Cell membrane regionalization and cytoplasm polarization in the rat early embryo.

In rat pre-implantation embryos, we compared the polarization of cytoplasmic organelles with cell membrane regionalization as revealed by the cytochemical demonstration of 5'-nucleotidase and alkaline phosphatase. The polarization is shown at the eight-cell stage by columns of organelles extending from the nuclei to the embryo periphery. No consistent segregation of cytoplasmic components was recognized prior to the eight-cell stage. As to regionalization, both enzyme activities were demonstrated, from the late four-cell stage onwards on the cell surface between blastomeres of early and late morulae, while the external surface of the embryo lacked these activities. In early blastocysts, these enzyme activities were lost on the inner surface of the blastocyst cavity and in late blastocysts; 5'-nucleotidase activity appeared on the external surface of the embryonal trophoplast and extended to the abembryonal pole. From these observations we conclude that cell membrane regionalization precedes the polarization of the cytoplasm; however, this temporal relationship does not necessarily imply a causal relationship, since both phenomena may be independent expressions of an underlying morphonetic process.