User:Mya.ngozi/Primitive node

All structures are as yet considered as homologous, as supported by the common expression of several genes, including goosecoid, Cnot, noggin, nodal, as well as the sharing of strong axis-inducing properties upon transplantation. Cell fate studies have also revealed a surprising similarity in the overall temporal sequence in which groups of endomesodermal cells internalize along the frog blastopore and amniote primitive streak: the first cells that involute around the amphibian blastopore lip in the organizer region, and that immigrate through Hensen’s node, contribute to foregut endoderm and prechordal plate, While cells involuting further laterally in the blastopore, or entering via Hensen’s node and the anterior primitive streak contribute to gut, notochord and somites. Gastrulation then continues along the ventroposterior blastopore lip and posterior streak region, from where cells contribute to ventral and posterior mesoderm. In addition, Brachyury and caudal homologues are expressed circumferentially around the blastopore lips in the frog and along the primitive streak in chick and mouse. This suggests that, despite their different morphology, the amniote primitive streak and the amphibian blastopore are homologous structures, that have evolved from one and the same precursor structure by a continuous sequence of morphological modifications.

The primitive node plays a critical role in chick development, as it marks the beginning of gastrulation and the formation of the three germ layers that will give rise to all the organs and tissues of the developing embryo. The primitive node starts as a knot of cells on the blastodisc, which is the flattened surface of the yolk where the embryo will develop.

As the primitive streak forms, the node migrates towards its posterior end and becomes a compact assembly of cells known as Hensen's node. The center of Hensen's node contains a funnel-shaped depression called the primitive pit, where the cells of the upper layer of the embryonic cells, known as the epiblast, begin to invaginate. This invagination expands posteriorly into the primitive groove as the cell layers continue to move into the space between the embryonic cells and the yolk.

During this process, the node becomes non-homogeneous in cellular composition, as some cells continue to emigrate and become replaced by surrounding epiblast cells. This dynamic nature of the node is reflected in the different gene expression patterns of its cells and their fate.

One of the critical functions of the node is its ability to induce the formation of the neural plate, which will give rise to the nervous system. The node accomplishes this by secreting specific signaling molecules that influence the fate of the surrounding cells. However, the node's ability to induce neural plate formation is limited to a specific region of the embryo, known as the neural plate border, which is just anterior to the node.

The node's unique nature is guaranteed by the presence of a TGF-like factor called ADMP, which has an antidorsalizing activity. This means that it antagonizes any further anterior or lateral node inductions, ensuring that the node remains distinct and does not induce neural plate formation in other regions of the embryo.

As gastrulation proceeds, the primitive node migrates posteriorly and eventually becomes absorbed into the tail bud, marking the end of its function in chick development.