Figure 1.

Schematic diagram showing the testis and epididymis including the 4 regions of the epididymis (initial segment, caput, corpus and cauda epididymidis). The epididymis is a single convoluted tubule in which the epithelium regulates the composition of the lumen necessary for sperm to acquire the ability to swim and fertilize. The epithelium of the epididymis contains various cells types, including principal cells, clear cells, basal cells and halo cells. Apical and narrow cells are present exclusively in the initial segment. Apical tight junctions between the cells that line the lumen of the epididymis form the blood-epididymis barrier. The barrier regulates the content of the epididymal lumen by limiting paracellular movement of ions between the blood and the lumen. It also protects maturing spermatozoa from the immune system.