Genetic and embryological experiments have established the Caenorhabditis elegans adult hermaphrodite gonad as a paradigm for studying the control of germline development and the role of soma-germline interactions. We describe ultrastructural features relating to essential germline events and the soma-germline interactions upon which they depend, as revealed by electron and fluorescence microscopy. Gap junctions were observed between oocytes and proximal gonadal sheath cells that contract to ovulate the oocyte. These gap junctions must be evanescent since individual oocytes lose contact with sheath cells when they are ovulated. In addition, proximal sheath cells are coupled to each other by gap junctions. Within proximal sheath cells, actin/myosin bundles are anchored to the plasma membrane at plaque-like structures we have termed hemi-adherens junctions, which in turn are closely associated with the gonadal basal lamina. Gap junctions and hemi- adherens junctions are likely to function in the coordinated series of contractions required to ovulate the mature oocyte. Proximal sheath cells are fenestrated with multiple small pores forming conduits from the gonadal basal lamina to the surface of the oocyte, passing through the sheath cell. In most instances where pores occur, extracellular yolk particles penetrate the gonadal basal lamina to directly touch the underlying oocytes. Membrane- bounded yolk granules were generally not found in the sheath cytoplasm by either electron microscopy or fluorescence microscopy. Electron microscopic immunocytochemistry was used to confirm and characterize the appearance of yolk protein in cytoplasmic organelles within the oocyte and in free particles in the pseudocoelom. The primary route of yolk transport apparently proceeds from the intestine into the pseudocoelom, then through sheath pores to the surface of the oocyte, where endocytosis occurs. Scanning electron microscopy was used to directly visualize the distal tip cell which extends tentacle-like processes that directly contact distal germ cells. These distal tip cell processes are likely to play a critical role in promoting germline mitosis. Scanning electron microscopy also revealed thin filopodia extending from the distal sheath cells. Distal sheath filopodia were also visualized using a green fluorescent protein reporter gene fusion and confocal microscopy. Distal sheath filopodia may function to stretch the sheath over the distal arm.
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This paper is dedicated to the memory of Dr. Loren H. Hoffman, a wonderful cell biologist, teacher, and friend. We thank Peg MacMorris and Barth Grant for antibodies to yolk proteins. We thank Barth Grant for providing the vit-2::gfp strain and Marie-Christine Paupard and Agness Miller for help with immunoEM. We thank Iva Greenwald and Tim Schedl for providing the lag-2::gfp strain and helpful discussions. Ed Hedgecock, Todd Starich, Jocelyn Shaw, Bill Wadsworth, and Ben Williams generously shared unpublished data. The authors are grateful to Jane Hubbard, Jay Kirchner, David Miller, and Chris Wright for helpful discussions and comments on the manuscript. We thank the two anonymous reviewers for providing helpful comments. This work was supported by grants from the American Cancer Society (DB-149 and JFRA-630) and the National Institutes of Health (GM57173) to D.G. The Center for C. elegans Anatomy is supported by NIH RR12596 to D.H.H. Core facilities for electron microscopy were provided through NIH Center Grant HD05797 to L.H.H. Laser-scanning confocal microscopy was performed at the VUMC Cell Imaging Resource Center (supported by CA68485 and DK20593) with the help of Jonathan Sheehan. Some strains used in this study were provided by the Caenorhabditis Genetics Center which is supported by the National Institutes of Health’s National Center for Research Resources.
- Caenorhabditis elegans
- Cell-cell interaction
- Electron microscopy
- Gap junction
- Germ line