Male Reproductive Function

The testes are the source of both germ cells and hormones essential for male reproductive function. The production of both sperm and steroid hormones is under complex feedback control by the hypothalamic-pituitary system. The testis consists of a network of tubules for the production and transport of sperm to the excretory ducts and a system of interstitial cells (called Leydig cells) that express the enzymes required for the synthesis of androgens. The spermatogenic or seminiferous tubules are lined by a columnar epithelium composed of the germ cells themselves as well as supporting Sertoli cells surrounded by peritubular tissue made up of collagen, elastic fibers, and myofibrillar cells. Tight junctions between Sertoli cells at a site between the spermatogonia and the primary spermatocyte form a diffusion barrier that divides the testis into two functional compartments, basal and adluminal. The basal compartment consists of the Leydig cells surrounding the tubule, the peritubular tissue, and the outer layer of the tubule containing the spermatogonia. The adluminal compartment consists of the inner two-thirds of the tubules containing primary spermatocytes and germ cells in more advanced stages of development. The base of the Sertoli cell is adjacent to the basement membrane of the spermatogenic tubule, with the inner portion of the cell engulfing the developing germ cells so that spermatogenesis actually takes place within a network of Sertoli cell cytoplasm. The mechanism by which spermatogonia pass through the tight junctions between Sertoli cells to begin spermatogenesis is unknown. The close proximity of the Leydig cell to the Sertoli cell with its embedded germ cells is thought to be critical for normal male reproductive function. The seminiferous tubules empty into a network of ducts termed the rete testis. Sperm are then transported into a single duct, the epididymis. Anatomically, the epididymis can be divided into the caput, the corpus, and the cauda regions. The caput epididymidis consists of 8 to 12 ductuli efferentes, which have a larger lumen tapering to a narrower diameter at the junction of the ductus epididymidis.

Serum erythropoietin levels in healthy humans after a short period of normobaric and hyperbaric oxygen breathing: the “normobaric oxygen paradox”

Renal (peritubular) tissue hypoxia is a well-known physiological trigger for erythropoietin (EPO) production. We investigated the effect of rebound relative hypoxia after hyperoxia obtained under normo- and hyperbaric oxygen breathing conditions. A group of 16 healthy volunteers were investigated before and after a period of breathing 100% normobaric oxygen for 2 h and a period of breathing 100% oxygen at 2.5 ATA for 90 min (hyperbaric oxygen). Serum EPO concentration was measured using a radioimmunoassay at various time points during 24–36 h. A 60% increase ( P < 0.001) in serum EPO was observed 36 h after normobaric oxygen. In contrast, a 53% decrease in serum EPO was observed at 24 h after hyperbaric oxygen. Those changes were not related to the circadian rhythm of serum EPO of the subjects. These results indicate that a sudden and sustained decrease in tissue oxygen tension, even above hypoxia thresholds (e.g., after a period of normobaric oxygen breathing), may act as a trigger for EPO serum level. This EPO trigger, the “normobaric oxygen paradox,” does not appear to be present after hyperbaric oxygen breathing.