Concept

Hypoblast

In amniote embryology, the hypoblast, is one of two distinct layers arising from the inner cell mass in the mammalian blastocyst, or from the blastodisc in reptiles and birds. The hypoblast gives rise to the yolk sac, which in turn gives rise to the chorion. The hypoblast is a layer of cells in fish and amniote embryos. The hypoblast helps determine the embryo's body axes, and its migration determines the cell movements that accompany the formation of the primitive streak, and helps to orient the embryo, and create bilateral symmetry. The other layer of the inner cell mass, the epiblast, differentiates into the three primary germ layers, ectoderm, mesoderm, and endoderm. The hypoblast lies beneath the epiblast and consists of small cuboidal cells. The hypoblast in fish (but not in birds and mammals) contains the precursors of both the endoderm and mesoderm. In birds and mammals, it contains precursors to the extraembryonic endoderm of the yolk sac. In chick embryos, early cleavage forms an area opaca and an area pellucida, and the region between these is called the marginal zone. Area opaca is the blastoderm's peripheral part where the cells remain unseparated from the yolk. It is a white area that transmits light. Although the hypoblast does not contribute to the embryo, it influences the orientation of the embryo. The hypoblast also inhibits primitive streak formation. The absence of hypoblast results in multiple primitive streaks in chicken embryos. The primitive endoderm derived yolk sac ensures the proper organogenesis of the fetus and the exchange of nutrients, gases, and wastes. Hypoblast cells also provide chemical signals that specify the migration of epiblast cells. In birds, the primitive streak formation is generated by a thickening of the epiblast called the Koller's sickle The Koller's sickle is created at the posterior edge of the area pellucida while the rest of the cells of the area pellucida remain at the surface, forming the epiblast.

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Related concepts (9)
Cell potency
Cell potency is a cell's ability to differentiate into other cell types. The more cell types a cell can differentiate into, the greater its potency. Potency is also described as the gene activation potential within a cell, which like a continuum, begins with totipotency to designate a cell with the most differentiation potential, pluripotency, multipotency, oligopotency, and finally unipotency. Totipotency (Lat. totipotentia, "ability for all [things]") is the ability of a single cell to divide and produce all of the differentiated cells in an organism.
Epiblast
In amniote embryonic development, the epiblast (also known as the primitive ectoderm) is one of two distinct cell layers arising from the inner cell mass in the mammalian blastocyst, or from the blastula in reptiles and birds, the other layer is the hypoblast. It derives the embryo proper through its differentiation into the three primary germ layers, ectoderm, mesoderm and endoderm, during gastrulation. The amnionic ectoderm and extraembryonic mesoderm also originate from the epiblast.
Primitive streak
The primitive streak is a structure that forms in the early embryo in amniotes. In amphibians the equivalent structure is the blastopore. During early embryonic development, the embryonic disc becomes oval shaped, and then pear-shaped with the broad end towards the anterior, and the narrower region projected to the posterior. The primitive streak forms a longitudinal midline structure in the narrower posterior (caudal) region of the developing embryo on its dorsal side.
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