GUIDANCE OF THALAMOCORTICAL AXONS BY GROWTH-PROMOTING MOLECULES IN DEVELOPING RAT CEREBRAL-CORTEX

Substrate-bound guidance cues play an important role during the develo pment of thalamocortical projections. We used time-lapse video microsc opy to study the growth behaviour of thalamic axons on different subst rates. On embryonic cortical membranes and on a pure laminin substrate , thalamic fibres advanced relatively slowly (similar to 15 mu m/h) an d on average their growth cones retracted transiently every similar to 5 h. In contrast, on membranes prepared from early postnatal cortex, thalamic fibres grew twice as fast and spontaneous growth cone collaps e occurred similar to 8 times less often. Experiments in which we used the sugar-binding lectin peanut agglutinin or heat inactivation to ch ange the membrane properties indicated that these differences are due to growth-supporting molecules on postnatal cortical membranes. When o ffered a choice between embryonic and postnatal cortical membranes, th alamic axons preferred the postnatal membrane substrate. Time-lapse im aging revealed that borders between these two substrates effectively g uided thalamic fibres, and in most cases axons changed their direction without collapse of the growth cone. Our results suggest that thalami c axons can be guided by the spatial distribution of growth-promoting molecules in the developing cortex.