Structure of the Ovary and “Nurse Cells” in a Freshwater Ostracod, Cyprinotus uenoi Brehm (Podocopida: Cypridoidea)

Abstract

The adult female of the freshwater ostracod Cyprinotus uenoi Brehm, 1936 (Podocopida: Cypridoidea) has a pair of long, sac-like ovaries separately lying in the posterior part of the left and the right carapace valves. Oogonia and very early previtellogenic oocytes are located in the terminal germarium of each ovary. In the germarium, the oogonia occur in the most terminal region, and the very early previtellogenic oocytes are located in the remainder, arranged in order of size, the larger ones nearer the ovarian lumen. Most of the growing oocytes, previtellogenic and vitellogenic, are found in the ovarian lumen, the larger ones farther from the germarium. In the germarium, a cytoplasmic bridge connects a pair of adjoining germ cells, resulting from an incomplete cytokinesis of oogonial division. Among the previtellogenic and early vitellogenic oocytes in the ovarian lumen, "nurse cells" are found as small, spherical cells in mostly the same number as these oocytes. A cytoplasmic bridge connects each "nurse cell" to an adjoining oocyte. Based on the manner of connection and some morphological features, we consider that each "nurse cell" originates from one of each pair of adjoining germ cells connected by a cytoplasmic bridge in the germarium, as in the true nurse cells of several branchiopod crustaceans and insects with meroistic ovarioles.