Aryl hydrocarbon receptor regulates growth, but not atresia, of mouse preantral and antral follicles

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that binds various environmental contaminants. Despite our knowledge regarding the role of the AhR in mediating toxicity, little is known about the physiological role of the AhR. Previous studies indicate that the AhR may regulate folliculogenesis, because AhR-deficient (AhRKO) mice have fewer preantral and antral follicles than wild-type (WT) mice during postnatal life. Thus, the first objective of the present study was to test the hypothesis that AhR deficiency reduces the numbers of preantral and antral follicles by slowing growth and/or increasing atresia of follicles. Because alterations in follicular growth or atresia can affect the ability to ovulate, the second objective was to test whether AhR deficiency reduces the number of ovulated eggs. To test these hypotheses, follicular growth was compared in WT and AhRKO ovaries using morphometric techniques and by measuring the ability of the ovary and follicles to grow in response to eCG. Atresia was compared in WT and AhRKO ovaries using morphometric techniques, TUNEL assays, and 3'-end labeling of fragmented DNA. Ovulation was compared in WT and AhRKO mice by assessing the number of corpora lutea per ovary. The results indicate that follicular growth and ovulation were reduced in AhRKO ovaries compared to WT ovaries. The WT ovaries had a 1.5-fold increase in the number of preantral and antral follicles between Postnatal Days 32 and 45, were more responsive to eCG, and contained more corpora lutea than AhRKO ovaries. In contrast, no significant difference was observed in the incidence of atresia in WT and AhRKO ovaries. Taken together, these results suggest that the AhR may regulate growth, but not atresia, of preantral and antral follicles in the mouse ovary.