Incorporation of Endogenous Prolactin by Granulosa Cells and Dictyate Oocytes in the Postpartum Rat: Effects of Estrogen1

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Abstract
Ovarian follicles were examined for the presence and distribution of endogenous PRL in normally lactating control rats and comparable rats which had been subjected to a variety of antilactational, luteotropic estrogen treatments. Control ovaries contained both preantral follicles and antral follicles up to 500 μm in diameter. In healthy antral follicles, PRL was present in the cytoplasm and nuclei of granulosa cells (GC), in follicular fluid (FF) and also in the cytoplasm of the maturing dictyane oocytes (OC), where its lattice-like distribution resembled that of, and suggested an association with, yolk material. PRL was distributed similarly in OC of healthy preantral follicles, but in was absent from the surrounding GC. In atresia, both GC and OC showed disturbances of PRL distribution (absence from GC nuclei, irregular staining of yolk lattice) or complete absence of staining for PRL. The changes in OC were seen in rats given only 4 μg estradiol benzoate/day. Stronger estrogen treatments produced atretic changes in GC as well. These findings suggest at least two periods of PRL incorporation by healthy dictyate OC: one prior to, or early in, the stage of meiotic arrest, the other during preovulatory maturational enlargement. During the later period, when PRL is transported to the OC through GC cytoplasm and FF, it also enters GC nuclei. The prolonged retention of PRL within the OC and its renewed incorporation as this cell approaches interphase, together with recent evidence that PRL may play a role in the inhibition of meiosis in mammalian oocytes, suggest that the specific local resistance to ovulation-inducing stimuli, which is needed for both selective maintenance of the dictyate state and fine control of preovulatory maturation, might be a function of the PRL stored within the OC itself.