Morphometric characteristics of the primordial to primary follicle transition in the human ovary in relation to age.

Background: The growth pattern of the smallest ovarian follicles in humans is still incompletely documented. Using follicle hemispheres in thick histological sections, morphometric changes of primordial to primary follicles and possible age-related effects were evaluated.

Methods: In ovarian sections from 25 females aged 4-39 years a total of 1122 primordial, transitory or primary follicles were assessed for the diameters of follicles, oocytes and oocyte nuclei and for number of granulosa cells (GCs).

Results: The number of GCs in primordial, transitory and primary follicles were approximately 30, 50 and 100, respectively. The diameters of primordial follicles and oocytes increased with the woman's age until the mid-30's, after which time they decreased in size. The number of GCs in primordial follicles showed a moderate increase with age, whereas the number of GCs in transitory follicles showed a clear increase with age . The oocyte nucleus diameter (14-23 microm) showed significant linear correlations with the oocyte and follicular diameters and number of GCs in the follicle, while the number of GCs in the whole follicle and in the largest cross-section were closely related to the oocyte diameter.

Conclusions: The number of GCs in small follicles is accurately estimated and shows an increase with age, indicating that the starting point of follicular development alters with female age. The age-related changes observed may be linked to the poorer reproductive performance of older women.