The cytoskeleton, endocytosis and cell polarity in the mouse preimplantation embryo

The process of cell polarization in mouse 8-cell embryos includes the formation of a polar cluster of cytoplasmic endocytotic organelles (endosomes) subjacent to an apical surface pole of microvilli. A similar polar morphology, supplemented by basally localized secondary lysosomes, is evident following division to the 16-cell stage in outside blastomeres, precursors of the trophectodermal lineage. The roles of microfilaments and microtubules in generating and stabilizing endocytotic and surface features of polarity (visualized by horseradish peroxidase incubation and indirect immunofluorescence labeling, respectively) have been evaluated by exposure of 8- and 16-cell embryos and 8-cell couplets to drugs (cytochalasin D, colcemid, nocodazole) that disrupt the cytoskeleton. The generation of endocytotic polarity is dependent upon intact microtubules and microfilaments, but the newly established endocytotic pole in blastomeres from compacted 8-cell embryos appears to be stabilized exclusively by microtubules. Polarized endocytotic organelles at the 16-cell stage are more resistant to drug treatment than at the 8-cell stage (probably due to microfilament interactions) indicating a maturation phase in the polar cell lineage. Microtubules are also responsible for the orientation of endocytotic clusters along the cell's axis of polarity. In contrast, the generation and stability of polarity at the cell surface appears relatively independent of cytoskeletal integrity. The results are discussed in relation to the mechanisms that may control the development and stabilization of polarization during cleavage.