A cytological analysis of artificial parthenogenesis in the surf clam Spisula solidissima

Dynein is required for spindle assembly in cytoplasmic extracts of Spisula solidissima oocytes

Meiosis I spindle assembly is induced in lysate-extract mixtures prepared from clam (Spisula solidissima) oocytes. Unactivated lysate prepared from unactivated oocytes contain nuclei (germinal vesicles, GVs) which house condensed chromosomes. Treatment of unactivated lysate with clarified activated extract prepared from oocytes induced to complete meiosis by treatment with KCl induces GV breakdown (GVBD) and assembly of monopolar, bipolar, and multipolar aster-chromosome complexes. The process of in vitro meiosis I spindle assembly involves the assembly of microtubule asters and the association of these asters with the surfaces of the GVs, followed by GVBD and spindle assembly. Monoclonal antibody m74-1, known to react specifically with the N terminus of the intermediate chain of cytoplasmic dynein, recognizes Spisula oocyte dynein and inhibits in vitro meiosis I spindle assembly. Control antibody has no affect on spindle assembly. A similar inhibitory effect on spindle assembly was observed in the presence of orthovanadate, a known inhibitor of dynein ATPase activity. Neither m74-1 nor orthovanadate has any obvious affect on GVBD or aster formation. We propose that dynein function is required for the association of chromosomes with astral microtubules during in vitro meiosis I spindle assembly in these lysate-extract mixtures. However, we conclude that dynein function is not required for centrosome assembly and maturation or for centrosome-dependent aster formation.

Centriole duplication in lysates of Spisula solidissima oocytes

A cell-free system has been developed that executes centriole duplication. Surf clam (Spisula solidissima) oocytes, arrested at late prophase of meiosis I, do not contain centrioles, centrosomes, or asters. Serial section high-voltage electron microscopy (HVEM) of asters and spindles isolated from potassium chloride-activated oocytes indicates that within 4 minutes oocytes assemble a single centriole that is duplicated by 15 minutes when assembly of the first meiotic spindle is complete. A mixture of lysates from unactivated oocytes and potassium chloride-activated oocytes induces centriole formation and duplication. Astral microtubule content in these lysate mixtures increases with time.

Ultrastructural analysis of artificially activated rabbit eggs

Ultrastructural investigations have been carried out on parthenogenetic rabbit eggs in an effort to elucidate events occurring during artificial activation and their similarity to processes of fertilization and embryogenesis. Rabbit eggs were artificially activated by culturing at 10 degrees C for 24 hours followed by incubation at 37 degrees C for 2 to 24 hours. Examination of eggs immediately after incubation at 10 degrees C for 24 hours indicates that activation is initiated when the chromosomes coalesce to form a reticulum which is either surrounded completely by two parallel membranes or incompletely by cisternae of smooth endoplasmic reticulum. Aggregation of the chromosomes occurs as a result of a reduction in the number of microtubules making up the meiotic spindle. When cold treated ova are subsequently incubated at 37 degrees C a nucleus is formed which moves central where it may participate in the cleavage of the egg. Formation of a second polar body and release of the contents of the cortical granules as reported for inseminated eggs was not found to be a part of activation of the egg by cold treatment. Approximately 95% of the ova cultured at 10 degrees C for 24 hours followed by 37 degrees C for 12 hours were activated, i.e., they possessed a nucleus or they had cleaved. Many of the activated eggs cultured for short periods at 37 degrees C were structurally similar to fertilized ova, with further incubation fragmented eggs and abnormal multicellular stages predominated.