Actin-dependent cytoplasmic streaming in C. elegans oogenesis.

Oocytes in the C. elegans gonad enlarge rapidly. During the stage of enlargement, they are transcriptionally quiescent, and it is not understood how they acquire large quantities of materials such as mRNA and protein. Enlarging oocytes are connected via cytoplasmic bridges to a large, younger population of transcriptionally active germ cells at various stages of mitosis and meiosis. We show here that there is a general streaming of gonad cytoplasm towards and into the enlarging oocytes, originating primarily from pachytene-stage germ cells. Because previous studies suggested that most or all of the pachytene germ cells have the potential to differentiate into oocytes, the pachytene cells appear to function transiently as nurse cells. Somatic gonadal cells that surround the germ cells do not appear essential for streaming. Instead, materials appear to be pulled into oocytes by forces generated either in, or adjacent to, the enlarging oocytes themselves. Streaming appears to be driven by the actomyosin cytoskeleton, although we show that populations of both microfilaments and microtubules are oriented in the direction of flow. Our study shows that oocyte enlargement in C. elegans differs significantly from that in Drosophila, where a small number of specialized nurse cells expel their contents into the enlarging oocyte.