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Inner cell mass
From Wikipedia, the free encyclopedia
In early embryogenesis, the inner cell mass (abbreviated ICM and also known as the embryoblast) is the mass of cells inside the primordial embryo that will eventually give rise to the definitive structures of the fetus. This structure forms in the earliest steps of development, before implantation into the endometrium of the uterus has occurred. The ICM lies within the blastocoele and is entirely surrounded by the single layer of cells called trophoblast.
The ICM differentiates into two types of cells, epiblast and hypoblast. Only the cells of the epiblast will give rise to cells inside the embryo, while hypoblast cells form only extraembryonic tissues (for example, the extraembryonic mesoderm forming parts of the placenta). Thus, all three germ layers of the definitive embryo arise from epiblast.
 Blastodermic vesicle of Vespertilio murinus. |
 Section through embryonic disk of Vespertilio murinus. |
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