The preimplantation mouse blastocyst consists of two differentiated tissues, the trophectoderm (a structurally and functionally polarized epithelium) and the inner cell mass. The divergence of these two cell types can be traced back to a contact dependent polarization of the surface and cytoplasm at the 8-cell stage. Membrane/cytocortical organization during this preimplantation period has been studied using freeze fracture in conjunction with the sterol-binding antibiotic filipin in an attempt to discern the molecular basis and origin of these surface asymmetries.

The distribution of filipin reactivity within the different membrane domains showed that the surface polarity exhibited by trophectoderm and by blastomeres of the 8-cell stage is underlain by a heterogeneity in molecular organization of the membrane/cytocortex which may originate prior to the appearance of any overt surface polarity.

The results are discussed in terms of the likely basis of this membrane/cytocortical asymmetry, its probable origins and the use of the preimplantation mouse embryo as a model system for studying the assembly of a polarized epithelium.

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