ROLES FOR GROWTH AND DIFFERENTIATION FACTORS IN AVIAN EMBRYONIC-DEVELOPMENT

We review the evidence for a role for transforming growth factor-beta (TGF-beta) and for tumor necrosis factor-alpha (TNF-alpha) in the deve lopment of the avian embryo. Transforming growth factor-beta is expres sed in a number of locations in the early embryo with a distribution c onsistent with function in epithelial-mesenchymal transformation and m odulation of the composition of the extracellular matrix. During gastr ulation, this factor is found in the mesoderm cell layer as well as in the endoderm underlying the primitive streak. In vivo and in vitro in vestigations suggest that TGF-beta may be involved in the regulation o f phenotypic transformation, matrix deposition, and cell proliferation . Tumor necrosis factor-alpha and its two receptors are also located w ith distributions that suggest important involvement for this pleiotro pic factor in early morphogenetic processes. Tumor necrosis factor-a i s found in several cell populations from the time of gastrulation onwa rds, including the lens. In vitro investigations, using tissue from th e gastrulating embryo as well as from the lens, suggest that this fact or may be associated with the extensive cell death that occurs through out the first 6 d of development, and with nuclear degeneration in the lens. We hypothesize that TNF-alpha, acting in a paracrine or autocri ne fashion, may be involved in the signalling pathways that effect the regulation of cell death in development.