These noninvasive types of placentation are probably a derived state, having evolved from ancestors whose placentas had a hemochorial interface. As a result, the placenta is either entirely or very largely of conceptus origin.

In other species, invasion of maternal tissue either fails to occur (e.g., swine and some prosimians) or is very limited (e.g., ruminants such as cattle and sheep). Consequently, the placenta is chimeric-comprised of intimately associated cells from both the conceptus and the mother. In hemochorial placentation, which occurs in humans and mice, there is extensive invasion of TR into the uterine wall. Many of the subsequent steps in placental development vary widely across species. Segregation of trophectoderm (TE), the precursor of placental TR cells, from the inner cell mass (ICM) at the blastocyst stage is the first obvious fate decision during formation of the mammalian conceptus. The diagrams are based on those of Tanaka et al. The insets in A and B show the regions illustrated in the main figures. FGF4 and NODAL are assumed to be produced by the ICM and epiblast. For simplicity, the polar TE, because of its transient nature, is regarded as the precursor of the “true” TSC population in its initial niche adjacent to the epiblast. Recent derivation of TSC from blastocysts of the rhesus monkey suggests that isolation of the human equivalents may be possible and will reveal the extent to which mechanisms uncovered by using animal models are true in our own species.ĭiagram to illustrate the dependence of the polar TE in the Day 3.5 blastocyst ( A) and TSC in a conceptus at Day 6.5 ( B) on factors produced by the ICM and epiblast, respectively, of the mouse conceptus. TSC have yet to be derived from domestic species in which remarkable TR growth precedes embryogenesis. The transition from ESC to TSC, which appears to be unidirectional, reveals important aspects of initial fate decisions in mice. Pluripotent cells that have acquired sublethal chromosomal alterations may be sequestered into TR for similar reasons. Disrupting the pluripotent network of embryonic stem cells (ESC) causes them to default to a TR ground state. Repression of the gene network that maintains pluripotency and emergence of the transcription factor pathways that specify a trophoblast (TR) fate enables TSC derivation in vitro and placental formation in vivo. Mouse TSC self-renewal can be sustained by culture on mitotically inactivated feeder cells, which provide one or more factors related to the NODAL pathway, and a medium supplemented with FGF4, heparin, and fetal bovine serum. The mouse TSC niche appears to be located within the E圎 adjacent to the epiblast, on which it depends for essential growth factors, but whether this cellular architecture is the same in other species remains to be determined. In the mouse, TSC can be derived from outgrowths of either blastocyst polar trophectoderm (TE) or extraembryonic ectoderm (E圎), which originates from polar TE after implantation. Trophoblast stem cells (TSC) are the precursors of the differentiated cells of the placenta.

Embryonic stem cells definition biology