Trophectoderm: the first epithelium to develop in the mammalian embryo.

The first epithelium to appear during mammalian embryogenesis is the trophectoderm, a polarized transporting single cell layer that comprises the wall of the blastocyst. The trophectoderm develops concurrently with blastocoele fluid production as the morula develops into a blastocyst. The process whereby the morula becomes a fluid-filled cyst is called 'cavitation', which can be regarded as the first functional expression of the trophectoderm phenotype. The outer layer of cells of the morula comprise the nascent trophectoderm and are already morphologically polarized prior to the onset of cavitation. A major working hypothesis in the field of mammalian embryogenesis is that such polarization is a prerequisite for the initiation of cavitation. To test this hypothesis we examined morulae for their ability to cavitate during treatments that modify morphological polarity in nascent trophectoderm cells. These treatments included ouabain, different concentrations of extracellular K and Na, cytochalasins and colcemid. Ouabain and extracellular K and Na affect the activity of Na/K-ATPase, which has been implicated in the maintenance of morphological polarity of nascent trophectoderm cells. Cytochalasins and colcemid also modify apical-basal polarity of nascent trophectoderm cells and impair cavitation. The endpoints that were monitored included incidence of cavitation, rate of cavitation and the status of morphological polarity of nascent trophectoderm cells. Collectively, all of these treatments indicate that there is a functional association between an asymmetric distribution of organelles along the apical-basal axis of nascent trophectoderm cells and the ability of the embryo to produce nascent blastocoele fluid efficiently. In addition, the preimplantation embryo appears to possess two mechanisms for accumulating blastocoele fluid.