128 Study of preantral ovarian follicular population in fetal alpaca (Vicugna pacos) ovaries

Abstract

In mammals, folliculogenesis begins at the fetal stage and is a complex, dynamic process that involves follicular quiescence, activation, growth, follicular migration, and cell interactions. At birth, the preantral ovarian follicular population, drastically reduced, constitutes >90% of all ovarian follicles, representing the ovarian reserve that will be used throughout the reproductive life. In alpacas, changes in follicular wave growth patterns and ovulation are different from other species; thus, it is important to know the ovarian reserve at fetal stage, as a starting point for future studies. Therefore, the aim of the present study was to establish the morphological characterisation and estimate the population of alpaca preantral follicles in the fetal stage. Ovaries from alpacas (n = 5) in the fetal stage (fetus during the last third of gestation) were collected at a local slaughterhouse. Whole ovaries were individually fixed in 4% paraformaldehyde phosphate-buffered saline overnight at room temperature for routine histology. Ovaries were dehydrated in alcohol, cleared with xylene, and embedded in paraffin, and all tissue was serially sectioned at 7 μm with a rotating microtome (Leica). The histological sections were mounted and stained with periodic acid-Schiff and hematoxylin. Preantral follicles were classified according to their developmental stage: primordial (one layer of flattened granulosa cells surrounding the oocyte), primary follicle transition (flattened cuboidal granulosa cells surrounding the oocyte), primary (single layer of cuboidal granulosa cells around the oocyte), or secondary (oocyte surrounded by more than one complete layer of cuboidal granulosa cells). We estimated the number of preantral follicles 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 = 40 per group), oocyte and follicle diameters were measured using an ocular micrometer. The variable means were compared using unpaired Student's t-test analysis, with significance set at P ≤ 0.05. Estimation of preantral follicular population (mean ± standard deviation) was 80 516 ± 14 575 in the ovaries of alpaca fetuses. Most of the follicles found belong to the primordial (49.2%) or primary follicle transition (39.2%) categories, followed by primary (10.8%) and secondary (0.8%) stages. Follicle and oocyte diameters of primordial (33.3 ± 7.2; 21.5 ± 4.6 μm) and primary follicle transition stages (36.7 ± 3.0; 23.4 ± 2.6 μm) were significantly (P < 0.05) smaller than those of primary-stage follicles (77.9 ± 15.8; 50.02 ± 11.1 μm). Finally, preantral follicles classified with normal morphological integrity appearance for each developmental stage were 98.2% (primordial), 96.7% (primary follicle transition), 91.5% (primary), and 88.1% (secondary), respectively. In conclusion, this study shows for the first time an estimation of the population of preantral follicles in alpaca fetal ovaries and establishes follicle and oocyte diameters and their normal morphological integrity.