Is guidance of chick retinal axons in vitro influenced by proteases?

Rhodamine-labeled explants of embryonic chick retinae were placed on a substratum consisting of alternating lanes of cell membranes derived from anterior and posterior chick optic tectum. Extending axons from temporal retinae prefer to grow in vitro on anterior tectal membranes because of a repulsive component within posterior membranes obtained from a tectum area which is not innervated by these axons in vivo. None of 16 protease inhibitors, specific for all known protease classes, when added to the culture medium, neutralized the repulsive activity of posterior membranes suggesting that the repulsive activity is not a protease. However, 2 metalloprotease inhibitors affect growth cone morphology and axon-axon interactions.