Local control systems within the testis.

A number of physiological and pathological observations cannot readily be explained unless one accepts that there exists within the testis some sort of local control system. This local network of regulatory interactions offers not only an additional level of fine regulation for individual testicular functions, but also creates an opportunity for co-ordination and integration of distinct activities such as germ cell development and androgen production. There is an overwhelming amount of data indicating that the testis produces a variety of regulatory molecules and that many of these agonists have marked effects on the function of testicular cells in vitro. Some of these molecules are identical with or are at least related to known hormonal and humoral agonists. Others are novel and require further characterization. The exact cellular origin of many of these regulatory factors remains unknown. This overview has been limited to regulatory interactions between somatic testicular cells. Particular attention has been paid to communications between the interstitial and the tubular compartment. It should be evident that the nature and the significance of these interactions is only beginning to emerge. The major difficulty remains to distinguish effects that are restricted to the specific and often artificial conditions of in vitro systems from phenomena that are relevant to testicular control in vivo. Further progress in this field will rely on the development of appropriate systems to study local interactions in vivo. Valuable attempts have been made in this direction: vitamin A induced synchronization of spermatogenesis may offer a model to study stage dependent alterations in the interstitial compartment (Morales and Griswold, 1987; Bartlett et al, 1989); destruction of Leydig cells followed by substitution with androgens might clarify the role of non-steroidal Leydig cell mediators on tubular function (Shape et al, 1988). Up to now these approaches have failed to demonstrate an important role for local regulatory interactions. It is obvious that both models are relatively crude, however, and that subtle changes may have been missed under the experimental conditions used. It should be stressed that some of the observed complexities may be inherent to local regulatory networks. In fact, such networks tend to display a certain level of redundancy. It is evident, for example, that a number of locally produced mediators can also reach the testis via the circulation. In this setting the relative contribution of circulating and locally produced factors may vary depending on developmental stages, physiological or pathological conditions. A relative redundancy may exist for distinct locally produced mediators.(ABSTRACT TRUNCATED AT 400 WORDS)