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Receptors in spermatozoa: are they real?

Sperm develop in the seminiferous tubules of testes following a complex developmental process called spermatogenesis. A series of morphologic remodeling occurs during spermiogenesis before the sperm are released into the lumen of the tubules by spermiation (Eddy and O’Brien, 1994). Mammalian testicular spermatozoa are relatively nonmotile and incapable of fertilizing an ovum. In most species, sperm attain progressive motility and fertilizing capacity during their transit through various regions of the epididymis. Changes in the sperm membrane and cytoplasm, by addition and deletion of several proteins and molecules, occur during the epididymal transit (Moore, 1990). During ejaculation, sperm are exposed to various components of the seminal plasma that adhere to the sperm membrane. The ejaculated sperm are incapable of fertilizing an oocyte. They must undergo physiologic changes that are collectively called capacitation, leading to acrosomal exocytosis during the transit through the female genital tract (Yanagimachi, 1994). One of the major steps besides tyrosine phosphorylation during capacitation is the removal of seminal plasma–adsorbed proteins from the sperm surface (Naz and Rajesh, 2004). These changes are a prerequisite for successful fertilization. It is generally believed that these processes are regulated by several hormones and cytokines/growth factors present in seminal plasma and the female genital tract. The receptors for these hormones and cytokines/ growth factors have been reported to be present on the sperm membrane, and the list is ever growing. The sperm are transcriptionally and translationally quiescent, and they can be successfully capacitated in vitro without any hormones and cytokines/growth factors. In lieu of these findings, the physiologic significance of several of these receptors seems enigmatic. The present article will review the receptors for well-known hormones, growth factors, and neurotransmitters that have been reported in the literature, and their authenticity and physiologic relevance will be discussed. The cognate proteins on the sperm surface that have been delineated by monoclonal and polyclonal antibodies (at least more than 100) and have been implicated in various sperm functions, especially oocyte zona pellucida binding, have been excluded from this review (Anderson et al, 1987; Eddy and O’Brien, 1994; Naz et al, 2005). Only receptors that are well known and have a definite role in other cell types are included.