Classification of small type B/C follicles as primordial follicles in mature rats

In the present study follicles were classified according to the morphology of their granulosa cells. Type B follicles contained only flattened granulo sa cells; type B/C follicles had a mixture of flattened and cuboidal granul osa cells in a single layer, and type C follicles had a single layer of cub oidal granulosa cells. The primary objectives of the study were to determin e whether 5-bromo-2-deoxyuridine incorporation into type B/C follicles was a marker for initiation of growth and how long type B/C follicles could rem ain at the same stage before transformation to type C follicles. Female Hol tzman rats received bromo-deoxyuridine for 7 days. After the infusion (day minipumps were removed = day 0), rats were ovariectomized on days 0 (n = 9) , 30 (n = 8), 90 (n = 8) and 150 (n = 9). The numbers of type B, B/C and C follicles within one ovary were determined using modified fractionator coun ting. Analysis over all times demonstrated that there were more (P < 0.0001 ) type B/C (941 +/- 61 per ovary) than type C (140 +/- 18 per ovary) or typ e B (159 +/- 19 per ovary) follicles. The numbers of type B and type C foll icles did not differ from each other at any time. Only one of 34 rats evalu ated had bromo-deoxyuridine-labelled type B follicles. On day 150, 57% of t he bromo-deoxyuridine-labelled type B/C follicles remained from day 0. It i s concluded that (1) DNA synthesis in granulosa cells of type B/C follicles is not a reliable indicator of impending growth; and (2) type B and type B /C follicles are both components of the pool of primordial follicles.