6

Directional Cues

in Angiogenesis

by Arie Horowitz

1.Introduction: Blood Vessels and Nerves Use Similar Guidance Cues

The investigation of guidance cues in the vascular system is a relatively new field — the first studies focusing on this topic came out only in the last six years. Guidance cues were no longer a new subject at that time, however, since they had been discovered earlier in the nervous system (reviewed in Ref. 1) and studied since intensively. It has become soon evident, however, that both systems largely use the same types of ligands and receptors for guidance. This commonality is not entirely surprising if one considers the morphological similarities between the two systems: they are both made up of a branching network which conducts bidirectionally either blood or electric signals, namely, arteries versus veins in the vascular system and sensory versus motor nerve fibers in the nervous system. Furthermore, the branches of both the vascular and the nervous systems are bi-layered and each is composed of two types of cells: mural and endothelial, or glia and neurons, respectively.

The structural similarity between the nervous and vascular systems extends to the growing neuron and the sprouting capillary. Axon

147