The structural morphology and epithelial association of spermatozoa at the uterotubal junction: a descriptive study of equine spermatozoa in situ using scanning electron microscopy

Uterotubal junction tissues (n=12) recovered from young nulliparous mares 4 h after insemination were examined for spermatozoa in situ using scanning electron microscopy. Mares were inseminated before ovulation with semen containing 66-85% morphologically abnormal spermatozoa and approximately 1 x 10(9) progressively motile spermatozoa. Spermatozoa were found in all the tissues and displayed a variety of relationships to the epithelium, depending on regional location, spatial constraints and epithelial surface characteristics. Spermatozoa were most abundant in the epithelial folds on the uterine side of the uterotubal junction papilla. More than 90% of the spermatozoa had normal morphology. These observations indicate that morphologically normal equine spermatozoa develop specific associations with the luminal epithelium of the reproductive tract during transport to the oviduct. Furthermore, the uterine papilla of the uterotubal junction may be a site of preovulatory sperm storage. The scarcity of morphologically abnormal spermatozoa, despite their abundance in the inseminate, indicates that most abnormal spermatozoa either do not reach the uterotubal junction or do not develop normal associations with the epithelium of the uterotubal junction. This observation provides evidence for a mechanism of sperm selection during transport. Hence, both normal morphology and progressive motility of spermatozoa should be considered in determining insemination doses, particularly for subfertile stallions.