Analysis of the equine ovarian structure during the first twelve months of life by three-dimensional internal structure microscopy

A three-dimensional internal structure microscopy (3D-ISM) can clarify the anatomical arrangement of internal structures of equine ovaries. In this study, morphological changes of the equine ovary over the first 12 months of life were investigated by 3D-ISM in 59 fillies and by histological analysis in 2 fillies. The weight and volume of the paired ovaries initially decreased from 0 to 1 months to 2 to 3 months of age and then significantly increased at 8 to 12 months of age. The ovulation fossa was first observed around the 3rd month and became evident after the 6th month. The number of follicles with a diameter of ≥10 mm and the diameter of the largest follicle increased gradually after 6 months of age. On a volume basis, the medulla accounted for nearly 90% of the whole ovary at 0 to 1 months of age, but significantly decreased from 2 to 3 months of age. The volume of the cortex increased progressively after birth and reached approximately 60% of the total volume at 8 to 12 months of age. This significant development of the cortex coincided with the increased number and size of large follicles observed from 6 months of age. These results suggest that the development of the cortex plays a role in the maturation of the follicles and the equine ovary undergoes substantial morphological changes postnatally until puberty.