Yutaka Koshida

Fine structure of dicyemid mesozoans, with special reference to cell junctions

The fine structure of the dicyemid mesozoan, Dicyema acuticephalum, from Octopus vulgaris, was studied with special attention to intercellular junctional complexes between various kinds of cells. Two types of intercellular junction, namely, adherens junctions and gap junctions, were found in both vermiform stages and in infusoriform embryos. Adherens junctions were classified into two types. Zonulae adherentes‐like junctions were observed between adjacent peripheral cells at vermiform stages, between adjacent external cells of infusoriform embryos, and between members of groups of internal cells that covered the urn in infusoriform embryos. Maculae adherentes‐like junctions were seen between a peripheral cell and an axial cell at vermiform stages. In infusoriform embryos, these junctions were observed between various types of cells, excluding urn cells. Gap junctions were found between adjacent peripheral cells at vermiform stages, whereas in infusoriform embryos these junctions were located between various types of cells excluding urn cells. Dicyemids might be the most primitive multicellular animals to possess these basic types of cell junctions. Ciliary rootlet systems at vermiform stages and in infusoriform embryos were unique in structure compared with those of other primitive multicellular animals. J Morphol 231:297–305, 1997.

Development of the Infusoriform Embryo of Dicyema japonicum (Mesozoa: Dicyemidae)

The cleavage pattern and cell lineage of the infusoriform embryo of the dicyemid mesozoan Dicyema japonicum were studied in fixed material with the aid of a light microscope. The early cleavages are holoblastic and spiral. At the 16-cell stage, the animal pole consists of four mesomeres, the equatorial region consists of four macromeres with four alternating sub-macromeres, and the vegetal pole is composed of four micromeres. At around the 20- to 24-cell stage, cleavage becomes asynchronous and its pattern changes from spiral to bilateral. The four micromeres, namely, the presumptive germinal cells, do not divide further and are finally incorporated into the cytoplasm of four urn cells, which are generated after divisions of the sub-macromeres. The blastomeres situated in the animal hemisphere give rise to ciliated cells that cover the posterior part of the embryo. Two blastomeres (2a2 and 2d2) undergo extremely unequal divisions and the much smaller sister blastomeres degenerate and ultimately disappear during embryogenesis. The fully formed embryo consists of 37 cells. These cells are produced after only four to eight rounds of cell division. The cell lineage appears to be invariant among embryos, apart from the derivation of the lateral cells.

Differentiation of lens and neural cells in chicken embryo is accompanied by simultaneous decay of DNA replication machinery

DNA polymerase alpha was detected in cells of developing chicken embryos by an immunofluorescent method using a monoclonal antibody specific for the high molecular weight polypeptide of chicken DNA polymerase alpha, and DNA polymerase beta was detected using a rabbit anti-chicken DNA polymerase beta antibody. In lens tissue of the 3- to 4-day chicken embryo, fluorescence with anti-DNA polymerase alpha antibody was detected in nuclei of lens epithelial cells but not in nuclei of lens fiber cells which had differentiated from epithelial cells. The localization of cells containing DNA polymerase alpha coincided with the distribution of cells capable of DNA replication as detected by [3H]thymidine autoradiography. Similar results were obtained during the differentiation of neural matrix cells to neuroblasts in the developing neural tube. In contrast to DNA polymerase alpha, DNA polymerase beta was detected in nuclei of both undifferentiated and differentiated cells of these tissues. Since the disappearance of DNA polymerase alpha was very rapid after the onset of differentiation, the DNA replication machinery in which DNA polymerase alpha plays a central role is thought to decay almost simultaneously with the onset of cellular differentiation in these tissues.

Polyclad turbellarians collected on the Osaka University Expedition to Viti Levu, Fiji, in 1985, with remarks on distribution and phylogeny of the genus Discoplana

We report the first records from Viti Levu, Fiji, for four species of polyclad turbellarians: Discoplana gigas (Schmarda), Paraplanocera oligoglena (Schmarda), Cestoplana cuneata Sopott-Ehlers et Schmidt, and Pericelis byerleyana (Collingwood). D. gigas has the widest distribution in the Indo-West Pacific among the six described species of Discoplana, and shows a wider range of color variation than has been attributed to it before. Analysis of morphological features in these species reveals that Discoplana can be divided hierarchically into several sister-species groups based upon differences in structure of the vagina and of the penis. Four species, occurring mainly in the Pacific, share an apomorphic feature of the vagina, a feature not seen in the other two from the Indian Ocean. This suggests that Discoplana originated in the Indo-West Pacific.