This research research was aimed at histologically characterizing the pre-natal ovarian development of cross-breed equine fetuses in the tropics, observing the rise and the quantification of oogonia and follicles. One hundred and thirty one fetuses were obtained from a slaughterhouse, their crown-rump length (CRL) was measured in centimeters (cm). Next, the gonads were sectioned and the samples were placed to rest in Bouin´s liquid for 24 hours. The ages were estimated by regression. 7µm thick histological ovarian cuts were prepared and stained with hematoxylin-eosin. Oogonia, primordial, growing and antral follicles were observed in fetuses whose CRL was between 4.1 and 6.5cm (40 to 49 gestation days (GD)); 9.1 and 12.0cm (60 to 69 GD); 23.1 and 29.0cm (116 to 135 GD); 50.1 and 65.0cm (201 to 245 GD) respectively. The maximum amount of oogonia and of primordial, growing and antral follicles occurred in fetuses whose CRL was between 4.1 and 6.0cm (40 to 49 GD); 16.1 and 23.0cm (90 to 115 GD); 29.1 and 36.0cm (136 to 155 GD); 50.1 and 97.0cm (201 to 320 GD), respectively. These results suggest that the development of cross-breed equine ovarian fetuses in the tropics is more premature than that described in the literature on studies of various equine breeds in temperate regions.
equine; fetuses; ovaries; gonadal development
