Preantral follicles are the largest ovarian follicle population and represent an important source of potentially competent oocytes. During the lifespan of the female this large population becomes atretic during their growth. In alpacas, there are few studies that estimate the number of preantral follicles. Therefore, the objective of the present study was to compare the population and morphology of preantral follicles in the ovaries of fetal and adult alpacas. Ovaries from alpacas in fetal (fetus during the last third of gestation, n=5) and adult stage (3–4 years, n=5) were collected at a local slaughterhouse. The whole ovaries were individually fixed overnight at room temperature, and later dehydrated in alcohol, cleared with xylene, and embedded in paraffin. Tissue were sectioned at 7μm with a rotating microtome. Then, sections were processed and stained with periodic acid Schiff and haematoxylin. Preantral follicles were classified for their development stage as primordial, transitional, primary, or secondary, according to the layer number and form of granulosa cells. Estimation of the number of preantral follicles was made by counting all follicles in each histological section. Only follicles in which the oocyte nucleus was visible were counted. In addition, for each follicle category (n=30 per group), oocyte and follicle diameters were measured using Motic Images Plus 2.0 software. The population estimate and follicular diameter were compared using Kruskal–Wallis test with significance set at P ≤ 0.05 using SPSS v.2 2 software (IBM Corp.). A total of 2174 histologic sections were analysed. The results showed a higher (P=0.045) number of preantral follicles (80 516.1±3623.9) for fetal alpacas compared with adult alpacas (67 870.8±2267.4). Also, primordial follicles population (31 543.4±2690) and morphologically normal follicles (98.2%) were higher (P=0.04) in fetus compared with those in the adult stage (2244.7±355.37; 76.35%) respectively. On the contrary, the diameters of primordial, transitional, and primary follicles (45.34±3.76; 52.38±6.22; 59.79±5.22µm) from adult alpaca were greater (P=0.04) than those of fetal preantral follicles (33.305±7.2; 36.715±3; 77.985±15.8µm). In conclusion, the preantral follicle population declines dramatically in adult alpaca and animals of this age show an increased percentage of degenerate primordial follicles.

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