Inner cell mass

In early embryogenesis of most eutherian mammals, the inner cell mass (abbreviated ICM and also known as the embryoblast or pluriblast, the latter term being applicable to all mammals) 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 (more correctly termed "blastocyst cavity," as it is not strictly homologous to the blastocoele of anamniote vertebrates) and is entirely surrounded by the single layer of cells called trophoblast.

Further development

The physical and functional separation of the inner cell mass from the trophectoderm (TE) is a special feature of mammalian development and is the first cell lineage specification in these embryos. Following fertilization in the oviduct, the mammalian embryo undergoes a relatively slow round of cleavages to produce an eight cell morula. Each cell of the morula, called a blastomere, increases surface contact with its neighbors in a process called compaction. This results in a polarization of the cells within the morula, and further cleavage yields a blastocyst of roughly 32 cells. In mice, about 12 internal cells comprise the new inner cell mass and 20 – 24 cells comprise the surrounding trophectoderm. There is variation between species of mammals as to number of cells at compaction with bovine embryos showing differences related to compaction as early as 9-15 cells and in rabbits not until after 32 cells. There is also interspecies variation in gene expression patterns in early embryos