While axon fasciculation plays a key role in the development of neural networks, very
little is known about its dynamics and the underlying biophysical mechanisms. In a model system
composed of neurons grown ex vivo from explants of embryonic mouse olfactory epithelia, we
observed that axons dynamically interact with each other through their shafts, leading to zippering
and unzippering behavior that regulates their fasciculation. Taking advantage of this new
preparation suitable for studying such interactions, we carried out a detailed biophysical analysis of
zippering, occurring either spontaneously or induced by micromanipulations and pharmacological
treatments. We show that zippering arises from the competition of axon-axon adhesion and
mechanical tension in the axons, and provide the first quantification of the force of axon-axon
adhesion. Furthermore, we introduce a biophysical model of the zippering dynamics, and we
quantitatively relate the individual zipper properties to global characteristics of the developing
axon network. Our study uncovers a new role of mechanical tension in neural development: the
regulation of axon fasciculation.