6.3 Directing the growth cone
The growth of the growth cone has been likened to the progress of a climber. The climber can only go where there are satisfactory hand and foot-holds and where progress is not blocked by physical obstacles (e.g. overhangs or ice). Furthermore, the climber is looking ahead for the best routes, from the current position to the top. Likewise the filopodia on the growth cones are extending outwards, adhering to the best holds and avoiding physical obstacles (e.g. bone or cartilage). This contact-mediated guidance in which immediate (proximal) cues are used is called chemotactic guidance. The filopodia are also responsive to more distant cues. The climber uses vision as a distance sense to select the best routes, but this sensory modality is not available to the growth cone. Instead the receptors on the growth cone are responsive to certain molecules that diffuse through the extracellular matrix. These molecules are called chemotropic factors, and they may be attractants or repellents. A useful analogy for the interaction between growth cones and attractants, is the sniffer dog. The sniffer dog (growth cone) detects and follows a particular smell (attractant). In so doing, it advances towards the source of a particular smell, the target. For sniffer dogs the target may be explosives or a missing person. For growth cones the target may be a group of muscle cells, a gland or a sensory receptor. The target releases the attractant and the growth cone advances towards the source of the attractant and so towards the target.