UC Davis

Superficial blastomeres of mid-cleavage stage amphibian embryos (32- and 64-cell stage) display regional cell-surface differences in adhesiveness. The cells are adhesive on the lateral and basal cell surfaces and non-adhesive on the apical surface. These adhesive differences are maintained by single cells which have been dissociated from the intact embryo. Pigmentation differences afford a means of distinguishing apical surface from basal/lateral surface in dissociated cells. The apical surface is underlain by melanin granules and so appears dark, whereas the lateral and basal surfaces lack pigment and are white. Scanning electron-micrograph observations of isolated superficial cells reveal striking morphological differences between the adhesive and non-adhesive regions. The apical (non-adhesive) portion of the cell surface has a convoluted appearance and is almost entirely devoid of microvilli. The lateral and basal (adhesive) surfaces lack the convolutions of the apical surface and bear scattered microvilli. The isolated superficial cells continue to divide in culture. The cells display a unilateral or one-sided furrow which begins in the basal surface and progresses toward the apical surface. The plane of cleavage of most of the cells is oriented such that the pigmented cap is bisected, with very few cells having the plane of the furrow pass parallel to the apical surface. The direction of furrowing in the isolated cells from the 32- to 64-cell embryo represents a direction of furrow progression in the intact embryo from the interior to the surface. Since the early cleavages are initiated at the surface and progress into the interior, it is suggested that a reversal in direction of furrowing occurs by mid-cleavage stages.