Planar polarity of the extraembryonic epithelia in the preimplantation porcine conceptus.

The present review deals with the trophoblast structure during the free intrauterine life of the pig blastocyst. The term trophoblast is used here to describe the association of the first extraembryonic cell layers, the trophectoderm and the primitive endoderm that are polarized epithelia, a fact established by ultrastructural and immunocytochemical data. The aim of this synthesis is to gather the relative works dispersed in the litterature and to explain the implication of the planar polarity of these cell layers on their developmental fate and roles. These epithelia are intricately dependent on each other for the maintenance of their differentiated state and continuity. The modalities of their spectacular expansion can be explained in part by biomechanical concepts.