Tenascin in the developing chick visual system: distribution and potential role as a modulator of retinal axon growth

The distribution of the extracellular matrix protein tenascin was studied in the developing chick visual system to determine its possible regulatory role in retinotectal development. Little tenascin was present in the retinal optic fiber layer, and the optic nerve and tract, but was abundant in the stratum opticum of the tectum, the target of retinal axons in the brain. A high concentration of tenascin was found in areas bordering the developing visual pathway, such as the optic disc, the outer surface of the optic nerve, and the supraoptic commissure. In vitro studies showed that tenascin did not promote neurite outgrowth of retinal axons. When optic axons were confronted with a tenascin substrate in culture, they did not grow onto the tenascin suggesting that this protein inhibited optic axon outgrowth. Furthermore, the addition of tenascin to retinal explants in collagen gels slowed the growth rate of optic axons by as much as 50%. The distribution of tenascin in vivo and its inhibitory function in several in vitro systems suggest that this protein acts as a modulator of axonal growth in vivo. Tenascin may act as a barrier at specific sites along the visual pathway, and at the target, may slow the rate of axon outgrowth, and ultimately act as a stop molecule. The growth inhibitory activity of tenascin in retinal and tectal synaptic layers may also serve to stabilize synapses once appropriate connections have been made.