The molecules secreted by the floorplate that elicit this response are called netrins. Netrins were identified biochemically in vertebrates and genetically in C. elegans . Netrins are a family of laminin-related proteins secreted into the extracellular matrix. These molecules help to attract or repulse growth cones. UNC-6 is expressed in the ventral part of the worm.
Different neurons respond differently to each neurotropin.
A given neuron may be attracted, be repulsed, or not respond to a certain neurotropin. In many parts of the vertebrate nervous system, over half of the neurons die during normal development. A target tissue can regulate the number of neurons that innervate it by limiting the supply of a neurotropin. Neurotropins can function both as chemotactic factors and survival factors. Different neurons require different neurotropins.