Expression patterns of PSA-NCAM in the human ganglionic eminence and its vicinity: role of PSA-NCAM in neuronal migration and axonal growth?

The ganglionic eminence (GE) is a transient but conspicuous structure of the developing forebrain which not only gives rise to a large number of precursor neurons and glial cells for various structures of the forebrain, but in addition serves as an intermediate target for growing axons to the cerebral cortex. To investigate the roles of the highly polysialylated isoform of the neural cell adhesion molecule (PSA-NCAM) in cell migration and axonal growth within the GE and its neighbouring structures, the spatio-temporal expression pattern of PSA-NCAM was examined in human fetal forebrains between 14 and 36 weeks of gestation with a specific immunohistochemical method. Scattered PSA-NCAM-positive cells were found in the centre but more frequently in the marginal zone of the GE. Intensely labelled cells were also identified in the gangliothalamic body, basolateral nuclei of the amygdala and the subventricular and intermediate zone adjacent to the GE. This cellular immunoreactivity started to appear in various structures during the period from 14 to 19 weeks and gradually diminished after 25-28 weeks. Strong immunoreactivity was also detected in fibres running from the intermediate zone of the neocortex to the internal capsule from 16 weeks onwards, and after 24 weeks, the immunoreactivity was gradually decreased. In the vicinity of the GE, between 16 and 22 weeks, short fibre bundles were observed to leave the longitudinally oriented axons of the internal capsule to reach the marginal zone of the GE. Our results suggest a close relationship between PSA-NCAM expression and neuronal migration (over short distances) and transitory axonal projections (target recognition and axonal fasciculation) in the region of the GE.