Intrafollicular concentrations of steroids and steroidogenic enzymes in relation to follicular development in the mare

The objective of the present study was to determine the changes in follicul ar fluid steroid concentrations and in granulosa cell steroidogenic enzyme expression during the follicular phase, in relation to follicular size and physiological status in the mare. Follicular fluid and follicular cells were recovered by ultrasound-guided f ollicular punctures either around the time of emergence of the dominant fol licle, at the end of the dominant follicle growth, or at the preovulatory s tage, after injection of gonadotropin to induce ovulation. Cellular relativ e amounts of steroidogenic acute regulatory protein (StAR), P450-side chain cleavage (P450(scc)), 3 beta-hydroxysteroid dehydrogenase (3 beta HSD), 17 alpha-hydroxylase, and aromatase were assessed by semiquantitative Western blot and densitometry. Follicular fluid was assayed for cholesterol concen trations by colorimetric assay and for progesterone, testosterone, and estr adiol-17 beta concentrations by RIA. Intrafollicular concentrations of progesterone and estradiol-17 beta signif icantly increased in the dominant follicle during growth. After injection o f gonadotropin, follicular maturation was characterized by a decrease in es tradiol-17 beta concentrations and a further increase in progesterone conce ntrations. Granulosa cells from dominant follicles had increased levels of StAR, P450(scc), 3 beta HSD, and aromatase during growth, but decreased lev els during maturation. Levels of StAR, P450(scc), 3 beta HSD, and aromatase , as well as progesterone and estradiol-17 beta, were lower in granulosa ce lls from subordinate than from dominant follicles. We did not observe a rel ationship between the steroidogenic activity of follicles and the capacity of their enclosed oocytes to complete meiosis in vitro.