Transport Mechanisms for Endocrine and Paracrine Factors in the Testis

Abstract

As in every organ in the body, mechanisms for the delivery and distribution of nutrients, hormones, and other messengers operate within the testis. In contrast to other organs (excepting the brain), the testis has a much greater dependence on these transport systems because of its unusual anatomy. The fact that the bulk of the testis is composed of the avascular seminiferous tubules, which have a very high energy/nutritional demand because of the proliferating germ cells, means that extravascular transport systems have to be highly developed if normal testicular function is to be maintained. This is probably why the intertubular spaces of the testes of most species contain abundant interstitial fluid (IF) (1), as it is this fluid that must transport factors from the bloodstream to the seminiferous tubules, Leydig cells, etc. However, IF cannot deliver nutrients and other factors directly to most of the developing germ cells because these are sequestered behind the “closed doors” of the inter-Sertoli cell tight junctions. Therefore, another delivery system, seminiferous tubule fluid (STF), is brought into play and is responsible for the transport of most factors from the Sertoli cells to the germ cells as well as the transport of spermatozoa out of the testis. There is also a fourth transport highway, testicular lymph, although its precise functions in the testis are not very clear (2).