Stage‐and cell‐specific gene expression and hormone regulation of the seminiferous epithelium

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Abstract
The regulation of spermatogenesis seems to involve complex cell interactions in the testis. Little is known about these cellular communication events. Advances in molecular technology and cell or cell group separation methods have made it possible to analyze function of defined spermatogenic and Sertoli cells, thereby giving some insights into the paracrine regulation of spermatogenesis. In this review we will describe how seminiferous tubule segments with distinct cell associations can be rapidly isolated and how the cell composition can be modified by high‐energy X‐irradiation. Results of the recent studies performed using these techniques will be briefly summarized. Spermatogenic cells at defined stages of their development can be isolated in living condition for morphological and biochemical studies by the transillumination technique. For accurate identification of the stages of the seminiferous epithelial cycle, phase contrast microscopy of live cell squashes has been used. The criteria described by Leblond and Clermont (Am. NY Acad. Sci., 55:548–573, 1952) can be used for accurate recognition of most of the stages of the cycle. However, stages I and II and substages of VII that are important in several studies are difficult to distinguish. Therefore, in addition to the morphology of early spermatids, development of the flagella at step 16 of spermiogenesis and the changing morphology of the cytoplasmic lobes (residual bodies) at stage VII of the cycle were used as criteria for rapid identification and isolation (preparative) of the seminiferous tubule segments. Expression of nucleoprotein and heat shock protein 70‐related protein genes was analyzed with Northern blot, slot blot, and in situ hybridization techniques in accurately staged seminiferous tubules. Accurate stage‐dependent timing of the onset of transcription, followed by storage and disappearance of the messages was demonstrated. The chromatoid body (cb) has been proposed to have a specific function in storage of the long‐lived mRNAs in the spermatids. It is an actively moving cytoplasmic organelle that interacts with Golgi complex during formation of the acrosomic system. The chromatoid body is apparently also dependent on cytoplasmic microtubules, since its movements are inhibited and its structure becomes abnormal in the presence of vincristin, an inhibitor of tubulin polymerization. Follicle‐stimulating hormone (FSH) is an important regulator of Sertoli cell function. Since both basal and FSH‐dependent cyclic AMP (cAMP) production by seminiferous tubules showed marked stage dependency, Sertoli cells are apparently influenced by spermatogenic cells. Thus, Sertoli cell function varies cyclically depending on the stage of the seminiferous epithelial cycle to provide an optimal microenvironment for spermatogenesis. Further evidence of the interactions between Sertoli cells and spermatogenic cells was obtained by investigating stage‐dependent responses of the seminiferous tubules to FSH after local x‐irradiation, when defined germ cells were low in number in the seminiferous epithelium. The absence of specific spermatogenic cells changed FSH‐stimulated cAMP production. Studies of the underlying cellular and molecular mechanisms are important for our understanding of the hormonal regulation of spermatogenesis.