Development of cellular polarity of hamster embryos during compaction

Development of cellular polarity is an important event during early mammali an embryo development and differentiation. Blastomeres of hamster embryos a t various stages were examined by scanning electron microscopy (SEM) and im munocytochemical staining. SEM observations revealed that 1- to 7-cell-stag e embryos showed a uniform distribution of microvilli throughout the cell s urface. Microvillous polarization was initially noted in the blastomeres (1 0-35%) of 8-cell-stage embryos. The polarized microvilli were observed most ly in the basal region of cell-cell contact and occasionally at the apical, outward-facing surface of the blastomere. Fluorescein-isothiocyanate-conju gated concanavalin A failed to reveal any polarity in the blastomeres regar dless of the stages of the embryos. Actin staining showed that microfilamen ts were present beneath the cell surface, and in addition, areas of cell co ntact were more heavily stained, indicating a thick microfilament domain. M icrotubules were located throughout the cytoplasm and were heavily concentr ated near the nucleus during interphase, although they became redistributed in the region of the mitotic spindle during karyokinesis. The position of nucleus changed from the cell center to the apical, outward-facing surface of the cell, and it distanced itself from the basal microvillous pole. It i s suggested that the changes in the cell surface and nuclear position are t he first manifestations of cell polarity in peri-compacted hamster embryos, which appear as early as the 8-cell stage; furthermore, the outward migrat ion of the nuclei may parallel the redistribution of microtubules in the cy toplasm.