Expression pattern of messenger ribonucleic acid for follistatin and the inhibin/activin subunits during follicular and testicular development in Gallus domesticus

The expression of mRNA for follistatin and the inhibin/activin subunits was investigated in the follicles of laying hens and the testes of immature and mature roosters. Total RNA was isolated from immature and mature testes and from individual granulosa layers of the F-1 through F-5 follicles, from a pool of the F-6-F-8 follicles, from the small yellow follicles, and from the combined granulosa and theca layers of the targe white follicles. Northern blot analysis was performed, and a follistatin mRNA transcript of approximately 2.4 kilobases (kb) was detected in immature testes and in the following follicles: F-5, the pool of F-6-F-8, and small yellow and large white follicles. The greatest expression of follistatin occurred in the small yellow follicles, and in this tissue two minor transcripts of approximately 1.7 and 3.7 kb were also detected. The inhibin alpha subunit was expressed in both testes samples and in all of the follicles except the large white follicles. Expression of the alpha subunit was greatest in the F-5 follicle, and expression decreased with follicle maturity. As previously reported, the inhibin/activin beta(A) subunit was found in the greatest abundance in the F-1 follicle, with lesser amounts detected in the other hierarchical follicles and immature testes. In contrast to the beta(A) subunit, the inhibin/activin beta(B) subunit was not detected in the four largest hierarchical follicles but was expressed in greatest abundance in the pool of F-6-F-8 and small yellow follicles. This represents the first report, to our knowledge, of the detection of follistatin mRNA in the hen ovary and rooster testes. The restriction of follistatin mRNA expression to the small follicles suggests that follistatin, by regulating activin and/or inhibin availability, may play a critical role in early follicular development.