Inner cell mass

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 the 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. The ICM and the TE will generate distinctly different cell types as implantation starts and embryogenesis continues. Trophectoderm cells form extraembryonic tissues, which act in a supporting role for the embryo proper. Furthermore, these cells pump fluid into the interior of the blastocyst, causing the formation of a polarized blastocyst with the ICM attached to the trophectoderm at one end (see figure). This difference in cellular localization causes the ICM cells exposed to the fluid cavity to adopt a primitive endoderm (or hypoblast) fate, while the remaining cells adopt a primitive ectoderm (or epiblast) fate. The hypoblast contributes to extraembryonic membranes and the epiblast will give rise to the ultimate embryo proper as well as some extraembryonic tissues. ==Regulation of cellular specification==

Since segregation of pluripotent cells of the inner cell mass from the remainder of the blastocyst is integral to mammalian development, considerable research has been performed to elucidate the corresponding cellular and molecular mechanisms of this process. There is primary interest in which transcription factors and signaling molecules direct blastomere asymmetric divisions leading to what are known as inside and outside cells and thus cell lineage specification. However, due to the variability and regulative nature of mammalian embryos, experimental evidence for establishing these early fates remains incomplete. Oct4 genetic knockout cells both in vivo and in culture display TE morphological characteristics. It has been shown that one transcriptional target of Oct4 is the Fgf4 gene. This gene normally encodes a ligand secreted by the ICM, which induces proliferation in the adjacent polar TE. • Cdx2: Cdx2 is strongly expressed in the TE and is required for maintaining its specification. Knockout mice for the Cdx2 gene undergo compaction, but lose the TE epithelial integrity during the late blastocyst stage. Furthermore, Oct4 expression is subsequently raised in these TE cells, indicating Cdx2 plays a role in suppressing Oct4 in this cell lineage. Moreover, embryonic stem cells can be generated from Cdx2-null mice, demonstrating that Cdx2 is not essential for ICM specification. • Tead4: Like Cdx2, Tead4 is required for TE function, although the transcription factor is expressed ubiquitously. Tead4-null mice similarly undergo compaction, but fail to generate the blastocoel cavity. Like Cdx2-null embryos, the Tead4-null embryos can yield embryonic stem cells, indicating that Tead4 is dispensable for ICM specification. Recent work has shown that Tead4 may help to upregulate Cdx2 in the TE and its transcriptional activity depends on the coactivator Yap. Yap's nuclear localization in outside cells allows it to contribute to TE specificity, whereas inside cells sequester Yap in the cytoplasm through a phosphorylation event. Together these transcription factors function in a positive feedback loop that strengthens the ICM to TE cellular allocation. Initial polarization of blastomeres occurs at the 8-16 cell stage. An apical-basolateral polarity is visible through the visualization of apical markers such as Par3, Par6, and aPKC as well as the basal marker E-Cadherin. ==Stem cells==

Blastomeres isolated from the ICM of mammalian embryos and grown in culture are known as embryonic stem cells (ESCs)s. These pluripotent cells, when grown in a carefully coordinated media, can give rise to all three germ layers (ectoderm, endoderm, and mesoderm) of the adult body. For example, the transcription factor LIF4 is required for mouse ESCs to be maintained in vitro. Blastomeres are dissociated from an isolated ICM in an early blastocyst, and their transcriptional code governed by Oct4, Sox2, and Nanog helps maintain an undifferentiated state. One benefit to the regulative nature in which mammalian embryos develop is the manipulation of blastomeres of the ICM to generate knockout mice. In mouse, mutations in a gene of interest can be introduced retrovirally into cultured ES cells, and these can be reintroduced into the ICM of an intact embryo. The result is a chimeric mouse, which develops with a portion of its cells containing the ES cell genome. The aim of such a procedure is to incorporate the mutated gene into the germ line of the mouse such that its progeny will be missing one or both alleles of the gene of interest. Geneticists widely take advantage of this ICM manipulation technique in studying the function of genes in the mammalian system. ==Additional images==

File:Gray10.png|Blastodermic vesicle of Vespertilio murinus. File:Gray11.png|Section through embryonic disk of Vespertilio murinus. ==See also==