Androgens are considered the main hormones that regulate spermatogenesis, but a lot of evidence confers to estrogen a key role in this process. In the testis of the seasonal breeder lizard Podarcis sicula we analyzed by means of in situ hybridization the expression of the androgen receptor (AR) and of the two types, α and β, of the estrogen receptors (ERs) throughout the three periods of the annual cycle: mating (spring-early summer), postmating-refractory (late summer), and autumnal recrudescence. The results show that during the mating period AR and ERs are expressed in all germ cells present in the tubules from spermatogonia to spermatozoa. During the postmating-refractory period, when only spermatogonia are present in the tubules, almost all express ERs and very few AR mRNA. During the autumnal recrudescence the localization of AR and ERs is the same of the mating period except for the population of the primary spermatocytes. The expression of the investigated receptors is peculiar in these cells that are positive in the middle-late pachytene stage of the meiotic prophase and negative in preleptotene. A possible functional role of the observed differences during spermatogenesis and in the spermatozoa is also discussed. 1. Introduction The androgen receptor (AR) and the two types of the estrogen receptors (ERα and ERβ) are members of the large super-family of ligand-activated hormone receptors and act as ligand-inducible transcription factors [1, 2]. Even though the expression of these three types of steroid receptors is reported in the testis of many mammalian species [3–9] their role is not well defined in male reproductive system and even if the male tract is an example where these receptors are expressed [6, 10] the reports about their localization are controversial [11]. More than 20 years ago in mammals, it was thought that AR was not expressed in male germ cells [12, 13], but later some evidence showed the expression of AR in spermatogonia, spermatocytes, elongated spermatids, and spermatozoa [14–16]. Regarding ERs, in all mouse germ cells only ERβ is detected [6, 17]; in the human some authors reported a complete absence of ERα [18, 19] while some others suggested its expression in primary spermatocytes and spermatids and in the ejaculated sperm [4, 20–22]. Among lower vertebrates, in the amphibians that show a cystic structure of the testis, the estrogen guarantees in Rana esculenta the progression of spermatogenesis and release of spermatozoa [23–25] and in Triturus marmoratus immunoreactive AR, ERα, and ERβ are reported in the germ cells
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