Further Studies on Cell Division Without Mitotic Apparatus in Sea Urchin Eggs

Abstract

A largc quantity of paraffin oil, sucrose solution, or sea water was injccted into the eggs of the heart urchin Clypeaster japonicus shortly before the onset of the first cleavage. The injected oil became spherical, pushing the mitotic apparatus aside. The sucrose solution mixed with the protoplasm and caused disintegration of the mitotic apparatus, and the sea water formed a vacuole at the center of the cell. In all these cases, cleavage may take place almost normally in spite of the absence of the mitotic apparatus or its displacement within the cell. In some eggs, furrowing may take place when more than fifty per cent of the endoplasm has been replaced with sea water before onset of cleavage.In a previous paper (3), it was shown that cleavage may take place in sea urchin eggs, from which the mitotic spindle or both the spindle and the asters have been removed before onset of cleavage (metaphase or anaphase of mitosis), and it was shown further that the position of the furrow is not affected by the removal or by the displacement of the mitotic apparatus within the egg. From these results, it was concluded that, after a definite stage, the furrowing results from mechanical activity of the cortex which is not modified by presence or absence of the mitotic apparatus. Recently, the author succeeded in some experiments further confirming the above conclusion, and these are described in the present paper. I METHODS AND RESULTSAs material, the fertilized eggs of thc heart urchin, Clypeaster japonicus, were used. The eggs were deprived of the fertilization membranes and the hyaline layers by treatment with 1 ~ urea solution shortly after insemination, and they were then left in a Syracuse dish filled with sea water. Before experimentation, the eggs were put into a manipu-1 A preliminary account of the results of this paper has been given in Japanese (6). lation chamber devised by the autho r (3). Unless otherwise stated, manipulation was carried out in eggs shortly before onset of cleavage (anaphase). The methods of the microinjection and of the suction of the egg protoplasm have already been mentioned elsewhere (3,5).

A FINE STRUCTURAL ANALYSIS OF CLEAVAGE INDUCTION AND FURROWING IN THE EGGS OF ARBACIA PUNCTULATA

“…Later, during active furrowing, the band is restricted to the walls of the furrow itself . This view has received support from a number of experimental studies (Hiramoto, 1956(Hiramoto, , 1965Rappaport, 1966 ;Scott, 1960 ;Dan and Kojima, 1963) . Mercer and Wolpert (1958) have described in thin sections a dense layer immediately below the surface membrane .…”

Influence of centriole number on mitotic spindle length and symmetry

“…As with mitosis, it is well established that cleavage can progress without centrioles and even without spindles at all [Hiramoto 1965], so if centrioles contribute to the process at all, they would only be expected to act in a regulatory manner. Defects in cytokinesis have been observed in some instances after centriole ablation [Khodjakov and Rieder, 2001], but only in a fraction of cells.…”

Geometrical Relations of the Cleavage Stimulus in Flattened, Perforated Sea Urchin Eggs

“…He assumed that surface differentiation had not yet occurred when he made slit punctures about ten minutes before cleavage began. Since cells divide despite removal of the mitotic apparatus during anaphase while they are yet spherical (HIRAMOTO, 1956). determination takes place earlier than SCOTT proposed and ntay well have been in progress while he performed his operations.…”

“…Existing information strongly suggests that the role of the mitotic apparatus in cytokinesis is confined to the release of a stimulus which elicits the functional differentiation of the surface required for division. The possibiiity that cytokinesis is mechanically dependent upon the mitotic apparatus was eliminated (YATSU, 1912;HIRAMOTO, 1956). Furrowing also proceeds despite replacement or drastic rearrangement of the endoplasm between the mitotic apparatus and the surface (CHAMBERS and KOPAC, 1937) HIRAMOTO, 1965; RAPPAPORT, 1966 ).…”

“…The possibiiity that cytokinesis is mechanically dependent upon the mitotic apparatus was eliminated (YATSU, 1912;HIRAMOTO, 1956). Furrowing also proceeds despite replacement or drastic rearrangement of the endoplasm between the mitotic apparatus and the surface (CHAMBERS and KOPAC, 1937) HIRAMOTO, 1965; RAPPAPORT, 1966 ).…”