Estrogen Synthesis in Leydig Cells: Structural-Functional Correlations in Necturus Testis1

Abstract

The intratesticular site of estrogen biosynthesis is still somewhat controversial. Since the testis of the urodele amphibian Necturus maculosus has exceptionally high aromatase activity and is organized into a distinct glandular region that can be readily separated from another component made up mainly of seminiferous lobules, a study was undertaken to correlate function and structure of these two regions. When microsomal preparations were incubated with radiohabeled substrates, two key enzymes in estrogen biosynthesis, aromatase and 1 7a-hydroxylase, were found to be concentrated in the glandular tissue. Also, spectral measurements showed that eytochrome P450 speciesthatbind androstenedione and progesterone, respective steroidal substrates for aromataseand 17cs-hydroxylase, were distributed between the two zones in ratios similar to their steroidogenic activities. The relative yields of radioimmunoassayable estrogen and androgen from cultured cells derived from each zone was further evidence fot the higher steroidogenic capacity of the glandular tissue. Observations with the electron microscope indicate that active steroid synthesis in the glandular tissue could be attributed solely to numerous, highly differentiated Leydig cells, which are characterized by enormous volumes of smooth endoplasmic reticulum and mitochondria with tubular cristae. The presence of structural heterogeneities in the cell population in this region further suggests there may be functional differences among the various Leydig cell types. Although differentiating Leydig cells were seen surrounding the seminiferous lobules, they had little smooth endoplasmic reticulum. Based on structural features, it is improbable that differentiating Leydig cells, Sertoli cells, or elements other than differentiated Leydig cells in either the glandular or lobular tissues are steroidogenically active. The functional significance of testicular estrogen is still unknown. Because of the uniquely high Leydig cell aromatase aetivity, the specialized anatomy of the urodele testis, and an annual cycle in which temporal changes are protracted, we conclude that Necturus is a useful animal model for further investigation of this problem.