We have compared the expression of the neural cell adhesion molecule (N-CAM) and the neuron-glial cell adhesion molecule (Ng-CAM) during histogenesis of the chick nervous system. Data from immunohistochemistry and photometry were combined to construct maps of the overall distribution and dynamics of CAM appearance and disappearance. Each CAM appeared in a characteristic spatial and temporal pattern in various areas during cell movement, fiber outgrowth, tract formation, and myelination. N-CAM was more uniformly distributed than Ng-CAM and was present on all neural cell bodies and processes of the CNS and PNS. In the adult, the staining pattern of N-CAM remained similar to that in the embryo, although the staining intensity was diminished. During embryonic development, Ng-CAM was expressed on extending neurites and migrating neurons. The appearance Ng-CAM in the CNS was correlated particularly with times of cell migration in spinal cord and cerebellum, and in regions undergoing neurite extension, such as the developing white matter of the spinal cord, the optic nerve, and the medial longitudinal fasciculus. Cell bodies not undergoing migration were negative for Ng-CAM. In the adult CNS, Ng-CAM was markedly decreased in myelinated fiber tracts like the white matter of the spinal cord but persisted in unmyelinated regions such as the olfactory bulb. In contrast, in the PNS (for example, the dorsal root ganglion and sciatic nerve), Ng-CAM appeared early on both cell bodies and neurites, and it continued to be present on both in the adult, even in the presence of myelin. Maps comparing the relative distribution of Ng-CAM and N-CAM showed dynamic reversals as the nervous system developed and, as a result, the pattern of CAM expression was markedly different in embryos and adults. This difference appears to reflect changes in the roles of selective adhesion and of the two neuronal CAMs at different times of development.

The migration of cerebellar granule cells from the external granular layer to the internal granular layer is mediated by the radical Bergmann glial fiber. Recent works have shown that cell adhesion molecules, extra-cellular matrix proteins and proteolytic enzymes or their activators are involved in this process. Immuno-localization studies showed differential temporal and spatial expression patterns of different adhesion molecules, their isoforms, and post-translational modification during different stages of granule cell migration. Functional perturbation experiments using cerebellar explant cultures demonstrated that several adhesion molecules as well as plasminogen activator are involved in granule cell migration and are required in different stages. Other systems used to study granule cell migration including dissociated microwell cultures and granule cell deficient mouse mutants are discussed in the context of adhesion molecules. The results accumulated so far suggest that the migration of granule cells is a complex process in which the cooperation of a group of molecules with different functions, some for adhesion some for de-adhesion, are required to fulfill the different needs during the migratory course.

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