EPITHELIAL-CELL POLARITY IN EARLY XENOPUS DEVELOPMENT

The Xenopus blastula consists of two morphologically distinct cell typ es. Polarized epithelial cells build up the embryonic surface and fenc e off an inner non-polarized cell population. We examined the establis hment of this early functional cell diversification in the embryo by s ingle cell analysis, in vitro cell culture, and transplantation experi ments. Single blastomeres from a 64-cell embryo (1/64 cells) exhibit s everal features of polarized cells. The plasma membrane of 1/64 cells consists of an apical domain, which is inherited from the original egg membrane, and a basolateral domain derived from newly formed membrane during cleavage. These are inherent, cell-autonomous properties of th e blastomeres, as they form and are maintained in blastomeres raised i n the absence of any cell interactions in calcium free medium. Upon in vitro culture a single 1/64 cell gives rise to an aggregate of two di fferent cell types. Cells carrying a part of the former egg membrane d omain differentiate into polarized epithelial cells, whereas cells lac king this membrane domain are not polarized. These results demonstrate that the inclusion of the egg membrane, rather than external signals related to the position of a cell in the intact embryo, is required fo r the apical/basolateral differentiation of the surface epithelium. Th is view is supported by cell transplantation studies. A single 1/64 ce ll was implanted into the blastocoel of a stage 8 blastula embryo. The progeny of the implanted cell proliferate within the host embryo and split into two morphologically distinct populations with different cel l behaviours. Cells incorporating a part of the egg membrane form cohe rent patches of polarized epithelial cell sheets in the interior of th e host embryo. In contrast, cells lacking egg membrane do not exhibit any characteristics of polarized cells and eventually spread into diff erent regions of the host embryo. Our results show that the egg membra ne and/or components of the submembrane cortex play a determinative ro le in the formation of the blastula epithelium. (C) 1995 Wiley-Liss, I nc.