Morphological characterization of bovine primordial follicles and their environment in vivo.

Classically, primordial follicles have flattened pre-granulosa cells that become cuboidal and divide during follicular activation. This change in shape is thus an index of activation. To examine this and follicular morphology in cattle, ovaries were processed for light and electron microscopy (n = 21). In single sections (5 microns thick), primordial follicles had a mean ratio of maximum to minimum diameters of 1.33 +/- 0.18 (mean +/- SD; n = 317, randomly selected), indicating that they had a prolate rather than a spherical shape. The prolate shape was attributable to clustering of granulosa cells at two opposite poles on the long axis of each follicle; and in each histological section, 82.5% of primordial follicles (n = 317) had at least one cuboidal granulosa cell. More accurate measurements of primordial follicles indicated that they had three dimensions measured as length (45.4 +/- 2.4 microns), breadth (26.8 +/- 1.5 microns), and depth (< 30.4 +/- 1.4 microns) (mean +/- SEM; n = 12, constructed from serial sections); a surface area of 0.0032 mm2; and 24 +/- 2 granulosa cells. The ultrastructural morphology of the cells of the bovine primordial follicle (n = 31 examined by electron microscopy) was similar to that of other species. The ovarian cortex was composed of discrete zones. The zones containing primordial follicles were substantially avascular, as observed by localizing von Willebrand factor, and rich in collagen fibrils. In conclusion, since primordial follicles of bovine ovaries contain some cuboidal pre-granulosa cells, changes in shape cannot be used with certainty as a marker of activation of bovine follicles.