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.