Epithelial cell polarity in early Xenopus development

Abstract

The Xenopus blastula consists of two morphologically distinct cell types. Polarized epithelial cells build up the embryonic surface and fence off an inner non-polarized cell population. We examined the establishment of this early functional cell diversification in the embryo by single cell analysis, in vitro cell culture, and transplantation experiments. Single blastomeres from a 64-cell embryo (1/64 cells) exhibit several 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 the blastomeres, as they form and are maintained in blastomeres raised in 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 different cell types. Cells carrying a part of the former egg membrane domain differentiate into polarized epithelial cells, whereas cells lacking 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 for the apicalhasolateral differentiation of the surface epithelium. This view is supported by cell transplantation studies. A single 1/64 cell 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 cell behaviours. Cells incorporating a part of the egg membrane form coherent patches of polarized epithelial cell sheets in the interior of the host embryo. In contrast, cells lacking egg membrane do not exhibit any characteristics of polarized cells and eventually spread into different regions of the host embryo. Our results show that the egg membrane and/or components of the submembrane cortex play a determinative role in the formation of the blastula epithelium.

Ca2+ signaling differentiation during oocyte maturation

“…Several proteins that localize to the cell membrane in oocytes are internalized during maturation, including bintegrin and the Na-K-ATPase (Schmalzing et al, 1990;Muller et al, 1993). The internalization of ionic transport pathways during oocyte maturation is thought to be critical for the formation of the fluid-filled blastocoel in the embryo (Muller and Hausen, 1995;Muller, 2001). However, the mechanisms targeting PMCA, or any other membrane proteins, for internalization during oocyte maturation are unknown.…”

Membrane dynamics of cleavage furrow closure in Xenopus laevis

“…The exocytosis of tight junction precursors is evidently a separate process, occurring after the bulk membrane expansion itself, involving the stepwise insertion of cingulin into the apical surface and then occludin and ZO-1 into the basolateral domain (Cardellini et al, 1996;Fesenko et al, 2000). Assembly of junctional complexes begins at the 2-cell stage, but a functional seal may not occur until as late as the 32-cell stage (Muller and Hausen, 1995;Merzdorf et al, 1998;Fesenko et al, 2000). Our time-lapse observations of membrane-labeled embryos revealed metachronal waves of surface contractility, generated by protrusive activity, sweeping rapidly across free basolateral surfaces.…”

A default mechanism of spindle orientation based on cell shape is sufficient to generate cell fate diversity in polarisedXenopusblastomeres

“…Dissociated blastomeres were obtained by transferring embryos in Ca 2+ /Mg 2+ -free medium (CAMFM) as described (Muller and Hausen, 1995). Isolation of single blastomeres, filming and compression was carried out in 1ϫMMR.…”

“…2B). They also maintain apicobasal polarity: pigment markers, molecular markers and tight junction components at the boundary of the apical and basolateral membrane domains remain localised correctly (Fesenko et al, 2000;Muller and Hausen, 1995;Chalmers et al, 2003).…”

“…As in other examples of fate-asymmetric divisions, the cells of the Xenopus blastula are morphologically and molecularly polarised along the apicobasal axis. Orientation of the mitotic spindle into this axis of polarity, perpendicular to the surface of the embryo, generates inner and outer cells, which are intrinsically programmed to follow different fates in development (Muller and Hausen, 1995;Chalmers et al, 2005;Chalmers et al, 2003;Chalmers et al, 2002). In the animal ectoderm region, outer cells give rise to neuronal progenitors, the ultimate fate of which is presently unknown, while inner cells give rise to early differentiating primary neurons (Chalmers et al, 2002;Hartenstein, 1989).…”