2 STEROIDOGENIC PATHWAYS PRESENT IN THE CHICKEN OVARY - THECA LAYER PREFERS DELTA(5) PATHWAY AND GRANULOSA LAYER PREFERS DELTA(4) PATHWAY

It has been suggested that there is a shift in steroidogenic pathways of the theca layer from Delta(5) to Delta(4) during follicular maturat ion in the chicken ovary. However, the preferred steroid metabolic pat hway in the theca and granulosa layers of small and preovulatory folli cles has not been identified. Therefore, the goal of our study was to identify the preferred steroidogenic pathways used by these follicles. Previously, we found that small follicles that had not entered the fo llicular hierarchy did not use cholesterol but did use pregnenolone (P -5) to produce testosterone and estradiol. For this reason, P-5 was us ed as the substrate for steroid production in the present study. The t heca layer of small follicles (2-4 mm in diameter), and the theca and granulosa layers of the fifth largest (F5), third largest (F3), and la rgest (F1) preovulatory follicles were isolated from the chicken ovary 22 hr before ovulation. The granulosa layer of small follicles was ex cluded from the study because it is steroidogenically incompetent. Tis sues were incubated with [H-3]-P-5 for 1 hr at 37 degrees C. Productio n of steroid metabolites was determined on the basis of comparison wit h known steroid standards using two-dimensional thin layer chromatogra phy (2D-TLC). Conversion of [H-3]-P-5 to [H-3]-DHEA was greater than t hat of [H-3]-P-4 in the theca layer of small and preovulatory follicle s. Even though the amount of [H-3]-P-4 was less than that of [H-3]-DHE A, theca cells from FZ and Fl follicles did produce [H-3]-P-4. In cont rast to the theca layer, conversion of [H-3]-P-5 to [H-3]-P-4 was grea ter than that of [H-3]-DHEA in the granulosa layer of preovulatory fol licles. Conversion of [H-3]-P-5 to [H-3]-DHEA by the granulosa layer g radually decreased as the follicle matured and approached ovulation. W e conclude that both Delta(5) and Delta(4) pathways are functional in theca and granulosa cells. However, in contrast to earlier speculation , the theca layer preferentially metabolizes steroids via the Delta(5) pathway regardless of the maturational stage of the follicle, whereas the granulosa layer of preovulatory follicles preferentially metaboli zes steroids via the Delta(4) pathway. (C) Elsevier Science Inc. 1998.