Characterization of the ovarian preantral follicle populations and its correlation with age and nutritional status in Brazilian Northeastern donkeys (Equus assinus)

This study aimed to characterize the ovarian follicular population and to determine its correlations with the age and nutritional status of donkeys of the Northeastern Brazilian breed. A total of 10 females with a mean age of 5.1 ± 3.2 years were submitted to ovariectomy by videolaparoscopy to obtain the ovaries. In the laboratory, the ovaries were sectioned into 6-12 fragments of approximately 7 mm in diameter, which were fixed in Carnoy, dehydrated in increasing concentrations of alcohol (85%, 95% and absolute), clarified using xylol and embedded in blocks of histological paraffin. The blocks were cut in sections of 7 μm and each 120th section was mounted on a slide for observation using optical microscopy. The follicle counting and identification allowed the characterization of the population of the preantral follicles. A total of 21.135 ± 10.821 preantral follicles was counted, of which, 91.3% were primordial, 8.3% were primary follicles and 0.4% were secondary follicles. There were no differences between the two ovaries of each animal regarding the follicular population (P > 0.05). There was a rate of 9.77% degenerated follicles. Values of 0.99% follicles containing two oocytes were also identified and classified as multi-oocyte follicles, always of the primordial category. The thickness of the granulosa cell layer was 1.85 μm ± 1.39, 3.56 μm ± 2.08 and 21.85 μm ± 17.27, for primordial, primary and secondary follicles, respectively. There was a significant inverse correlation (r = −0.66; P < 0.001) between the age of the animals and the population of ovarian follicles. A negative influence of the weight and body score on the ovarian follicular population was also observed, when donkeys had very little or a great amount of body condition. This is the first study to describe the morphometric characteristics and estimation of the population of preantral ovarian follicles in Northeastern Brazilian donkey, showing that number of preantral follicles decreased with increasing age of the animals and this finding may be affected by nutritional status of the animals.