The cellular anatomy of embryos of the nematode Caenorhabditis elegans: Analysis and reconstruction of serial section electron micrographs

Abstract

As described from light microscopy, embryogenesis of the free-living soil nematode Caenorhabditis elegans follows a strictly determinate cleavage pattern, producing a newly hatched juvenile with about 550 cells arranged quite predictably. In this communication we present results on the electron microscopy of C. elegans embryos and introduce methods for fixing, embedding, and serially sectioning embryos encased in the egg shell. Fixation at elevated temperature either with osmium tetroxide alone or with glutaraldehyde followed by osmium tetroxide gives reproducible results with embryos in all developmental stages so far tested, from the fertilized egg to hatching. Eighteen wild-type eggs at various stages have been sectioned to date. We have achieved using newly developed procedures for analyzing electron micrographs of serial sections detailed reconstructions of the cellular anatomy of complete embryos of a metazoan organism. Three-dimensional serial section reconstructions were made with a computer system. We characterize and map the 24 cells of an early-stage embryo in this report. Additionally, we can specify the lineage history of all cells of this embyro by matching the reconstructed three-dimensional arrangement of this series to a living embryo at this stage, where cell lineage has been observed with Nomarski optics and analyzed using videotape ( U. Deppe, E. Schierenberg, T. Cole, C. Krieg, D. Schmitt, B. Yoder, and G. von Ehrenstein, 1978, Proc. Nat. Acad. Sci. USA 75, 376–380). In addition, cytoplasmic and nuclear morphological features such as incomplete membranes between sister cells, rounding-off of the cytoplasm, and chromatin condensation patterns have been correlated with cell division. Mapping of such structures presents a new method by which supplementary lineage information can be obtained directly from an electron micrographic series.