[Kinetic aspects of follicle development in the rat]

The eighth and ninth generations of follicle growth in rats represent a turning point in development. This stage is characterized by establishing a complete regulatory control based on feedback in follicle development. The feedback between follicles and gonadotropin secretion regulates a number of follicles maturing for ovulation. Gonadotropins seem to play a permissive, and not a directive part in regulation of follicle development in the eighth and ninth generations. By this stage of development, possibilities for theca and granule cells become sharply limited. Whereas expression of many maturation features may be hastened or hindered by changes in hormonal status, the result of development cannot be changed. Although only last generations of theca and granule cells exhibit mature functional features, their precursors seem to become committed to a single direction of development at early stages of follicle development. Neither the stage when precursor cells become irreversibly committed to differentiation into granule or theca cells, neither regulatory factors which determine this process have been identified yet. We suppose that precursor cells become committed to thecal tissue compartment when the follicle is at a primordial stage of development. Precursors of all the follicle components may already be assembled into one unit by the beginning of follicle growth. Accumulation of sufficient number of precursor cells around the primordial follicle may serve as a signal for follicle growth initiation. We think that the understanding of follicle postnatal growth and development should be based on understanding of origins, destiny, and possibilities of cells which form ovary and its compartments. First generations of follicle growth seem to be most promising for future research.