CRITICAL PERIOD-DEPENDENT REDUCTION OF THE PERMISSIVENESS OF CAT VISUAL-CORTEX TISSUE FOR NEURONAL ADHESION AND NEURITE GROWTH

During postnatal development, the visual cortex undergoes an experienc e-dependent refinement of its circuitry. This process includes synapse formation, as well as synapse elimination. Both mechanisms appear to be restricted to a limited 'critical period' which lasts for similar t o 2 months in cats. We tested whether the termination of the critical period for cortical malleability is paralleled by changes in the growt h permissiveness of the tissue. These changes may inhibit progressive reorganization of functional circuitries mediated by axon growth. Embr yonic cortical neurons were cultured on unfixed cryostat sections of t he visual cortex obtained from cats aged 2-50 weeks. After 2-3 days in vitro the distribution of viable cells and the percentage of neurite- bearing cells were determined and analysed with respect to the develop mental age and subdivisions of the underlying tissue substrate. It was shown that cell adhesion and neurite formation are correlated with th e developmental age of the substrate tissue and the time period of mye lination. While embryonic neurons adhered and survived on grey and whi te matter tissue from 2- and 4-week-old kittens, there was a significa nt reduction in cell adhesion on the myelinated white matter regions o f the tissue sections of older animals. Quantitative analyses showed t hat neurite formation by cultured neurons also became successively imp aired on grey and white matter areas of tissue substrates, correspondi ng to the time course of the critical period for cortical malleability . On grey matter tissue this effect was most pronounced between the se cond and sixth postnatal weeks. The effects were not antagonized by co ating the substrate sections with the growth-promoting molecule lamini n, It is therefore proposed that neurite growth-inhibiting factors, mo st probably associated with central nervous system myelin, are gradual ly expressed postnatally and may contribute to the termination of the critical period in the visual cortex of cats.