CONTRAST BETWEEN THE ENVIRONMENTAL pH DEPENDENCIES OF PROPHASING AND NUCLEAR MEMBRANE FORMATION IN INTERPHASE-METAPHASE CELLS

In Chinese hamster Don cells, fusion of an interphase cell with a metaphase cell resulted either in prophasing of the interphase nucleus, including loss of the nuclear envelope (NE), or in the formation of a double membrane around the metaphase chromosomes. Only one of these phenomena occurred in a given interphase-metaphase (I–M) binucleate cell. At pH 7.4, there was about an equal probability that either event could occur amongst the population of I–M cells. The effect of pH changes in the medium containing the fused cells was examined. At pH 6.6, prophasing was the predominant event; at pH 8.0, membrane formation predominated. It was found that the rate of progression of a mononucleate cell from G2 to metaphase was appreciably faster at pH 6.6 than at pH 8.0. Conversely, the progression from metaphase to G1 was faster at pH 8.0 than at pH 6.6. These results with the mononucleate cells strengthen the hypothesis that structural changes in I–M cells are reflections of normal mitotic phenomena. Additional evidence for this hypothesis was produced by electron microscope examination after direct fixation in chrom-osmium. The double membrane around the chromosomes of the I–M cell was indistinguishable from the normal NE. The results obtained by varying the pH of the medium containing the fused cells provide an indication that disruption or formation of the NE of Don cells depends on the balance reached between disruptive and formative processes.

The fine structure of the cecal epithelium of Megalodiscus temperatus

The cecal epithelium of Megalodiscus temperatus (Stafford 1905) contains two cell types. Although the major component of the epithelium is a syncytium there are also isolated, small, mononucleate cells located in the basal region. The mononucleate cells are always in contact with the underlying basal lamina and show no signs of secretory activity. The lumenal surface is extended in the form of numerous long, closely packed, cylindrical microvilli with tapering tips. Each microvillus may be up to 25 μ long and possesses a central fibrillar core. The cytoplasm of the cecal syncytium contains numerous Golgi complexes which produce membrane-delimited granules containing a dense, homogeneous matrix. These granules appear to be releasing their contents either at the lumenal surface or immediately beneath it. The base of the cecal syncytium but not that of the mononucleate cell type is penetrated by numerous projections of underlying muscle cells. No evidence of endocytotic activity by the cecum can be detected by incubation in thorium dioxide.