Is the Epididymis a Series of Organs Placed Side By Side?

Abstract

The mammalian epididymis is more than a highly convoluted tube divided into four regions: initial segment, caput, corpus and cauda. It is a highly segmented structure with each segment expressing its own and overlapping genes, proteins, and signal transduction pathways. Therefore, the epididymis may be viewed as a series of organs placed side by side. In this review we discuss the contributions of septa that divide the epididymis into segments and present hypotheses as to the mechanism by which septa form. The mechanisms of Wolffian duct segmentation are likened to the mechanisms of segmentation of the renal nephron and somites. The renal nephron may provide valuable clues as to how the Wolffian duct is patterned during development, whereas somitogenesis may provide clues as to the timing of the development of each segment. Emphasis is also placed upon how segments are differentially regulated, in support of the idea that the epididymis can be considered a series of multiple organs placed side by side. One region in particular, the initial segment, which consists of 2 or 4 segments in mice and rats, respectively, is unique with respect to its regulation and vascularity compared to other segments; loss of development of these segments leads to male infertility. Different ways of thinking about how the epididymis functions may provide new directions and ideas as to how sperm maturation takes place.

Keywords: Wolffian duct; epididymal segments; epididymis; gene expression; male fertility.

FIG. 1

Comparison of epididymal regions with epididymal segments in the rat. Segments 1–4 correspond to the initial segment, intermediate zone (IZ) segments 5–11 correspond to the caput region, segments 12–13 and 14 correspond to the corpus region, and segments 14 and 15–19 correspond to the cauda region (diagram adapted from Robaire et al. [113]; epididymal segments illustration courtesy of Prof. YL Zhang, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Shanghai, China).

FIG. 2

Expression of defensin mRNA in different epididymal segments of the rat (figure was modified from Zhang et al. [8], with permission; data from Jelinsky et al. [5]). The 6 epididymides to the left show sequential expression, and the 2 on the right show one example of 1 defensin mRNA expressed in multiple segments in a row (Defb30), and the other shows a mosaic pattern of expression (Defb33). The color-coded bar below show degree of expression, with red having the highest expression. (Epididymis figures courtesy of Prof. YL Zhang [Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Shanghai, China] with permission of American Society of Andrology.)