Folliculogenesis in the Argentine brown bat Eptesicus furinalis (Vespertilionidae: Chiroptera)

Bats belong to the second largest mammalian Order and exhibit a great variety of reproductive strategies. Although tropical regions have the greater variety of bats, the majority studies of reproductive aspects in bats are done just with temperate species of bats. Eptesicus furinalis is a neotropical bat from the Vespertilionidae family that shows a great distribution, and the males are known to have a process of testicular regression, but no studies were previously done with the females. Our aims were to analyze the ovarian morphology and the folliculogenesis process in this species, to increase the knowledge about the reproductive diversity of tropical bats. Sexually mature females of E. furinalis were collected in northwestern Sao Paulo State – Brazil (20°49’11’’ S, 49°22’45’’W), and the ovaries were carefully removed and prepared for histological analysis. The diameter (µm) of each ovary, follicles (without theca layers and just the sections that showed the oocyte nucleus), and corpus luteum were measured - the diameter was calculated by the formula: (Dµm) = √ab, where ‘a’ is the measure of the ovary/follicles/corpus luteum diameter, and ‘b’ is the diameter in 90o angle. The ovary (~878,8 µm) is round to oval, it is divided into cortex and medulla, shows an abundance of stromal tissue, and there was no difference in size between right and left ovaries. Five follicle stages were observed in the ovaries: 1. primordial (~15,6 µm) - surrounded by a single layer of flattened granulosa cells, 2. primary (~62,4 µm) - the single layer of granulosa cells becomes now cubic cells; 3. secondary (~101,3 µm) - two or more layers of granulosa cells surrounding the oocyte; 4. tertiary (~180,1 µm) - the granulosa cells start to secret the antrum fluid, which accumulate between the cells of the granulosa layer, forming small cavities; 5. antral follicle (~214,1 µm) - the cavities start to increase and merge into each other, until it became a major cavity, known as antrum. We observed more than one dominant follicle in the ovaries and two corpora lutea in the same ovary of some females. Besides, atresia was present in all follicle stages, except in the primordial follicles, and we recognized two types of follicular atresia: Type I – degeneration and fragmentation of the oocyte before starts atresia in the granulosa layers; type II – degeneration of the granulosa cells before starts atresia in the oocyte. The process of folliculogenesis and the morphology of the ovaries in Eptesicus furinalis are similar to the mammal general pattern. Furthermore, E. furinalis presents the ovary morphology similar to other species from the same family: bilateral symmetry of both ovaries and abundance of the interstitial tissue, they present more than one dominant follicle and can ovulates more than one ovum. Polytocous species of bats are generally related with bilaterally symmetry of the female reproductive organs, i.e., both ovaries presenting the same size. Although the folliculogenesis process is well preserved between mammals, bats show different specializations between different taxa, and here we described, for the first time, the folliculogenesis process in this neotropical species.