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Studies have shown that the transplantation of stem Leydig cells (SLCs) in Leydig cell-disrupted or aging models can help restore testosterone production, thereby accelerating meiosis and germ cell recovery 18,19. The proper development of Leydig cells during puberty is essential for initiating spermatogenesis and promoting secondary sexual characteristics in males . Spermatogenesis is a complex process that involves the development of male germ cells, known as spermatogonia, into fully mature spermatozoa, or sperm. Proper hormone regulation is essential for maintaining male reproductive health, which includes sexual maturation, germ cell production, and steroidogenesis. Also, the process of differentiation from SSCs to the eventual release of sperm is guaranteed by AR signaling. The classic pathways guarantee Sertoli cell maturation, BTB integrity, SSC differentiation, and spermatocyte meiosis. This pathway predominantly functions by promoting the phosphorylation and activation of ERK1/2 and CREB . Since STAC3 plays a key role in calcium signaling, it may act as an important factor in the calcium signaling pathway regulating testosterone synthesis. In addition, calcium regulates steroidogenesis by activating the early orphan nuclear receptor NUR77 via calmodulin-dependent protein kinase (CaMK), which in turn promotes the expression of StAR 114,115. It has recently been shown that ERK5 knockdown decreases steroidogenesis and decreases the expression of StAR and Nr4a1 (nuclear receptor subfamily 4, group A, member 1, also known as NUR77/NGFI-B) in human chorionic gonadotrophin (HCG)-treated MA-10 LCs, through phosphorylation of MEF2 . This also demonstrated the regulatory role of ERK signaling in testosterone secretion in LCs. In depth studies, pertaining to the role of androgen-binding protein(s) in sequestration, retention and bioavailability of T/DHT are required to understand male fertility regulation. Although, recent in vivo studies explained the involvement of FSH, androgen, estrogen and IGF-1R to be essential for Sertoli cell development, still the complete scenario of this complex process is unresolved. It has been described that thyroid hormones regulate lactate production, glucose transporter type 1 mRNA levels, aromatase activity, Sertoli cell proliferation and other processes of Sertoli cells in various mammalian species (128–130). For example, interleukin-1, 6 (IL-1 and IL-6) and tumor necrosis factor α (TNF-α) are produced by Sertoli cells and in vitro studies demonstrated that all of these cytokines are involved in Sertoli cell metabolism by activating the production of transferrin. Various studies reported that inflammatory cytokines are not only produced by macrophages in response to inflammatory signals but these cytokines are also secreted from Sertoli cells and appear to take part in the regulation of Sertoli cell proliferation (1, 19). Different studies have reported the function of insulin in testicular development, in modulating testicular cell function (38, 124, 125), or even influencing HPG axis function (28, 29). When testosterone enters the seminiferous tubules, it binds to androgen receptor (AR) proteins located in peripheral vascular cells such vascular endothelial cells and smooth muscle cells, as well as support cells like Sertoli cells and peritubular myoid cells 54,55. Finally, ILCs differentiate into ALCs, intracellular lipid droplets disappear, and the levels of testosterone production and the number of LHR receptors reach their maximum . Androgen receptors are present on T-cells (Benten et al. 1999), but their numbers might be dynamically regulated in response to baseline androgen levels and other feedback mechanisms.
