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ES cells show diversity

 
The first few days of embryonic development in the mouse witness the zygote undergo a transformation from a single cell, to a raspberry-like cluster (morula), to a hollow ball (blastocyst) before implanting in the uterine wall. Cell differentiation begins by about the third day of embryogenesis, with exterior cells entering the trophectodermal lineage, which forms the placenta and other extraembryonic tissues, and interior cells, which take the first step of the path leading to various somatic fates. The inner cell mass (ICM) of the blastocyst is famous as the source of embryonic stem cells, although it has been shown that ES-like pluripotent cells can also be obtained from the epiblast, a transient structure that arises slightly later in development, after implantation. The epiblast in turn gives rise to primitive ectoderm, which is considered to be the source of the somatic lineages that yield the many differentiated cell types in the adult body. ES cells are of great interest because they show similar ability to give rise to all somatic lineages in culture, but it remains unclear whether all ES cells are truly created equal, or whether they show diversity among themselves.