The document discusses key aspects of early chicken development, focusing on the role of the primitive streak, the posterior marginal zone, and Hensen's node. The primitive streak defines the embryo's axes, initiates gastrulation, and establishes bilateral symmetry. Hensen's node acts as a crucial organizer for gastrulation, analogous to Spemann's organizer in amphibians, facilitating the differentiation of germ layers.

1. Thank you 1. What is responsible for anterior:posterior axis formation in the chicken? How is
dorsal ventral patterning accomplished? 2. What is the role of the Posterior Marginal zone in
early chicken development? 3. What is the primitive streak and why is it important in chicken
development? 4. What is the function of Henson's Node? To what is it analogous in amphibian
development?
Solution
1.The primitive streak defines the axes of the embryo. It extends from posterior to anterior;
migrating cells enter through its dorsal side and move to its ventral side; and it separates the left
portion of the embryo from the right. Those elements close to the streak will be the medial
(central) structures, while those farther from it will be the distal (lateral) structures
2.Primitive streak is induced by the posterior marginal zone (PMZ) of Koller's sickle, which can
also induce Hensen's node. If cell movement in the PMZ is blocked, primitive streak does not
form.Thus, the PMZ acts as an organizer.Cells in marginal zones of the embryo, like the PMZ,
are key to development and cell fate determination in chick embryos.
3.The primitive streak is a structure that forms in the blastula during the early stages of avian,
reptilian and mammalian embryonic development. It forms on the dorsal (back) face of the
developing embryo, toward the caudal or posterior end.The presence of the primitive streak will
establish bilateral symmetry, determine the site of gastrulation and initiate germ layer formation.
To form the streak, reptiles, birds and mammals arrange mesenchymal cells along the
prospective midline, establishing the second embryonic axis, as well as the place where cells will
ingress and migrate during the process of gastrulation and germ layer formation.The primitive
streak extends through this midline and creates the left–right and cranial–caudal body axes,and
marks the beginning of gastrulation.This process involves the ingression of mesoderm
progenitors and their migration to their ultimate position,where they will differentiate into the
mesoderm germ layer that, together with endoderm and ectoderm germ layers, will give rise to
all the tissues of the adult organism.
4.
The primitive knot (or primitive node) is the organizer for gastrulation in the vertebrate embryo.
Diversity
In birds, it is known as "Hensen's node", and is named after its discoverer Victor Hensen.
In amphibians, it is known as "Spemann's organizer", and is named after Hans Spemann
In chick development, the primitive knot starts as a regional knot of cells that forms on the
blastodisc immediately anterior to where the outer layer of cells will begin to migrate inwards -

2. an area known as the primitive streak, which is involved with Koller's sickle. Posterior to the
node is the primitive pit, where the cells of the epiblast (the upper layer of embryonic cells)
initially begin to invaginate. This invagination expands posteriorly into the primitive groove as
the cells layers continue to move into the space between the embryonic cells and the yolk. This
differentiates the embryo into the three germ layers - endoderm, mesoderm, and ectoderm. The
primitive knot migrates posteriorly as gastrulation proceeds, eventually being absorbed into the
tail bud.