BASIC FIBROBLAST GROWTH-FACTOR INDUCES DIFFERENTIATION OF NEURAL-TUBEAND NEURAL CREST LINEAGES OF CULTURED ECTODERM CELLS FROM XENOPUS-GASTRULA

The vertebrate nervous system is initially induced from a section of d orsal ectoderm by signal(s) from the underlying dorsal mesoderm during gastrulation. In an effort to identify the neural inducing factor(s) emanating from the dorsal mesoderm, we have examined the inductive act ion of various growth factors by applying them to ectoderm cells from Xenopus gastrulae (8- to 12.5-hour age; embryonic stage 9+ to 11 1/2) in a microculture system. Monoclonal antibodies that specifically reco gnize cellular differentiation antigens from three distinct ectoderm l ineages (Nl for CNS neurons from neural tube, Mel for melanophores fro m neural crest and E3 for skin epidermal cells from epidermal lineages , respectively) and a mesoderm lineage (Mul for muscle cells) were use d as markers to monitor the differentiation of cultured ectoderm cells . We found that basic fibroblast growth factor (bFGF) was capable of s pecifically and reproducibly inducing gastrula ectoderm cells to produ ce CNS neurons and melanophores at concentrations as low as 5 pM, a va lue about 50-fold lower than that required to induce the formation of muscle cells from blastula animal cap cells (6-hour age; stage 8+). Th e induction of neural lineages by bFGF was correlated with a suppressi on of epidermal differentiation in a dose-dependent manner. bFGF never induced the formation of muscle cells from gastrula ectoderm cells ev en at concentrations as high as 5 nM. The response of ectoderm cells t o bFGF changed dramatically during gastrulation. Ectoderm cells from e arly (8- to 9-hour age; stage 9+ to 10) gastrula gave rise to CNS neur ons, but yielded few melanophores. As ectoderm cells were prepared fro m gastrulae of increasing age, their competence to form neurons was gr adually lost, whereas the production of melanophores was enhanced and peaked in 11-hour gastrula (stage 10 1/2). The ability to form both ne urons and melanophores was substantially reduced in 12.5-hour gastrula (stage 11 1/2). By examining ectoderm cells from the ventral and dors al sides independently, it was also shown that during gastrulation the change in response to bFGF of the ventral ectoderm preceded that of t he dorsal ectoderm. The state of competence of the ectoderm changed pr imarily due to intrinsic factors rather than by instruction from other parts of the gastrula embryo. This was shown by adding bFGF to cultur es of ectoderm cells that were isolated at 9-hour (stage 10) and cultu red for increasing periods to allow their autonomous development. The time course of both loss of neuronal competence and gain and loss of m elanophore competence closely paralleled that observed in vivo during gastrulation. Further, we showed that the sensitivity of ectoderm cell s to bFGF in the neuronal and melanophore induction processes decrease d during later gastrula stages; i.e., higher doses of bFGF were requir ed for older ectoderm cells to be fully induced. A possible role of bF GF in neural induction during normal development is discussed.