Removal of enveloping follicle cells can trigger resumption of meiotic maturation in Fundulus heteroclitus oocytes

The various somatic cell layers surrounding large (1.3–1.4 mm in diameter) meiosis‐arrested oocytes of the saltmarsh teleost Fundulus heteroclitus were successively removed by a combination of manual dissection and exposure to calcium/magnesium‐free medium. The efficacy of these methods was verified by light and scanning electron microscopy. Oocytes within intact follicles, and oocytes in follicles with the outermost surface epithelium and theca layers removed, typically resumed meiosis only when treated with either a steroid, 17α‐hydroxy‐20β‐dihydroprogesterone, or an aqueous extract of Fundulus pituitaries. In contrast, removal of the follicle cell layer immediately surrounding the oocyte frequently triggered the resumption of meiosis in the absence of exogenous hormonal stimulation. These results suggest that the follicle cells may somehow be responsible for the maintenance of meiotic arrest in non‐hormone‐treated oocytes.