Koller's sickle

In avian gastrulation, Koller's sickle is a local thickening of cells at the posterior edge of the upper layer of the area pellucida called the epiblast. Koller's sickle is crucial for avian development, due to its critical role in inducing the differentiation of various avian body parts. Koller's sickle induces primitive streak and Hensen's node, which are major components of avian gastrulation. Avian gastrulation is a process by which developing cells in an avian embryo move relative to one another in order to form the three germ layers (endoderm, mesoderm, and ectoderm).

The thickening of the epiblast in Koller's sickle acts as a margin separating sheets of cells from posterior side of avian blastoderms from hypoblasts and area opaca endoderm. The blastoderm is a single layer of cells, and the hypoblast and area opaca endoderm cells lie directly below the blastoderm. Koller's sickle arises from the midpoint, between the hypoblast cells and the area opaca endoderm. As blastoderm cells migrate anteriorly they push primary hypoblast cells and form a secondary hypoblast known as the endoblast. Also during this migration, Koller's sickle prevents the hypoblast cells and the area opaca cells from making contact with the blastoderm, which allows the primitive streak to form.

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.

Avian gastrulation occurs as cells move though the primitive streak. Hence, primitive streak is analogous to the blastopore lip in amphibian gastrulation. The primitive streak develops from Koller’s sickle and the epiblast of the avian embryo. As the cells of Koller’s sickle migrate during gastrulation, they form different portions of the primitive streak. The anterior cells of Koller’s sickle become the anterior region of the primitive streak, known as Hensen's node. Similarly, the posterior cells of Koller’s sickle form the posterior region of the primitive streak. This differential movement is due to expression of different mesodermal marker genes among the cells located in different areas of Koller’s sickle. Chordin is expressed in cells of the anterior streak, while Wnt8c is expressed in cells of the posterior streak. The movement is coordinated by a Wnt signaling pathway which is activated by fibroblast growth factors from the hypoblast.

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