Populations of granulosa cells in small follicles of the sheep ovary

The aim of this study in sheep ovaries was to determine the total number of granulosa cells in primordial follicles and at subsequent stages of growth to early antrum formation. The second aim was to examine the interrelation ships among the total number of granulosa cells in the follicles, the numbe r of granulosa cells in the section through the oocyte nucleolus, and the d iameter of the oocyte. A third aim was to examine whether proliferating cel l nuclear antigen labelling occurred in flattened granulosa cells in primor dial follicles or was confined to follicles containing cuboidal granulosa c ells. The follicles were classified using the section through the oocyte nu cleolus by the configuration of granulosa cells around the oocyte as type 1 (primordial), type la (transitory), type 2 (primary), type 3 (small preant ral), type 4 (large preantral), and type 5 (small antral). In type 1 follic les, the number of granulosa cells and oocyte diameter were highly variable in both fetal and adult ovaries. Each type of follicle was significantly d ifferent from the others (all P < 0.05) with respect to oocyte diameter, nu mber of granulosa cells in tl-le section through the oocyte nucleolus and t otal number of granulosa cells. Follicles classified as type 2, 3, 4 or 5 e ach corresponded to two doublings of the total granulosa cell population. T he relationships between oocyte diameter and the number of granulosa cells (that is, in the section through the oocyte nucleolus or total population p er follicle) could all be described by the regression equation log(e) X = a + b log(e) Y with the correlation coefficients X always > 0.93. For each p air of variables the slopes (b) for each type of follicle were not differen t from the overall slope for all types of follicle pooled. Immunostaining f or proliferating cell nuclear antigen was observed in granulosa cells in ty pe 1 follicles, as well as in the other types of follicle. These findings i ndicate that 'flattened' granulosa cells in type 1 follicles express an ess ential nuclear protein involved in cell proliferation before assuming the c uboidal shape. Thus, when considering factors that regulate specific phases of early follicular growth, it is important to consider: (i) the follicle classification system used; (ii) the animal model studied; (iii) whether ty pe 1 follicles are all quiescent; and (iv) the likelihood that each follicl e type represents more than one doubling of the population of granulosa cel ls.