Anatomic distribution of the growth-associated protein GAP-43 in the developing human brainstem.

GAP-43 is a membrane phosphoprotein whose expression is high in neurons undergoing development or remodeling of axonal connections. This study used a monospecific antibody to GAP-43 to investigate the sequences of fiber tract elongation and synaptic development in the human brainstem. Immunocytochemistry was performed in 14 fetal and infant brainstems; two child and adult cases were also examined for comparison. At midgestation, GAP-43 immunostaining was moderately intense across nuclei and fiber tracts, except for the corticospinal tract, where levels were higher, and cranial nerve nucleus VII, superior olive, inferior colliculus, inferior olivary hilum, inferior cerebellar peduncle, medial lemniscus, and medial longitudinal fasciculus, where staining was nearly absent. By the end of the neonatal period, the relative distribution of GAP-43 immunostaining appeared well-established and similar, although not identical, to that in the child and adult brainstem. Immunostaining was absent or negligible in almost all the cranial nerve somato- and branchiomotor nuclei, auditory-relay nuclei, and vestibular nuclei, while remaining intense in visceral-related nuclei, reticular formation, cochlear nucleus, and periaqueductal gray. Staining was also virtually absent in all fiber tracts at birth, except for the corticospinal tract and central tegmental tract. Persistence of GAP-43 staining in the corticospinal tract past the fetal period suggests that this tract remains in a plastic state beyond initial axonal elongation. Intense immunostaining in visceral-related nuclei into adulthood suggests that these regions may continue to undergo synaptic reorganization. This study provides baseline information relevant to understanding developmental brainstem disorders in early human life.