Equine Endometrial Development: Setting the Stage for Reproductive Success

Abstract

Uterine development in the foal is initiated prenatally; endometrial adenogenesis, or gland development, is initiated in the fetus and completed postnatally. To date, there is little data describing this process in the horse. The objective of this study was to characterize uterine development in normal foals and to provide a systematic assessment of equine uterine endometrial development during fetal and neonatal periods. Uterine tissue was acquired from a total of 38 foals of varying ages, ranging from gestational day (GD) 300 to postnatal day (PND) 180. Twenty-eight of the 38 original tissue samples were snap-frozen in optimum cutting temperature (OCT) gel and sectioned at 6µm. Tissue sections were processed for immunofluorescent analysis of estrogen receptor, glucocorticoid receptor, and Ki-67 in situ. Additional tissue samples were stored in Universal Molecular Fixative (UM) for immunohistochemical evaluation of progesterone receptor expression, and for assessment of endometrial histogenesis. Histological evaluation revealed that nascent endometrial glands were present in all perinatal samples, consistent with previous descriptions. Labeling index (LI), indicative of cell proliferation, was calculated as a percentage of cells expressing Ki67 for each endometrial cell compartment including luminal epithelium (LE), glandular epithelium (GE) and stroma (ST). Overall, main effects of age (P= 0.005), cell compartment (GE, LE, ST; P < 0.0001) and an age by cell compartment interaction (P < 0.0001) were identified for Ki67 labeling index. Consistent with observations based on results of immunohistochemistry (IHC) studies, Ki67 LI increased (P < 0.0001) from the prenatal to the postnatal period, and was higher overall (P < 0.0001) in epithelium (GE and LE) versus ST. Postnatally, Ki67 LI increased (P < 0.0001) from week 1 to week 24. Progesterone receptor (PR) was observed in LE, GE, and ST at all ages. Overall, a main effect of cell compartment (GE, LE; P < 0.0001) and an age by cell compartment interaction (P =0.01) were identified for PR labeling index. Consistent with observations based on results of IHC studies, PR LI was higher in GE than LE for all age groups. Results indicate that during late fetal and early neonatal life, equine endometrium demonstrated a low degree of proliferation, as defined by a low Ki67 LI. This is in contrast to other species where proliferation is rapid in early neonatal life. This may indicate that endometrial development in the horse is a less organized event during the time periods evaluated relative to other species. Results also demonstrate that multi-spectral imaging is a valuable analytical methodology that can be used in equine tissues.