Some Structural and Functional Aspects of the Mitotic Apparatus in Sea Urchin Embryos

Abstract

The mitotic figures in dividing cells of sea urchin embryos, from first division to the onset of cilia formation, were studied with regard to the filament system and its relation to kinetochores, chromosomes, and poles, as well as to fixation conditions which would best preserve these structures. With regard to fixation, variations in the salt concentration and pH of the fixative indicated that an extraction effect on the chromosomes noted in earlier work was probably due to a combination of neutral pH and salt concentration equivalent to sea water. The presence of the 15 m# filaments depended on the presence of either of two stabilizing conditions: pH 6.1 or presence of the salts of sea water, presumably the divalent cations of Ca and Mg. Kinetochores and centrioles were unaffected by the fixative variations. The 15 m# filaments, reported earlier in the central spindle, are also found in great numbers in the asters of early cleavage divisions. However, with successive divisions and reduction in cell size, the aster disappears at about the 32 to 64 cell stage, and the 15 m~ filaments are entirely associated with the central spindle. This disappearance of the aster suggests that it may be, in fact, merely a specialization of large cells for cytokinesis.

Some Observations Concerning Metakinesis in Sea Urchin Eggs

“…These filaments, having the appearance of tubes with a diameter of 15 to 20 m#, become aligned in massive array to form the central spindle, along with the continuous filaments that extend from pole to pole. The chromosomal filaments, as described earlier (Harris, 1962), appear to attach directly to a dense kinetochore region at the surface of the chromosome (Fig. 1).…”

“…In recent years, however, evidence for the actual existence of spindle fibers has accumulated, notably Inou6's studies showing the birefringence of the spindle in living material (Inou6, 1953). Recently improved electron microscope techniques have now revealed great numbers of fine filaments having a tubular appearance and a diameter of approximately 15 to 20 m/z in osmium tetroxide-fixed material (Roth et al, 1960;Roth and Daniels, 1962;Harris, 1962; and others). The present paper describes the fine structure of the mitotic apparatus at prometaphase of the first cleavage division, showing the early attachment of these filaments to the chromosomes and their insertion into the centrosphere region at the poles.…”

Protein synthesis and the cell cycle: centrosome reproduction in sea urchin eggs is not under translational control.

“…At times beginning 90 min after fertilization, aliquots of zygotes were fixed for 90 min in 2% osmium tetroxide in 0.4 M sodium acetate (12,27). After dehydration and embedment in Epon-Araldite, zygotes were serially semithick (0.25-0.5 ~m) sectioned as previously described (21,23).…”

Observations on the Cytology and Fine Structure of Mitosis in the Fern, <i>Ophioglossum petiolatum</i> Hook

“…Some appear in nearly every section through mitotic figures. We were unable to observe any differentiation of this structure, although in animal cells the centromere may exhibit staining characteristics which differ from the rest of the chromosome (Harris 1962, Mota 1962, Luykx 1965, Murray et al 1965, Brinkley and Stubblefield 1966. Pickett-Heaps and Northcote (1966) also have observed no particular centromere structure in wheat, but Dietrich (1966) has observed that the centromere in Lilium candidum definitely stains lighter than the rest of the chromosome.…”