Gastrulation

In human development, the process of gastrulation transforms components of the blastocyst into three embryonic germ layers.

The process starts after blastocyst implantation into the uterine wall. At which point, the cells of the inner cell mass separate into a bilaminar embryonic disc consisting of the epiblast, cells that eventually form the actual embryo, and the hypoblast, which generates extra embryonic structures.

Within the epiblast, the amniotic cavity forms while the cells of the hypoblast migrate to form the yolk sac. Near the midline of the embryonic disc, epiblast cells condense to create the primitive streak through which cells migrate below in a process called ingression. The primitive node also forms, which will be important for the organization of neural tissue.

During ingression, migrating cells infiltrate and displace cells of the hypoblast and over time create the endoderm that eventually forms components of the respiratory and digestive systems.

The process continues and a second layer manifests between the epiblast and primitive endoderm. This is the mesoderm, which contributes to the skeletal, circulatory, and muscular systems.

Eventually, ingression stops and the primitive streak vanishes. Any cells that do not migrate and stay within the epiblast constitute a new third tissue, the ectoderm, the precursor of the nervous system and skin.

Thus, the end result of human gastrulation is an embryo composed of three stacked germ layers. Ectoderm, mesoderm, and endoderm, each of which will contribute to unique components of the body.