CELL-CELL ASSOCIATION DIRECTED MITOTIC SPINDLE ORIENTATION IN THE EARLY DEVELOPMENT OF THE MARINE SHRIMP SICYONIA-INGENTIS

During early cleavages of Sicyonia ingentis embryos, mitotic spindle o rientations differ between blastomeres and change in a predictable man ner with each successive mitosis, From 2(nd) through 7(th) cleavages, spindles orient at a 90 degrees angle with respect to the spindle of t he parent blastomere, Thus, spindle orientation is parallel to the cle avage plane that formed the blastomere, To determine if specific spind le orientations were intrinsic properties of individual blastomeres, w e altered blastomere associations and asked how mitotic spindle orient ation was affected in successive cleavages using laser scanning confoc al microscopy, Linear embryos were constructed by dissociating 4-cell embryos and recombining the blastomeres in a linear array, The ensuing cleavage (3(rd) embryonic cleavage) of these linear embryos was paral lel to the long axis of the embryo, resulting in four parallel pairs o f blastomeres which lay in a common plane that was parallel to the sub stratum, The 4(th) cleavage produced a linear embryo with the 16 blast omeres arranged in four parallel quartets, Then, in preparation for 5( th) cleavage, spindles oriented at a 45 degrees angle (not parallel as in normal development) with respect to the previous cleavage plane, W hen 8-cell linear embryos were separated into linear half-embryos, sub sequent spindle orientations were not like those observed for intact 8 -cell linear embryos, but rather regressed to the orientation seen in 4-cell linear embryos, We suggest that the reorientation of mitotic sp indles during early cleavage of S. ingentis is neither an intrinsic pr operty nor age dependent, but rather is cell contact related, Further, these results in conjunction with observations of non-manipulated emb ryos suggest that spindle poles (centrosomes) avoid cytoplasmic region s adjacent to where there is cell-cell contact during early developmen t.