Human testicular peritubular cells – novel insights into an underestimated testicular somatic cell

The crucial contributions of Sertoli and Leydig cells to the functions of the testis are undisputed, but the roles of a third, testis-specific somatic cell type, peritubular cells, were not well known. In the adult man, unlike in rodents, a true peritubular compartment exists. Like in other species it is strategically positioned between the tubular and the interstitial compartment. However, its specific architecture, namely several layers of slender, elongated peritubular cells, extracellular matrix, few capillaries, nerve fibers and immune cells, indicates a human-specific situation. Insights into the human situation are derived, in part, from studies using a culture system of human testicular peritubular cells (HTPCs), established in 2006, which opened an experimental window into the otherwise inaccessible human testis. Here, I will neither reiterate topics of previous reviews, nor attempt to cover all studies performed since then. Rather, I will focus on recent insights into the cellular nature of these testis-specific smooth muscle cells and the appropriateness of HTPCs to model the adult testicular peritubular compartment. The available data indicate that their functions go beyond the assumed sole role in sperm transport. They include immunological roles, contributions to spermatogenesis, regulation of testicular microcirculation and more. The evidence for plasticity of HTPCs, together with the observed changes associated with cellular senescence, bear witness that all roles of peritubular cells are, however, not fixed and can change. Regulators of such changes may include hormones and drugs, e.g. dexamethasone and cannabinoids. These unexpected results may be of relevance to human reproductive health.