Roles for Growth and Differentiation Factors in Avian Embryonic Development

We review the evidence for a role for transforming growth factor-β (TGF-β) and for tumor necrosis factor-α (TNF-α) in the development of the avian embryo. Transforming growth factor-β is expressed in a number of locations in the early embryo with a distribution consistent with a function in epithelial-mesenchymal transformation and modulation of the composition of the extracellular matrix. During gastrulation, this factor is found in the mesoderm cell layer as well as in the endoderm underlying the primitive streak. In vivo and in vitro investigations suggest that TGF-β may be involved in the regulation of phenotypic transformation, matrix deposition, and cell proliferation. Tumor necrosis factor-α and its two receptors are also located with distributions that suggest important involvement for this pleiotropic factor in early morphogenetic processes. Tumor necrosis factor-α is found in several cell populations from the time of gastrulation onwards, including the lens. In vitro investigations, using tissue from the gastrulating embryo as well as from the lens, suggest that this factor may be associated with the extensive cell death that occurs throughout the first 6 d of development, and with nuclear degeneration in the lens. We hypothesize that TNF-a, acting in a paracrine or autocrine fashion, may be involved in the signalling pathways that effect the regulation of cell death in development.