A dogma of mammalian reproduction states that primordial germ cells in females are restricted to the intrauterine phase and only small portions of oocytes are available for ovulation during the adult life. Among the rare exceptions to this rule is the plains viscacha. It polyovulates up to 800 oocytes per cycle, from which 10 to 12 are implanted, but only 1 to 2 conceptuses survive. To better understand the main mechanisms involved in these patterns of super-ovulation, super-implantation, and embryonic loss in the viscacha, we conducted an analysis of the ovary and uterus of pregnant females and their conceptuses. Pregnant females (n = 16) of ~50 and 90 days of gestation (early to mid-gestation) were selected for conceptus recovery. Hemi-ovariohysterectomy was performed following surgical and anaesthetic protocols used for laboratory animals and the conceptuses collected. Female fetuses of Day 50 (n = 2) and 90 (n = 1) of gestation were obtained from Estación de Cría de Animales Silvestres, Buenos Aires, Argentina. The reproductive organs were investigated by means of gross morphology, histology (hematoxilin and eosin), stereology (quantification of the volume of the ovary and number of ovary follicles), immunohistochemistry (PCNA, Oct-4, VEGF, and Caspase-3), and transmission electron microscopy. In the Day 50 fetal samples, the ovaries had an ovoid shape with smooth surface without apparent folds. First steps of subdivision were observed in the ovary of fetus of 90 days. The total volume of all fetal ovaries was of 4.8 mm2 and a coefficient of variation (CV) of 0.32. The ovaries of adult individuals had remarkable invaginations with surface projections and were small, asymmetrical, and dorsoventrally flattened with a mean of 77.6 mm3 (CV = 0.47). Only adult females had differentiation of germ cells. Primordial follicles had a mean of 9.9 × 105 (CV = 0.19), representing 93% of the total number of ovarian follicles. The mean of primary follicles was 3.05 × 104 (CV = 0.36), whereas for secondary follicles it was 2.75 × 104 (CV = 0.50), each representing 3% of all ovarian follicles. The number of antral follicles in several stages of development was 8.64 × 103 (CV = 0.75), representing 1% of the follicles. Primordial follicles expressed pluripotency (Oct-4+) and proliferation (PCNA+) markers, as well as the primary follicles. The cells did not react for Caspase-3 as marker for apoptosis. Variations regarding to the vascularization of the different regions of the uterine horn were observed, which were more intense and efficient near to the cervix. Data showed that a specialised, highly convoluted structure of the ovarian cortex developed in the intrauterine phase as a prerequisite for massive super-ovulation, associated with the inhibition of apoptosis and continued proliferation of germ cells, as well as maintenance of several corpora lutea during the adult life. Thus, a highly complex pattern of polyovulation, polyimplantation, and controlling mechanisms has evolved in the female reproductive system of the viscacha that mainly was associated with the maternal side. After an in-depth analysis of the arterial and venous vascularization of the uterine horns and uteri, we speculate that specializations regarding the vasculature and musculature evolved first and then contributed as a compensatory or controlling mechanism for polyovulation and polyimplantation. In conclusion, polyovulation in the viscacha represents a unique enigma in reproductive biology.
Mammalian foetal ovarian development: consequences for health and disease