Surface polarization in loach eggs and two-cell embryos: correlations between surface relief, endocytosis and cortex contractility.

The aim of this study was to examine the reorganization of the microfilamentous cortical layer (MC) accompanying ooplasmic segregation in loach eggs. Using scanning (SEM) and transmission electron microscopy (TEM), we found that the MC is thicker in folded areas. Prior to fertilization, surface microvilli are distributed more or less uniformly throughout the egg. A similar, more or less uniform, distribution of endocytotic events was observed in the eggs 5-15 min after insemination using fluorescence microscopy of Lucifer yellow CH uptake. During ooplasmic segregation, the surface is progressively polarized so that before the first cleavage onset (50-60 min after insemination) only the blastodisc surface is folded and undergoes endocytosis, whereas the vegetal surface is smooth and does not show internalization. In two-cell embryos, the blastomeric surface is also regionalized according to its relief and endocytosis. When surface tension was lowered by sucking most yolk granules out of the egg, we observed contractile responses only in the animal folded surface. These data suggest that a polar distribution of contractile structures is established in the loach egg undergoing ooplasmic segregation.

The effect of local cortical microfilament disorganization on ooplasmic segregation in the loach (Misgurnus fossilis) egg.

Injections of cytochalasin D (CD) or DNase I under the surface of fertilized loach egg result in local disorganization of microfilamentous cortex (MC) as revealed by transmission electron microscopy. This effect correlates with the loss of the cortex ability to contract in vitro. The disorganization of MC in the vegetal hemisphere of the egg does not affect the ooplasm segregation or blastodisk cleavage. Injection under the animal pole suppresses blastodisk formation and results in the autonomous separation of ooplasm in the central part of the egg. The experiments suggest that (1) autonomous separation of ooplasm from the yolk granules can proceed in the central part of the egg without the participation of MC; (2) normal segregation of ooplasm at the animal pole requires that the structures of microfilaments in the animal hemisphere (but not in the vegetal one) be preserved.

Asymmetrical rotations of blastomeres in early cleavage of gastropoda.

The movements of blastomere surfaces marked with carbon particles during cytokinesis of the Ist-IVth cleavage divisions in the eggs of the gastropodsLymnaea stagnalis, L. palustris, Physa acuta and Ph. fontinalis have been studied by time-lapse cinematographic methods. The vitelline membrane was removed with trypsin. At 2- and 4-cell stages shifts of nuclei have also been studied.Symmetrical as well as asymmetrical surface movements (in respect to the furrow plane) have been revealed. Symmetrical surface movements at the beginning of cytokinesis consist mainly in contraction of the furrow zone and in expansion of the more peripheral regions; between these there is a stationary zone. After the end of cytokinesis the furrow region expands.Considerableasymmetrical surface movements have also been observed in all four divisions. From anaphase until the end of cytokinesis each of the two sister blastomeres rotates with respect to the other in such a way, that if viewed along the spindle axis, the blastomere nearest to the observer rotates dexiotropically in a dextral species and laeotropically in a sinistral species (primary rotations). After the completion of cytokinesis the blastomeres may rotate in a reverse direction. The latter rotations are less pronounced in the IInd and IIIrd divisions and most pronounced in the IVth division. Blastomeres with the vitelline membrane intact retain a slight capacity for primary rotations. In normal conditions nuclei of the first two blastomeres shift mainly laeotropically in dextral species, but dexiotropically in sinistral species, being carried along by the reverse surface rotations.The invariable primary asymmetrical rotations of blastomeres seem to be the basis of enantiomorphism in molluscan cleavage. They are assumed to be determined by an asymmetrical structure of the contractile ring carrying out the cytokinesis.