Hensen's node
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Hensen's Node is the organizer for gastrulation in birds. It is the equivalent of the dorsal lip of the blastopore found in the embryos of more primitive vertebrates, such as amphibians and fish. It is named after its discoverer Victor Hensen.
Hensen's node 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 - an area known as the primitive streak. 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 and differentiate the embryo into the germ layers - endoderm, mesoderm, and ectoderm. Hensen's node migrates posteriorly as gastrulation proceeds, eventually being absorbed into the tail bud.
The cells of Hensen's node secrete many cellular signals essential for gastrulation - including Fibroblast Growth Factors, Sonic hedgehog, and Retinoic Acid. Differential secretion of factors by the node also causes development of the right-left axis in the embryo.
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