PERITUBULAR MYOID CELLS IN THE TESTIS - THEIR STRUCTURE AND FUNCTION

Peritubular myoid cells, surrounding the seminiferous tubules in the t estis, have been found in all mammalian species, but their organizatio n in the peritubular interstitial tissue varies by species. In laborat ory rodents, including rats, hamsters and mice, only one layer of myoi d cells is seen in the testis. The cells in these animals are joined b y junctional complexes as are epithelial cells. On the other hand, sev eral cellular layers exist in the lamina propria of the seminiferous t ubule in the human and some other animals. Myoid cells contain abundan t actin filaments which are distributed in the cells in a species-spec ific manner. In the rat, the filaments within one myoid cell run both longitudinally and circularly to the long axis of the seminiferous tub ule, exhibiting a lattice-work pattern. The arrangement of the actin f ilaments in the cells changes during postnatal development, and the di sruption of spermatogenesis, such as cryptorchidism, seems to affect f urther the arrangement of the filaments. Other cytoskeletal proteins, including myosin, desmin/vimentin and alpha-actinin, are also found in the cells. Myoid cells have been shown to be contractile, involved in the transport of spermatozoa and testicular fluid in the tubule. Seve ral substances (prostaglandins, oxytocin, TGF beta, NO/cGMP) have been suggested to affect the contraction of the cell, though the mechanism s of the contraction are still unknown. Recent in vitro studies have d emonstrated that the cells secrete a number of substances including ex tracellular matrix components (fibronectin, type I and IV collagens, p roteoglycans) and growth factors (PModS, TGF beta, IGF-I, activin-A). Some of these substances are known to affect the Sertoli cell. functio n. Furthermore, it has been reported that myoid cells contain androgen receptors and are involved in retinol processing. Considering all thi s, it is evident that peritubular myoid cells not only provide structu ral integrity to the tubule but also take part in the regulation of sp ermatogenesis and the testicular function. Their precise roles, howeve r, remain to be solved.