An ultrastructural study of the ovary cord organization and oogenesis in Erpobdella johanssoni (Annelida, Clitellata: Hirudinida).

The aims of the present study were to analyze the ovary cord structure and oogenesis in Erpobdella johanssoni under light, fluorescent and transmission electron microscopy and to compare the obtained results with other clitellate annelids, especially with other arhynchobdellid leeches. Each of the paired ovaries is composed of the ovary wall (ovisac) and several (7-8) short, cone-shaped ovary cords. The ovary cords are of the "Erpobdella" type, i.e. they are short and polarized and five zones containing germ cells at consecutives stages of their development can be distinguished along their long axis. One, huge somatic cell (the apical cell), oogonia and premeiotic germ cells occur at the tip of the apical part of the ovary cord - zone I. Below, in zone II germ cells enter meiosis, whereas in zone III only a few cells continue meiosis and gather nutrients (oocytes), while the rest become nurse cells. In zone IV, huge vitellogenic oocytes form protuberances on the surface of the cord, and degenerating germ cells were observed at the base of the ovary cord (zones IV and V). The germline cells form syncytial cysts in zones I-III. The germline cysts have broadly the same architecture as in the ovaries of all of the clitellate annelids that have been described to date. Each germ cell in a cyst has only one cytoplasmic bridge connecting it to the common cytoplasmic mass - the cytophore. The cytophore is poorly developed, and it has the form of thin, long cytoplasmic strands. The presence of two categories of germ cells suggests a meroistic mode of oogenesis. The germline cysts are closely associated with somatic, follicular cells. There are two subpopulations of follicular cells: one envelops the growing oocytes, while the second is distributed between other germ cells. The entire ovary cord is additionally enveloped by a layer of somatic cells with a spongy appearance - the spongiosa cells. A characteristic feature of vitellogenic oocytes is the condensation of the chromosomes into a karyosome. Fully grown oocytes are excluded from the ovary cords and float freely in the ovisac lumen.