Hypoblast

An article about the hypoblast in embryonic development

The hypoblast is a layer of cells that forms during the early stages of embryonic development in vertebrates. It is one of the two layers that make up the bilaminar disc, the other being the epiblast. The hypoblast plays a crucial role in the formation of the extraembryonic endoderm and contributes to the development of the yolk sac.

Human embryo at day 9, showing the hypoblast.

Formation

The hypoblast forms during the second week of development, following the implantation of the blastocyst into the uterine wall. Initially, the inner cell mass of the blastocyst differentiates into two layers: the epiblast and the hypoblast. The hypoblast is situated beneath the epiblast and faces the blastocoel.

Function

The primary function of the hypoblast is to contribute to the formation of the yolk sac, which is an essential structure for nutrient transfer during early development. The hypoblast cells migrate to line the blastocoel, forming the Heuser's membrane, which eventually becomes part of the yolk sac.

Role in Development

Although the hypoblast does not directly contribute to the formation of the embryo proper, it plays an indirect role in embryonic patterning. The hypoblast helps to establish the anterior-posterior axis of the embryo by influencing the positioning of the primitive streak on the epiblast.

Hypoblast and Gastrulation

During gastrulation, the hypoblast is displaced by the migrating epiblast cells that form the definitive endoderm. The hypoblast cells are eventually pushed to the periphery and contribute to the extraembryonic tissues.

Related pages

• Epiblast
• Gastrulation
• Yolk sac
• Embryonic development