Light- and electromicroscopic examinations were performed on granulosa and theca of primordial-, primary-, secondary- and resting tertiary follicles of human ovaries. These examinations were intended to clarify how far correlation exist between the structural components of the different tissue formations of the follicles and their determined functions. Remarkably many intraplasmatic filaments were found in the cytoplasm of granulosa cells of primordial-, primary- and secondary follicles. In the resting tertiary follicles the electronmicroscopy defines the majority of the follicle granulosa cells as proteinsynthetic active cells with abundant rough endoplasmatic reticulum. Most of the nuclei contain several nucleoli. An interesting finding compared with the granulosa cells of earlier developing stages of the follicle is the presence of single or grouped fat droplets in the cytoplasm, whereas metaplastic structures like filaments and/or microtubules are rare. The theca cells aroung the primordial-, primary- and secondary follicle were characterized by electromicroscopy as typical stroma cells. These cells of the resting tertiary follicles in the theca interna and externa show characteristic submicroscopic criteria of active steroidbiosynthesis. Their cytoplasm is especially rich of smooth endoplasmatic reticulum about from that there are tubular mitochondria and diffus fat droplets. Regarding the functional meaning of the different tissue formation of the follicles the existence of filamentous material in the membrana granulosa of primordial-, primary- and secondary follicles demonstrates an important finding. Apparently the presence of these metaplastic structures in the follicle granulosa cells play a role in the formal development of the zona pellucida and the Call-Exner-bodies. The structural organisation of the granulosa cells of resting tertiary follicles shows a high proteinsynthetic activity which plays a role in the metabolism of oocyte and the follicular fluid production. So far there are no definite submicroscopic criteria for steroidbiosynthesis. The structural differentiation of the normal stroma cells around primordial-, primary- and secondary follicles leads to definite submicroscopic steroidcells in the resting tertiary follicle. According to our results the process of the transformation of follicular granulosa cells in steroidbiosynthetic active cells in the resting tertiary follicle is not complete.
