THE EXTRACELLULAR-MATRIX MOLECULE TENASCIN - EXPRESSION IN THE DEVELOPING CHICK RETINOTECTAL SYSTEM AND SUBSTRATE PROPERTIES FOR RETINAL GANGLION-CELL NEURITES IN-VITRO

To investigate the molecular mechanisms involved in the outgrowth of r etinal ganglion cell axons in the tectum, the expression of the extrac ellular matrix molecule tenascin was analysed in the tectum and retina of chickens by immunocytochemistry and in situ hybridization. Tissue was analysed between embryonic days 4 and 12, just before and during t he period when retinal ganglion cell axons innervate their target regi on, the optic tectum. In the tectum, tenascin immunoreactivity becomes detectable at the anterior pole at embryonic day 4, 2 days before ret inal ganglion cell axons arrive, and spreads caudally with increasing age, At early stages, tenascin is predominantly accumulated in the str atum opticum, the zone of ingrowing retinal ganglion cell axons, and a long their prospective pathway. In the stratum opticum, the molecule i s associated with radial glial fibres, glial endfeet and retinal gangl ion cell axons located in the immediate neighbourhood of radial glial fibres. At all ages investigated, tenascin mRNA is mainly restricted t o cells located in the periventricular region, suggesting that the mol ecule is synthesized by radial glial cells, In the retina, tenascin is expressed by amacrine, displaced amacrine and horizontal cells but no t by retinal ganglion cells. To investigate whether the accumulation o f tenascin in the developing and prospective pathway of retinal gangli on cell axons may affect their rate of growth we assayed the substrate properties of tenascin for retinal ganglion cell neurites in vitro. W hen retinal ganglion cell suspensions from 6-day-old chick embryos wer e maintained on homogeneous mouse or chick tenascin/polyornithine subs trates, neurite length was significantly increased when compared to po lyornithine substrates at coating concentrations of 10 or 20 mu g/ml. Higher coating concentrations (35 or 70 mu g/ml) resulted in neurite l engths comparable to control values. Together, these observations sugg est that tenascin in the developing and prospective stratum opticum mi ght serve as a preformed pathway to support growth of retinal ganglion cell axons in the tectum.