Synaptophysin immunocytochemistry with thermal intensification: a marker of terminal axonal maturation in the human fetal nervous system

Synaptophysin is a protein of synaptic vesicles and may be demonstrated in tissue sections of human brain and spinal cord by immunocytochemistry using a monoclonal antibody. Synaptophysin immunoreactivity was studied in paraf fin-embedded sections of the central nervous system (CNS) in 14 normal huma n fetuses and neonates ranging in age from 8 to 41 weeks gestation, and in three brains with heterotopic neurons or malformations. A progressive expre ssion of synaptophysin is seen in axonal terminals within grey matter in va rious parts of the CNS, beginning in the ventral horns of the spinal cord a nd brainstem tegmentum at 12-14 weeks. In the cerebellum, the molecular lay er shows a band of reactivity from 18 weeks; by term two parallel bands of synaptophysin are seen in the molecular layer and reactivity also is demons trated in the Purkinje and internal granular layers. In the cerebral neocor tex, the molecular zone has weak synaptophysin reactivity as early as 10 we eks, though reactivity is not detected in the deep layers of the cortical p late until 19 weeks and in layers 2-4 until 25 weeks gestation. Synaptophys in reactivity is strong at the surface of neurons but not detected in their somatic cytoplasm; coarsely beaded reactivity within the neuropil probably corresponds to synaptic vesicles in terminal axone. Similar granular synap tophysin reactivity is seen around heterotopic neurons in the subcortical w hite matter, in dysgenesis of the cerebellar cortex and in the residual ane ncephalic forebrain. Thermal intensification by heating the incubating solu tion in a microwave oven often enhances immuno-reactivity because of more c omplete antigen retrieval and is recommended for tissue stored in formalin or in paraffin for long periods. Synaptophysin provides a useful tissue mar ker of synaptogenesis during normal development and in cerebral dysgeneses, and may provide useful correlations with functional imaging of the brain i n living patients. Used in conjunction with other neuronal markers, the exp ression of synaptophysin in terminal axons of distant neurons, in temporal relation to the maturation of the neurons they innervate, may provide dues to the pathogenesis of epilepsy in early infancy. (C) 1999 Elsevier Science B.V, All rights reserved.