Epididymal cell secretory activities and the role of proteins in boar sperm maturation
Introduction
The formation of the male gamete is the result of extensive cellular differentiation that occurs during the transformation of a round spermatid into a highly polarized and fully motile cell. Most of these complex biochemical, physiological and morphological events of cellular differentiation take place in the testis during the process of spermiogenesis. During this testicular processing, cellular events are mostly under genomic regulation of the gamete. However, when DNA begins to condense in the elongated spermatids, the transcription process in the germinal DNA decreases and then stops. At the final testicular phase of male gamete differentiation, the spermatozoa that are neither motile nor fertile require additional discrete and essential post-gonadal modification to be able to fertilize eggs. Thus, the presence of a specific sperm environment during the subsequent differentiation stages in the epididymis is believed to play an essential role in controlling or inducing the final sperm changes.
The maturation stage of spermatozoa in the epididymis is out of the control of the germinal genome and is therefore largely the consequence of their interactions with the epididymal fluid, mostly with specific proteins present in the lumen of the epididymal tubule (Fig. 1). This specific microenvironment, which is isolated from the blood by the epididymal-blood barrier, also ensures protection of the gametes until ejaculation as well as regulation of the functionality and integrity of the epididymis.
Understanding of these post-testicular effects on the sperm and on the epididymal environment is essential to obtain good markers for fertility prediction for the animal and for sperm fertility and conservation.
Section snippets
Post-testicular environment of the spermatozoa
From the testis, the spermatozoa are transported into an epididymal tube that forms an organ composed of several regions, such as the caput, corpus and cauda (Fig. 2). The epididymis is formed by a polarized epithelium composed mostly of principal and basal cells and creating an environment around the spermatozoa that is certainly the most complex found in any exocrine gland. This complexity results from two peculiarities: (1) continuous and progressive changes in its composition throughout the
Morphological characteristics of the secretory activity of the epididymal cells
In order to provide the luminal changes required for sperm maturation to occur, the epithelium may resort to two basic mechanisms: absorption and secretion. Far from being a uniform channel, the epididymal duct is a canal with highly specialized regional differentiation of its epithelial ultrastructure.
Sperm membrane remodelling during epididymal transit
As the sperm enter the epididymis, the sperm plasma membrane undergoes substantial remodelling during epididymal transit, both in protein and phospholipid composition and in localization of the components on the gamete. In all the species studied to date, including the boar, it appears that specific testicular sperm surface proteins are removed or further processed as the gametes pass through the epididymis. The disappearance of some of them is clearly related to a specific mechanism of
Effect of the epididymal environment on sperm
The sequential changes associated with the sequential modifications observed in the sperm membrane are believed to be directly or indirectly involved in the maturation stages of sperm during epididymal transit.
Conclusions and perspectives
During their transit through the epididymis, spermatozoa are subjected to a continually changing luminal environment modified by the secretory and endocytic activities of the cells lining the epithelium. Ultimately, it is the coordinated activities of secretion and endocytosis of various substances by the epithelial cells along the duct that influence the final maturation of sperm as well as their concentration, protection and storage. These sequential changes are accompanied by modifications
Acknowledgements
This study was supported by grants from the Institut National de la Recherche Agronomique (INRA, France) and from the Région Centre (France).
References (54)
- et al.
Apocrine secretion-fact or artifact?
Anat Anz
(1999) - et al.
Epithelial inclusions in the vas deferens: an electron microscopic study
J Urol
(1974) - et al.
Evidence for distinct serine protease activities with a potential role in processing the sperm protein fertilin
Dev Biol
(1997) - et al.
The major maturation glycoprotein found on rat cauda epididymal sperm surface is linked to the membrane via phosphatidylinositol
Biochem Biophys Res Commun
(1989) - et al.
Molecular cloning and characterization of the epididymis-specific glutathione peroxidase-like protein secreted in the porcine epididymal fluid
Biochim Biophys Acta
(1997) - et al.
Protein secretion in the epididymis
- et al.
Aquaporin 9 expression along the male reproductive tract
Biol Reprod
(2001) - et al.
Role of epididymal secretory proteins in sperm maturation with particular reference to the boar
J Reprod Fertil Suppl
(1998) - et al.
Contribution of epididymal secretory proteins for spermatozoa maturation
Microsc Res Tech
(2003) - et al.
Characterization and identification of proteins secreted in the various regions of the adult boar epididymis
Biol Reprod
(1996)