Studies on the structure and function of the mammalian testis. 3. In vitro steroidogenesis by the seminiferous tubules of rat testis.

The seminiferous tubules of rat testes were separated from the adjacent interstitial tissue by dissection and incubated with [7$\alpha $-$^{3}$H]pregnenolone and [4-$^{14}$C]progesterone in an oxygenrich, Krebs-Ringer bicarbonate medium at 33 degrees C for a maximum period of 6 h. Samples of the incubation medium were taken at various time intervals throughout the experiment to provide a kinetic study of the metabolic processes occurring, and the radioactive steroid products were identified and estimated. There was conversion of the labelled substrates to androgens and to a series of hydroxylated and/or reduced C$_{21}$ compounds, namely 20$\alpha $-hydroxypregn-4-en-3-one, 5$\alpha $-pregnane-3$\beta $,20$\alpha $-diol, 5$\alpha $-pregnane-3$\beta $-ol-20-one, 5$\alpha $-pregnane-3$\alpha $-ol-20-one, and 5$\alpha $-pregnane-3,20-dione. The preferred pathway of androgen synthesis was via $\Delta ^{4}$ intermediates. Thus the two series of compounds originated from the common precursor, progesterone, and then diverged along separate pathways. A comparison with parallel studies carried out previously on isolated interstitial tissue shows that there are definite differences in the steroidogenic activity of the two main components of the testis which cannot be accounted for by contamination. Hence it is concluded that the tubules possess a steroidogenic capacity distinct from that of the interstitium. It is suggested that the Sertoli cells are the most likely source of the androgens and perhaps also of the C$_{21}$ compounds. On this basis, and evidence which suggests that a significant proportion of the labelled metabolites were retained by the tubules (e.g. by selective binding), the possibility is considered that the capacity of the Sertoli cells to elaborate steroids in vitro may be at least equal to that of the Leydig cells.