Cell membrane regionalization in early mouse embryos as demonstrated by 5'-nucleotidase activity
The distribution of 5'-nucleotidase activity in pre-implantation mouse embryos is studied by means ofa cytochemical method adapted from Uusitalo & Karnovsky (1977). The enzyme activity is detected, from the4-cell stage up to the morula stage, on discrete patches of the cell membane between blastomeres. Appropriatecontrols show that this distribution is not a localization artifact due to selective retention of the enzyme reaction product in the narrow interblastomeric spaces. In early blastocysts, as the blastocoel expands the enzyme activity on its lining disappears. The external surface of the trophectoderm in early blastocysts lacks any enzyme activity, whereas in late blastocysts a strong enzyme activity is detected at the embryonic trophectoderm, decreasing in intensity towards the opposite pole of the embryo. These results are compared to previous observations by other authors and the differences are mainly ascribed to differences in the cytochemical procedure employed. We conclude that during cleavage a gradual cell membrane regionalization unfolds, revealing a pattern that may be related to morphogenesis; in particular, to the localization of zonular tight junctions around the peripheral blastomeres of the morula (Izquierdo, 1977; Izquierdo, López & Marticorena, 1980).