The bat Corynorhinus mexicanus provides an interesting experimental model for the study of epididymal sperm maturation because after spermatogenesis and the regression of the testes, this bat stores sperm in the epididymal cauda for several months. Earlier research conducted by our group suggested that sperm maturation in this species must be completed in the caudal region of the epididymis. One of the major signal transduction events during sperm maturation is the tyrosine phosphorylation of sperm proteins. The aim of the present study was to comparatively evaluate tyrosine phosphorylation in spermatozoa obtained from the caput, corpus and cauda of the epididymis during the sperm storage period. The maturation status of the sperm was determined by the percentage of capacitation and tyrosine phosphorylation in sperm obtained from the epididymis. The highest proportion of tyrosine phosphorylation was registered after the sperm had reached the cauda epididymis during the middle of the storage period. In conclusion, in Corynorhinus mexicanus and most likely in other chiropteran species with an asynchronous male reproductive pattern, epididymal sperm maturation ends in the caudal region of the epididymis and is related to the time that the sperm remains in the epididymis before mating activity.

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Epididymal sperm maturation is the process through which sperm acquire the potential to fertilize the egg. For most mammals, studies report that this process is completed before the sperm enter the cauda region of the epididymis; however, in some bat species, such as the Mexican big-eared bat (Corynorhinus mexicanus), this does not occur, as the process does not end until the sperm are inside the cauda of the epididymis, and thus is associated with a long storage period.

The epididymis is an organ that performs all the biochemical changes responsible for sperm maturation. During ageing, histological alterations in the epididymis and decreased protein synthesis have been found. This might affect the sperm maturation process. The aim of this study was to determine if the changes in the epididymis during ageing might cause alterations in sperm maturation. Wistar rats of 3–4months old (young) and 18–21months old (old) were used. The testosterone concentration was determined and the epididymides were dissected and divided in three regions: caput, corpus, and cauda. The tissues were used for histological processing and sperm extraction. Testosterone concentration decreased 34% in the old animals compared to the young ones. The distribution of mannose, sialic acid, and N-acetylglucosamine in the glycocalyx of the sperm membrane of old animals was different from that of young animals. The same occurred with phosphatidylserine externalisation and protein phosphorylation at tyrosine residues. Epididymis histology in old animals showed tubular and cellular degeneration. Our results suggest that ageing affects maturational markers, likely due to alterations in the epididymis as a result of the testosterone decrease associated with ageing.

Mammalian sperm capacitation includes biochemical and physiological changes, such as (Ca2+)i increase, hyperpolarization of the plasma membrane potential, and sperm hyperactivation, among others. These changes provide the sperm the ability to fertilize. In the bat Corynorhinus mexicanus, there is an asynchrony between spermatogenesis and sperm storage in males, with the receptivity of the female. For instance, in C. mexicanus spermatogenesis occurs before the reproductive season. During the reproductive period, the sperm are stored in the epididymis for few months and the testis undergoes a regression, indicating low or almost null sperm production. Therefore, it is unclear if the required elements necessary to sperm fertilization success undergoes maturation or preparation during epididymis storage. Here, we characterized a pH-sensitive motility hyperactivation and a Ca2+ influx in sperm which was regulated by alkalinization and progesterone. In addition, by electrophysiological recordings we registered currents that were stimulated by alkalinization and inhibited by RU1968 (a specific CatSper inhibitor), strongly suggesting that these currents were evoked via CatSper, a sperm Ca2+ specific channel indispensable for mammalian fertilization. We also found a hyperpolarization of the membrane potential, such as in other mammalian species, that increased according to the month of capture, reaching the biggest hyperpolarization during the mating season. In conclusion, our results suggest that C. mexicanus sperm have a functional CatSper and undergoes a capacitation-like process such as other mammals, particularly a Ca2+ influx and a membrane potential hyperpolarization.