Ultrastructure of differentiating oocytes and vitellogenesis in the giant freshwater prawn, Macrobrachium rosenbergii (De Man).

The ultrastructure of oogenesis in Macrobrachium rosenbergii, with reference to vitellogenesis, has not been reported. We used light and electron microscopy, as well as vitellin (Vn) purification and antibody production, to study the temporal and spatial production of Vn in the ovary by immunofluorescence. Histologically, the ovary is subdivided into cone-shaped ovarian pouches with a central core containing layers of oogonia. These divide to produce oocytes that migrate outwardly and differentiate into mature oocytes. During the course of differentiation, oocytes undergo modifications, including the rearrangement of nuclear chromatin, the accumulation of ribosomes, rough endoplasmic reticulum (RER), and lipid, and the formation of secretory and yolk granules, resulting in four stages. Ultrastructurally, early previtellogenic oocytes (Oc(1)) are characterized by the accumulation of new ribosomal aggregates, translocated from the nucleus. Late previtellogenic oocytes (Oc(2)) show nuclear heterochromatin with a "clock face" pattern, the presence of RER, and three types of secretory granules. Follicular cells occupy the intercellular spaces and surround the Oc(2). Early vitellogenic oocytes (Oc(3)) are larger, with nuclei containing predominantly decondensed euchromatin, and cytoplasm with yolk and secretory granules, and few lipid droplets. Late vitellogenic oocytes (Oc(4)) are characterized by completely euchromatic nuclei, an indistinct plasma membrane, yolk platelets and secretory granules, and abundant lipid. Vitellogenin (Vg) in ovaries of M. rosenbergii consist of two main bands at MW 90 and 102 kDa. Our data indicates that Vn is present, and probably synthesized in Oc(3) and Oc(4), but there may be some undetected exogenous Vg production.