Building bridges for oocyte growth: regulation of C. elegans germline architecture and function by oriented cell divisions

In most animals, the growth of oocytes depends on the delivery of cytoplasm from other germ cells ("nurse" cells) via cytoplasmic bridges. In some cases, such as in mice and Drosophila, these bridges are formed via incomplete cytokinesis and connect the germ cells to the oocyte directly. In other animals, like the nematode Caenorhabditis elegans (C. elegans), germ cells are connected to an anucleate core of cytoplasm, termed the rachis, that supplies materials to the oocyte. This difference in germline architecture poses an interesting challenge for tissue development. Whereas in the first case, stabilization of the cytokinetic ring between dividing germ cells produces the final organization, with the total number of cytoplasmic bridges being one fewer than the total number of germ cells; in the second scenario, germ cell division must produce two daughter cells each with their own connection to the rachis, with the total number of cytoplasmic bridges being equal to the number of germ cells. The cellular and molecular mechanisms that enable germ cells to form and maintain this latter type of architecture are incompletely understood but have been under increasing scrutiny over the last years. Here we review the recent progress in understanding C. elegans germline development from a tissue architecture perspective.