In mammalian female reproduction, primordial follicles serve as stores to sustain the ovulation cycle. Regulations of primordial follicle development, activation, and dormancy in mice are summarized in a review by Nagamatsu (pp. 189–195). The importance of mechanical stress, especially extracellular matrix (ECM)-mediated pressure, for the maintenance of primordial follicle dormancy was recently demonstrated. Primordial follicles treated with collagenase, trypsin, and knockout serum replacement (KSR) (this mixture called CTK) to digest ECM were examined by immunohistochemistry. Ovaries treated solely with phosphate-buffered saline displayed primordial follicles composed of flat granulosa cells and complex stress fibers, as revealed by phalloidin. In contrast, CTK treatment resulted in fewer stress fibers and cuboidal-shaped granulosa cells, suggesting oocyte activation.