Destruction of the germinal disc region of an immature preovulatory chicken follicle induces atresia and apoptosis

The germinal disc region (GDR), which contains the germinal disc and overlying granulosa cells, is essential for completion of maturation of the preovulatory chicken follicle. The current study was conducted to test the hypothesis that destruction of the GDR (GDRX) of an immature preovulatory chicken follicle blocks ovulation, induces apoptosis, and causes atresia. The GDR of immature preovulatory follicles (F2) were destroyed by freezing with dry ice (3 mm in diameter) 48-50 h before ovulation. As a control for the effect of freezing, a nonGDR portion (a portion of the follicular wall opposite to the GDR relative to the follicular stalk) of other F2 follicles were destroyed (nonGDRX). Treatment of F2 follicles by GDRX caused atresia and blocked ovulation of all treated follicles (6 of 6), whereas none of the nonGDRX follicles (0 of 5) underwent atresia. Treatment of follicles by GDRX induced apoptotic DNA fragmentation (laddering) in theca and granulosa layers obtained from the frozen area and in the theca layer obtained from the follicular wall distal to the frozen area. In contrast, apoptosis was only present in theca and granulosa layers in the frozen area of the nonGDRX follicle. Furthermore, the in situ DNA end-labeling technique demonstrated that in the GDRX follicle 24 h after treatment, cells in the theca interna, endothelial cells in blood vessels of the theca externa, and a few granulosa cells underwent apoptosis. These results indicate that destruction of the GDR of an immature preovulatory follicle causes atresia and apoptosis and blocks ovulation. These novel findings suggest that the GDR maintains development of the chicken preovulatory follicle by producing one or more survival factors. Without the GDR, chicken follicles cannot develop further and they eventually die.

Epidermal growth factor in the germinal disc and its potential role in follicular development in the chicken

The germinal disc region (GDR; germinal disc + overlying granulosa cells) of the hen's ovarian follicle secretes one or more factors that stimulate proliferation of, and decrease progesterone (P4) production by, granulosa cells. Destruction of the GDR results in apoptosis and atresia of the follicle. These data suggested that the GDR produces a growth factor(s) to sustain the development of the follicle. These findings prompted us to investigate two questions: 1) Is epidermal growth factor (EGF) or transforming growth factor alpha (TGFalpha), which binds to the EGF receptor, present in the GDR? 2) Does EGF regulate granulosa cell functions in the hen? Immunocytochemistry revealed that EGF, but not TGFalpha, was present in the germinal disc of the four largest preovulatory follicles of the hen. TGFalpha was found only in the theca interna. To determine whether EGF regulates granulosa cell functions, granulosa layer explants (13 mm in diameter) from the second-largest preovulatory follicle were cultured for 36 h with 0,0.017, or 0.17 microM EGF. Proliferation, apoptosis, and P4 production of granulosa layer explants were then measured by using a colorimetric method for determining viable cell number, gel electrophoresis, and RIA, respectively. EGF regulates several functions of granulosa layer explants by stimulating proliferation, inhibiting apoptosis, and decreasing basal P4 production. These data indicate that EGF is present in the germinal disc and may be one of the factors that regulate follicular development in the hen.

Two 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 maturation in the chicken ovary. However, the preferred steroid metabolic pathway in the theca and granulosa layers of small and preovulatory follicles 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 follicular hierarchy did not use cholesterol but did use pregnenolone (P5) to produce testosterone and estradiol. For this reason, P5 was used as the substrate for steroid production in the present study. The theca layer of small follicles (2-4 mm in diameter), and the theca and granulosa layers of the fifth largest (F5), third largest (F3), and largest (F1) preovulatory follicles were isolated from the chicken ovary 22 hr before ovulation. The granulosa layer of small follicles was excluded from the study because it is steroidogenically incompetent. Tissues were incubated with [3H]-P5 for 1 hr at 37 degrees C. Production of steroid metabolites was determined on the basis of comparison with known steroid standards using two-dimensional thin layer chromatography (2D-TLC). Conversion of [3H]-P5 to [3H]-DHEA was greater than that of [3H]-P4 in the theca layer of small and preovulatory follicles. Even though the amount of [3H]-P4 was less than that of [3H]-DHEA, theca cells from F3 and F1 follicles did produce [3H]-P4. In contrast to the theca layer, conversion of [3H]-P4 was greater than that of [3H]-DHEA in the granulosa layer of preovulatory follicles. Conversion of [3H]-P5 to [3H]-DHEA by the granulosa layer gradually decreased as the follicle matured and approached ovulation. We 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 metabolizes steroids via the delta 4 pathway.