Ultrastructural localization of non-specific alkaline phosphatase during cleavage and blastocyst formation

The localization of non-specific alkaline phosphatase activity during cleavage and blastocyst formation has been investigated in the mouse by electron microscopy. The activity is detectable for the first time at the two-cell stage and is localized on the surface of the interblastomeric plasma membranes and on small cytoplasmic inclusions. It increases in the following stages, predominantly on the interblastomeric membranes, the outside membranes remaining devoid of reaction. From the four-cell stage on, small reactive grains are also observed in the crystalloid plates of the cytoplasm. At the morula stage, the plasma membranes of the inner mass cells are entirely marked by the reaction whereas the trophoblastic cells are polarized, with their inner surfaces positive and outside surfaces negative. At the blastocyst stage the enzyme is gradually eliminated from the membranes bordering the blastocoel and from the interblastomeric furrows of the trophoblast and primary endoderm. The significance of the differential localization of the enzyme is discussed, especially in relation to the differentiation mechanisms of the trophoblast and inner cell mass.