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Biology of Reproduction, Vol 23, 219-229, Copyright © 1980 by Society for the Study of Reproduction
1 Department of Endocrinology, Growth and Reproduction,
Faculty of Medicine, Erasmus University,
Rotterdam, The Netherlands
J.Th.J. Uilenbroek, Department of Endocrinology, Growth and Reproduction, Faculty of Medicine, Erasmus University, P.O. Box 1738, Rotterdam, The Netherlands.
Atresia of large preovulatory follicles was studied in rats with 5-day reproductive cycles in
which ovulation was blocked by two injections of sodium pentobarbital (Nembutal) at proestrus
and estrus, respectively. The first morphological signs of atresia appeared 3 days after the first
injection of Nembutal, After 4 days the oocytes of all atretic follicles showed resumption of
meiosis. Specific binding of hCG to expressed granulosa cells was high at proestrus and the next day
(Day 1), but was decreased at Day 2. Specific binding of hFSH to granulosa cells decreased gradually after proestrus. Autoradiographic analysis revealed localization of hCG binding to granulosa
and thecal cells of preovulatory follicles even in those which showed morphological signs of atresia
(Day 3). However, hCG binding to follicles in an advanced stage of atresia (Day 4) was considerably less. The steroidogenic activity of follicles undergoing atresia was studied by incubating single
follicles and measuring accumulation of steroids in the medium after 4 h. The main steroid produced by proestrous follicles (expressed in ng/4 h/follicle) was estradiol (2.53 ± 0.56). Estradiol
accumulation in the medium by follicles collected at Day 1 was considerably lower (0.48 ± 0.24).
Accumulation of androgen also decreased from 0.34 ± 0.05 ng at proestrus to 0.06 ± 0.02 ng at
Day 1. Progesterone accumulation was not different between proestrous follicles and follicles
obtained at Day 1 (0.41 ± 0.09 ng and 0.50 ± 0.13 ng, respectively). The decreased estradiol
accumulation by follicles that failed to ovulate was not due to a lack of aromatase activity, as
addition of testosterone to the medium (50 ng/ml) increased estradiol accumulation 5-fold. Addition of oLH (20 µg/ml) increased estradiol production 10-fold in proestrous follicles and only
4-fold in follicles obtained at Day 2. In contrast, progesterone production was increased over
100-fold in both types of follicles. It is concluded that before morphological signs of atresia are apparent, follicular estradiol
production is reduced. Since preovulatory follicles undergoing atnesia show gonadotropin binding
and are responsive to LH with increased progesterone production in vitro, and since aromatase
activity is still present, it is suggested that enzymes involved in the conversion of progesterone to
androgens are deficient in the early process of atresia of preovulatory follicles.
Note:
ACKNOWLEDGMENTS
Authors wish to thank Mr. D. H. van Gool and Mr.
Th. H. van Beneden for technical assistance, and Dr. S.
G. Hillier for constructive criticism during the preparation of the manuscript.
The investigation was supported in part by the
Foundation for Medical Research (FUNGO), which is
subsidized by the Netherlands Organization for the
Advancement of Pure Research (ZWO).
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