Oogenesis in the Siberian salamander, Salamandrella keyserlingii (Amphibia: Caudata, Hynobiidae)

Previous studies have described the possibility of biennial reproductive cycle in females of Salamandrella keyserlingii. To verify this statement, the detailed observation the females captured throughout the year or more and description of oogenic stages are necessary. In this study, we described all stages of oogenesis from oogonia to mature oocytes, also postovulatory and atretic follicles in S. keyserlingii. We studied females with a various reproductive system state captured in May-August 2005, 2006, and 2013 in the suburbs of Tomsk (the southeast of Western Siberia, Russia). We conducted histological and histochemical examinations of the ovaries using the Mayer's hematoxylin-eosin, modified azan, aclian blue (pH = 0.5 and 2.5), and PAS staining methods. We divided the oogenesis into 6 stages. Oogonia (stage 1) are the smallest of oogenic cells (Mean diameter: Mean, Range; 20.9 ^m, 14.2-26.4, n = 10), located singly or in groups in the ovarian wall. The stage 2 is previtellogenic oocytes (230.8 ^m, 93.7-372.9, n = 10). These cells did not have the yolk and zona pellucida. At stage 2, some regions of ooplasm are stained with alcian blue and PAS positive. At stage 3 (494.7 ^m, 389.4-598.6, n = 10), there are yolk granules in the peripheral parts of oocytes. The zona pellucida is visible and PAS positive. Females before spawning (gravid), during ovulation, and just after breeding had early vitellogenic follicles with dispersal distribution of yolk granules in the ooplasm. At stage 4 (634.5 |m, 527.8-711.7, n = 10), oocytes have two parts in the ooplasm: large peripheral region rich yolk granules and central region without them and contained the nucleus. Oocytes of stage 5 (875.3 |im, 647.6-1163.4, n = 10) are rich yolk granules. In the submembranous zone of these cells, numerous small melanin granules are present. Stage 6 is mature oocytes (1534.3 |m, 1258.6-1696.5, n = 10). These cells were mesolecithal and telolecithal. Early postovulatory follicles are vascularized, have a thick wall. Inside of these follicles, there are follicle cells and numerous macrophages. Late postovulatory follicles are smaller. Follicle cells inside of them have several signs of degeneration (e.g., karyorhexis, hyperchromia). In several females after spawning and during maturation of the ovary, we detected atretic follicles. During atresia, follicle cells entered into the damaged oocyte. Presence of yolk granules in several atretic follicles is indicated, that vitellogenic follicles were involved into the process. The late atretic follicles contain the pigment or follicular cells having adipocyte-like morphology.

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