Extraembryonic mesoderm is any mesoderm in the uterus that is not part of the embryo itself. These include mesoderm in the amnion, chorion, yolk sac and the connecting stalk. Mesoderm is found in the middle of three primary germ layers of the embryo. The embryonic mesoderm forms from the hypoblast (the contribution of trophoblast is also likely), while in mice, this arises from the caudal end of the primitive streak.
The implantation of blastocyst in the endometrium of the uterus is completed in the second week of the development. Embryoblast (the cells of the mass) differentiate into two layers; these are hypoblast and epiblast layers. The hypoblast layer consists of tiny cuboidal cells, while the epiblast layer consists of highly columnar cells. The differentiation of the two into the embryo forms an embryonic disc.
Exocoelomic membrane and the hypoblast form the lining of the exocoelomic cavity (primitive yolk sac). Extraembryonic mesoderm is formed from the cells derived from the yolk sac, and it occupies the space between the trophoblast from the outside and the exocoelomic membrane and the amnion from the inside. A large scale of events within the extraembryonic mesoderm differentiates extraembryonic mesoderm into two layers: the extraembryonic somatic mesoderm and the extraembryonic splanchnic mesoderm. The extraembryonic portion of the ectoderm and extraembryonic somatic mesoderm form the amnion.