Development of equine endometrial glands from fetal life to ovarian cyclicity

Branched tubular endometrial glands are key features of the uterine lining in mares. The epithelium in the neck regions of the glands is similar to that lining the lumen of the uterus, whereas the epithelium in the deeper, coiled portions of the glands secretes protein rich histotrophe or 'uterine milk' during dioestrus and pregnancy. The present study was performed to characterize endometrial histology in perinatal and peripubertal fillies and to investigate the influences of specific steroid treatments on endometrial gland development in the prepubertal period. Uteri obtained from equine fetuses (n=5) between days 150 and 320 of gestation, and from foals (n=5) during the first 2 months of life, were compared histologically with endometrial biopsies taken from yearling fillies (n = 8) around the onset of ovarian cyclicity. In addition, biopsies were obtained from two prepubertal fillies that were treated for sequential periods with injections of progesterone or oestradiol benzoate each day to mimic the peripheral hormone changes associated with puberty. At day 250 of gestation the luminal epithelium was starting to invaginate into the underlying stroma and, by day 300 of gestation, the first cross-sections of very simple tubular gland structures were visible. These tubular gland structures developed into clusters of rudimentary gland cross-sections during the early postnatal period but, thereafter, gland development was arrested until the epithelial invaginations became more obvious during the first oestrus at the onset of puberty. However, final differentiation and maturation of the secretory portions of the glands occurred only after the first ovulation. Furthermore, in prepubertal fillies this maturation process could not be elicited by treatment with progesterone only and an initial period of oestrogen priming was essential to induce full differentiation of the endometrial glands.