The formation of axonal pathways in developing cranial nerves

Abstract

The roles of a variety of molecules including cell adhesion molecules and growth factors in the development of cranial nerves have begun to be understood in detail. In the course of embryonic development, cranial nerves are differentiated in concordance with the development of the metameric facial structure called ‘ectomeres’. Each ectomere parallels the segmentation of the hindbrain called the ‘rhombomere’, in which pairs of metameric units cooperate to generate the repeating sequence of cranial branchiomotor nerves. A number of genes, including homeobox genes, are expressed in a rhombomere-specific pattern. For the formation of the olfactory nerve, it is suggested that several carbohydrate residues play important roles in receptor-target specificity. In the optic nerve, a combination of multiple cell adhesion molecules contributes to neurite growth in a developmental stage-specific manner. The development of the trigeminal nerve is under the control of both cell adhesion molecules and several growth factors. There is evidence that some of the adhesion molecules are expressed in a modality-specific way. There are also several molecules, such as 11p15 or TAG1/SNAP which are expressed only in selected cranial nerves. The growth rate of neurites also varies according to the individual nerves. Thus each cranial nerve has its own intrinsic properties and their outgrowth is the outcome of these properties and their interactions with surrounding non-neuronal tissues.