Immunolocalization of apelin receptor (APJ) in mouse seminiferous epithelium.

The apelin receptor (APJ) belongs to the member of the G protein‐coupled receptor family, and expression of APJ has been reported in the different cell types of testis. The seminiferous tubules in the testis can be identified as different stages (I–XII). It has been also suggested that different factors could be expressed in stage and cell‐specific manner in the seminiferous tubules. Recently, we also shown that expression of APJ is developmentally regulated in the testis from PND1 to PND42. Therefore, we analyzed the expression of APJ in the testis of adult mice by immunohistochemistry. Immunohistochemistry showed that the APJ was highly specific for the round and elongated spermatids with stage‐dependent changes. The seminiferous tubules at stages I–VII showed APJ immunostaining in the spermatid steps 1–8, not steps of 13–16. The seminiferous tubules at stages IX–XII showed APJ immunostaining in the spermatid steps 9–12. These results suggested the possible role of APJ in the spermiogenesis process. The intratesticular administration of APJ antagonist, ML221 showed a few round spermatids in the seminiferous tubules and some of the tubules with complete absence of round spermatid. Overall, we present evidence that APJ expression in spermatid is dependent on the stages of the seminiferous epithelium cycle and APJ could be involved in the differentiation of round spermatid to elongated spermatid. Research Highlights: The expression of APJ is stage‐ and cell‐specific.The apelin signaling is required for elongation of spermatid and might have a crucial role in spermiogenesis.The intratesticular inhibition of ML221 study suggests that apelin signaling plays important role in spermatid differentiation. [ABSTRACT FROM AUTHOR]