posterior epiblast that extends anteriorly, serving as the site where cells ingress into the blastocoel to become mesoderm and endoderm.

Hensen’s Node

The thickening at the anterior end of the primitive streak, functionally equivalent to the amphibian dorsal lip of the blastopore (the organizer).

Primitive Groove

A depression at the center of the primitive streak that is functionally equivalent to the blastopore; it serves as the opening through which cells ingress.

Chordamesoderm

The trail of cells, including the head process and the notochord, formed behind the Hensen’s node as it recedes from anterior to posterior.

Neurulation

The process of forming the neural tube, which begins anterior to the receding Hensen's node while gastrulation continues posteriorly.

Somites

Segments of tissue derived from segmental plate mesoderm that form in an anterior to posterior order as the primitive streak regresses.

Posterior Marginal Zone (PMZ)

The region determined by gravity as the egg rotates at $15\,rph$, causing lighter yolk components to shift and initiate the anterior-posterior axis.

Dorsal-Ventral Axis Establishment

Process involving pH differences ($pH=9.5$ above the blastoderm vs $pH=6.5$ in the subgerminal cavity) and a voltage of $25\,mV$ across the epiblast.

Noggin

A BMP7 antagonist secreted by the avian organizer (Hensen's node) to dorsalize the nearby ectoderm.

BMP7

Bone morphogenetic protein 7; a factor expressed in ventral regions of the ectoderm that promotes ventralization unless inhibited by the organizer.