Worm Breeder's Gazette 12(3): 111 (June 15, 1992)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Little is known about the mechanisms responsible for cell fusions in biological systems. Cell fusion is an universal process involved in sperm-egg fusion during fertilization, syncytia formation in myotubes, placenta and osteoclasts, conjugation in yeast, and entry of enveloped viruses. C. elegans is a good model system to study cell fusion in situ. Multiple cell fusions occur during different stages of development in some pharyngeal muscles, during the organogenesis of the vulva and uterus, and in the hypodermis. Fused cells were originally observed in C. elegans two decades ago using serial section electron microscopy. For example, hyp7 ,is formed in the embryo by the fusion of 23 cells. During postembryonic development 110 cells fuse with hyp7 forming the largest syncytium surrounding the adult body. Hypodermal cells other than hyp7 also fuse forming cylindrical syncytia linked by cell junctions called belt desmosomes, adherent junctions or zonula adherens (ZA). More than 10% of the somatic cells fuse to form syncytia. We are studying the process of cell fusion during development to try to understand the mechanism of fusion between cells as well as to address the role of fusion during morphogenesis and organogenesis. We are using three approaches: First, characterization of the sequence of events in which cells fuse during embryonic and postembryonic development. Second, screening for mutations specific to cell fusion. Third, reverse genetics to look for C. elegans genes homologous to fusion proteins in other species.
To define spatial and temporal parameters of cell fusion in C. elegans we studied the order of events in which epithelial cells fuse. Since Nomarski optics do not reliably resolve cell boundaries, it is necessary to stain cell junctions to study the behavior of cells in the epidermis. To study cell boundaries during migrations and fusions we used the MH27 monoclonal antibody [kindly provided by R. Francis and R. Waterston; Wash. U.] that stains the ZA of epithelial cells. Thus, we were able to visualize dynamic changes in cell boundaries taking place during embryonic and postembryonic development. Using confocal microscopy and 3D reconstructions we have followed the sequence of fusions and found that they start after migration and interdigitation of dorsal epithelial cells. The first fusion after fertilization occurs in the embryo before elongation. Two dorsal epidermal cells in the anterior part of the pre-comma stage embryo fuse Epidermal fusions start in the anterior dorsal hypodermis and continue posteriorly The fusions are ordered but not completely invariant. At the end of embryogenesis 23 cells have fused including 6 ventral cells that form hyp7 .Using the same methodology we analyzed the timing and location of postembryonic fusions in the hypodermis as well as fusions involved in organogenesis of the vulva and uterus. These studies complement the observations of the fusions obtained by electron microscopy and show intermediates in the sequence of fusions during development.
A complete description of cell-to-cell fusions in C. elegans will allow us to understand the mechanics involved in numerous morphogenetic processes. We are also screening for mutations specific to cell fusion using an immunofluorescence screen. Finally, we found a putative worm homologue to a recently sequenced sperm-egg fusion protein from guinea pig (Blobel, C.P. et al. Nature 356, 248-252 (1992)). The identification of mutants affecting cell fusion will permit cellular and molecular analyses of this elusive and universal process.