Activin beta A- and beta B-subunit expression in the developing chicken bursa of fabricius.

The bursa of Fabricius (BF) is a site for B-lymphocyte maturation in birds. The BF is also known to function as an endocrine organ, with regulatory effects on steroid-secreting glands (testes, ovaries, and adrenal glands) during embryonic development. To study the possible involvement of the growth and differentiation factors, inhibins and activins, in bursal differentiation and function, the expression of inhibin/activin subunits in chicken BF was examined using specific antibodies against inhibin/activin alpha-, beta A-, and beta B-subunits. Bursae from chickens from 11 days of embryonic development until 22 weeks after hatching were studied. Immunoreactive beta A- and beta B-subunits were demonstrated in bursal epithelial cells throughout the time period studied. Immunoreactivities for both subunits were most intense and widespread from day 18 of embryonation until 1 week after hatching. Inhibin alpha-subunits were not detected. Partially different locations were shown for beta A- and beta B-subunits, suggesting different roles for activin-A (beta A beta A homodimer) and activin-B (beta B beta B homodimer) in bursal development and function. As activin beta A-subunit immunoreactivity was predominantly localized in medullary epithelia, it may be assumed that the developing B-cells within follicular medullae might be the target for activin-A action in the chicken BF. The most conspicuous site for activin-B production, on the other hand, was the follicle-associated epithelium, which is able to take up antigens from the cloacal environment and pass them to medullary cells. The data suggest that activin-A and -B may have different roles in modulating bursal microenvironment during B-lymphocyte differentiation. The possible role of bursal activins in the regulation of steroidogenesis in birds is discussed.