Characteristics of the Epididymal Luminal Environment Responsible for Sperm Maturation and Storage

Abstract

The testicular spermatozoa of all mammalian species are considered functionally immature owing to their inability to swim in a progressive manner and engage in productive interactions with the cumulus-oocyte complex. The ability to express these key functional attributes develops progressively during the cells' descent through the epididymis, a highly specialized ductal system that forms an integral part of the male reproductive tract. The functional maturation of the spermatozoon is achieved via continuous interactions with the epididymal luminal microenvironment and remarkably, occurs in the complete absence of de novo gene transcription or protein translation. Compositional analysis of the luminal fluids collected from the epididymis of a variety of species has revealed the complexity of this milieu, with a diversity of inorganic ions, proteins, and small non-coding RNA transcripts having been identified to date. Notably, both the quantitative and qualitative profile of each of these different luminal elements display substantial segment-to-segment variation, which in turn contribute to the regionalized functionality of this long tubule. Thus, spermatozoa acquire functional maturity in the proximal segments before being stored in a quiescent state in the distal segment in preparation for ejaculation. Such marked division of labor is achieved via the combined secretory and absorptive activity of the epithelial cells lining each segment. Here, we review our current understanding of the molecular mechanisms that exert influence over the unique intraluminal environment of the epididymis, with a particular focus on vesicle-dependent mechanisms that facilitate intercellular communication between the epididymal soma and maturing sperm cell population.

Keywords: apocrine secretion; dynamin; epididymis; epididymosome; intracellular communication; merocrine secretion; protein trafficking; sperm maturation.

Figure 1

Regionalized structure and schematic distribution of the major cell types in the mouse epididymis. (A,B) The mouse epididymis is generally broadly divided into four unique anatomical segments: the initial segment, the caput, corpus, and cauda epididymis. The initial segment is a loosely coiled tubule with a wide diameter and a low concentration of spermatozoa. Epithelial cells in this segment are elongated and possess high stereocilia. The caput segment is characterized by a narrow luminal diameter, while both the luminal diameter and the sperm concentration increase distally within the corpus and cauda epididymis. Differing cell types within these segments are responsible for the creation of a specialized luminal microenvironment that promotes the sequential maturation of spermatozoa (caput and corpus epididymis) and their subsequent storage (cauda epididymis). (C) Principal cells dominate the soma along the entire length of epididymis and are particularly active in terms of protein biosynthesis and secretion in the proximal epididymal segments. In this context, an apocrine pathway of secretion, featuring the formation and eventual shedding of large bleb-like structures from the apical margin (i.e., apical blebs) of principal cells, appears to be a dominant secretory mechanism operating in all epididymal segments. Upon degradation within the epididymal lumen, apical blebs release a heterogeneous population of membranous extracellular vesicles, termed epididymosomes, which have been implicated in intracellular communication with spermatozoa and downstream epithelial cells. Aside from principal cells, clear cells are distributed sporadically throughout the epithelium of the caput, corpus, and cauda segments in most studied species and are primarily responsible for selective absorption of luminal components and conversely, the regulation the luminal pH. A suite of additional cell types have been described in the epididymis, including basal cells, apical cells, halo cells, narrow cells (only found in initial segment and intermediate zone), and immunological cells.

Figure 2

Schematic of vesicle-dependent mechanisms that contribute to the creation of the highly specialized epididymal intraluminal milieu. Principal cells represents the dominant cell type throughout the length of the epididymis and are particularly active in terms of protein biosynthesis and secretion. At least two active secretory pathways have been documented in these cells, namely classical merocrine secretion and an alternative apocrine secretory pathway. The former is characterized by secretory vesicles formed by the Golgi apparatus and leads to the release of a myriad of soluble proteins. In contrast, apocrine secretion provides a pathway for the secretion of glycosyl-phosphatidylinositol-linked proteins and other hydrophobic proteins. These proteins generally require posttranslational modification in the Golgi apparatus, prior to being selectively packaged into exosome-like vesicles referred to as epididymosomes. These small vesicles are sequestered into large bleb-like structures protruding from the apical margin of the epithelial cells. The attachment anchoring the blebs to their parent cell progressively narrows to form a stalk-like process (potentially through reorganization of cytoskeletal proteins such as myosin and β-actin) and eventually undergoes scission leading to their shedding into the lumen and eventual degradation. The epididymosomes so released provide a mechanism for intercellular communication, thereby enabling the delivery of a complex macromolecular payload to recipient cells in the form of luminal spermatozoa and/or downstream epithelial cells. Such cargo are known to include several hundred proteins as well as various species of small non-protein coding RNA (sncRNA). At present, the mechanisms by which epididymosomes are tethered, and deliver their cargo, to recipient cells remains to be equivocally determined although various proteins have been implicated in this process. Principal cells may also participate in endocytosis, involving the uptake of epididymal luminal contents via receptor (e.g., LRP2) mediated mechanism(s). A similar function has also been assigned to the clear cell population, which is mainly responsible for the recycling of luminal components.