It is well recognized that the primary function of the epididymis is to mature spermatozoa, allowing them to transition from nonfunctional immature cells entering from the testis to functionally mature cells in the cauda epididymis. During epididymal transit spermatozoa acquire progressive motility and the ability to fertilize an oocyte. Because spermatozoa are synthetically inactive the maturation process involves the interaction of spermatozoa with proteins that are synthesized and secreted in a region-dependent manner from the epididymal epithelium. Much emphasis has been placed on identifying and studying these epididymal secretory proteins to establish whether they interact with spermatozoa and/or to determine their roles in sperm maturation.
Perhaps equally important is understanding the complex epididymal luminal environment as a whole, since perturbations in this environment can result in abnormal sperm function. Indeed in addition to sperm maturation, a critical function of the epididymal lumen is to protect the spermatozoa while they undergo maturation. Thus many luminal proteins may not play direct roles in sperm maturation but rather help create the appropriate environment that is conducive for this process to occur. This can include proteins involved in the regulation of luminal pH, osmolality, regulation of oxidative stress, and regulation of protein folding/misfolding. In particular, the epididymal lumen is rich in chaperones with clusterin alone contributing 41% of the total protein secreted into the rat caput epididymidis suggesting an important role for this protein in maintaining protein solubility within the lumen (Dacheux and Dacheux 2002)
. Mechanisms also exist to remove secreted proteins from the lumen, presumably once their functions have been carried out or if misfolding occurs. While a number of proteins secreted by the initial segment, the most proximal region of the epididymis, are detected in the lumen throughout the rest of the epididymis and into the cauda region, other epididymal secretory proteins are removed from the lumen suggesting that a prolonged stay could be detrimental to epididymal function. While the uptake of luminal proteins is not well understood, the epididymal epithelium carries out fluid phase, adsorptive and receptor-mediated endocytosis (Andonian S and Hermo L, 1999)
. Components of a functional ubiquitin- proteosome pathway including ubiquitin activating enzyme E1, ubiquitin carrier enzyme E2, and the ubiquitin C-terminal hydrolase PGP9.5/UCHLI have also been detected in the epididymal lumen suggesting an alternative pathway for turnover of luminal proteins and/or removal of defective sperm (Baska et al., 2008). The highly regulated and regionalized secretion and removal of proteins from the lumen is indicative of a unique microenvironment that likely utilizes a variety of quality control mechanisms to maintain the appropriate conditions for sperm maturation.
Finally, it is important to realize that the epididymal luminal compartment is not a dilute sea of soluble proteins and spermatozoa but rather a dense and crowded milieu due to the loss of greater than 90% of the water from the luminal fluid as it moves through the efferent ducts and into the initial segment of the epididymis. This loss of water and the concentration of spermatozoa create a close association of luminal components with spermatozoa which is thought to be critical for normal sperm maturation. The loss of water can also create a situation described in other cellular systems as macromolecular crowding which can result in protein misfolding and aggregation (Rialdi and Barrisel 2007
; Minton 2005
). Indeed, examination of the luminal compartment by electron microscopy shows that, in addition to spermatozoa, the lumen is highly particulate in nature (). Some of these structures are small membrane bound vesicles termed epididymosomes which are released from the epithelium by apocrine secretion and are proposed to interact with spermatozoa as part of the maturation process (Sullivan et al., 2007
). Various other aggregate or fibrillar type structures have also been detected in the epididymal lumen; however, how they are formed and what their functions are is not known. These structures include large electron dense bodies which contain heat shock protein 1 (HSPD1, HSP60) and tumor rejection antigen (TRA1), a member of the heat shock protein 90 family (Asquith et al., 2005
). Because heat shock proteins have proposed roles in the folding of denatured proteins, these structures may function as a site of protein refolding or sequestration of proteins targeted for clearance from the lumen or may facilitate protein interactions with spermatozoa. Thus, soluble as well as insoluble components are an integral part of the epididymal lumen.
Transmission electron micrograph of the initial segment region of the mouse epididymis showing particulate material within the lumen