DIFFERENTIAL ADHESION AND THE INITIAL ASSEMBLY OF THE MAMMALIAN OLFACTORY NERVE

During the initial assembly of the olfactory pathway, the behavior of olfactory axons changes as they grow from the olfactory epithelium tow ard the telencephalic vesicle. The axons exit the epithelium singly or in small fascicles, and their growth cones are simple and bullet-shap ed. Outside the epithelium, they make a sharp dorsal turn and fascicul ate into a single nerve; the growth cones remain simple. Upon entering the ventromedial telencephalon, the axons defasciculate, branch exten sively, and end in complex, lamellate growth cones which extend toward the ventrolateral aspect of the telencephalic vesicle. The distributi on of laminin, collagen-IV, and fibronectin varies in register with th ese changes in olfactory axon and growth cone behavior. Each of these extracellular matrix molecules influences olfactory neurite outgrowth and growth cone morphology in vitro consistent with its distribution i n vivo. The distribution of E-cadherin, L1, neural cell adhesion molec ule (NCAM) and the polysialated form of NCAM also varies in register w ith changes in olfactory axon behavior. In vitro, L1 modulates embryon ic olfactory neurite outgrowth and growth cone morphology consistent w ith its distribution in vivo. Thus, olfactory axon trajectory, fascicu lation, and growth cone morphology change within distinct adhesive env ironments in the nascent olfactory pathway, and some of the molecules that characterize these environments have differential effects upon ol factory neurite growth and growth cone morphology. Consequently, the p atterned expression and activity of extracellular matrix and cell surf ace adhesion molecules may contribute to the initial assembly of the o lfactory pathway. (C) 1996 Wiley-Liss, Inc.