The pairing of nucleolar patterns in an epithelium as evidence for a conserved nuclear skeleton

Pattern Formation and Complexity in Single Cells

In the context of animal or plant development, we tend to think of cells as small, simple, building blocks, such that complex patterns or shapes can only be constructed from large numbers of cells, with cells in different parts of the organism taking on different fates. However, cells themselves are far from simple, and often take on complex shapes with a remarkable degree of intracellular patterning. How do these patterns arise? As in embryogenesis, the development of structure inside a cell can be broken down into a number of basic processes. For each part of the cell, morphogenetic processes create internal structures such as organelles, which might correspond to organs at the level of a whole organism. Given that mechanisms exist to generate parts, patterning processes are required to ensure that the parts are distributed in the correct arrangement relative to the rest of the cell. Such patterning processes make reference to global polarity axes, requiring mechanisms for axiation which, in turn, require processes to break symmetry. These fundamental processes of symmetry breaking, axiation, patterning, and morphogenesis have been extensively studied in developmental biology but less so at the subcellular level. This review will focus on developmental processes that give eukaryotic cells their complex structures, with a focus on cytoskeletal organization in free-living cells, ciliates in particular, in which these processes are most readily apparent.

The development of branched silk gland nuclei

Nuclei in the giant polyploid silk gland cells of Calpodes ethlius grow by endomitosis and can develop hundreds of branches during larval life. The shape of the these nuclei is characteristic for each region of the gland. We have found shape to be correlated with arrangement of the nuclear matrix. Scanning electron microscopy showed nuclear matrices with shapes similar to those of feulgen stained nuclei. Profiles of isolated matrices seen by transmission electron microscopy had filaments aligned parallel to the long axis of nuclear branches. DNA stained by Hoechst had a similar parallel alignment within the branches. Nuclear shape may be maintained by a small number of components, since electrophoretic analysis showed only a few abundant polypeptides in the matrix fraction. Silk gland nuclei have some of the same nuclear matrix antigens found in smaller, more regularly shaped, eukaryote nuclei.

A protein homologous to human Ku p70-protein is required for reconstitution of Xenopus sperm pronuclei

The monoclonal antibody mAbH6, which recognizes one human Ku-protein (p70), cross-reacted with the counterpart protein from Xenopus eggs. The Xenopus antigen purified with a mAbH6-Sepharose column was a complex of 88 kDa and 72 kDa proteins. The role of Ku protein in nuclear structure formation was studied using a cell-free nuclear assembly extract derived from Xenopus interphase eggs. The protein was distributed on the surface of demembranated sperm chromatin and in the membrane vesicle fraction of the nuclear assembly extract. Addition of mAbH6 to the assembly extracts prevented the completion of reconstitution of pronuclei from demembranated sperm chromatins, although partial decondensation mediated by nucleoplasmin was not impeded. As a result, the sperm chromatin remained in partially swollen structures or formed round but small anomalous nuclei. Nuclear membranes were formed on the nuclei in mAbH6-inhibited extract systems, but DNA synthesis was largely decreased, suggesting the incomplete reconstitution of pronuclei. The incorporation of lamin to the nuclei was inhibited by mAbH6. It is suggested that the Xenopus Ku-homologous protein has a role in the formation of the lamin layer after nuclear membrane reconstitution.

The planes of division in twin cell doublets

The fifth stage larval epidermis of Calpodes ethlius (Lepidoptera, Hesperiidae) is a syncytium of doublets where sibling cells remain connected by residual midbodies between mitoses. These twins resemble one another more than their other neighbours in features such as the shape and number of nucleolar particles, the number of actin bundles, the position of condensed chromosomes in female cells and the timing of mitosis. Although the patterns of arrangement of structures may be similar in twinned nuclei they differ in their orientation. We have now found that twins also differ in the orientation of their division planes with respect to the previous plane of division. Similarity of structural pattern but not orientation is most easily explained if nuclei, together with determinants for the plane of division, are free to rotate in the plane of the epithelium.

The timing of division in twin cell doublets

The fifth stage larval epidermis of Calpodes ethlius (Lepidoptera, Hesperiidae) is a syncytium of doublets where sibling cells are twins connected by residual midbodies between divisions. Twin cells resemble one another more than their other neighbours in such features as the shape and number of nucleolar particles, the number of actin bundles and the position of condensed female chromosomes. We have now found that they also resemble one another in the timing of cell division in preparation for pupation. Twins are more likely to divide together than at random. The paired timing of mitosis is presumed to be a result of the twins sharing a common cytoplasm.

The relative positions of condensed chromosomes are maintained between divisions in the epidermis of Calpodes ethlius

The larval epidermis of Calpodes ethlius (Lepidoptera, Hesperiidae) is a syncytium of doublets where sibling cells are twins that remain connected by residual midbodies between mitoses. Twins resemble one another more than their other neighbours in such structural features as the shape and number of nucleolar particles and the number of actin bundles. We have now found that they also resemble one another in the position of the condensed chromosomes that occur in female cells. Female lepidopteran cells contain one or more particles of condensed chromatin, depending on their ploidy. In the epidermis, nuclei with two condensed chromosomes are found in pairs and are separated by the same distances. However, clones of cells with multiple condensed chromosomes are not all alike, suggesting that chromosomes are repositioned at mitosis. Separation distances between chromosomes remain the same between but not through cell divisions, suggesting that determinants for nuclear structure are conserved through interphase and relaxed at mitosis. Although the condensed chromosomes of sibling nuclei resemble one another in their separation, they differ in their orientation, as would be expected if whole nuclei rotate in the plane of the epithelium.