Abstract
A monoclonal antibody, 4D7, obtained with embryonic rat brain as an immunogen, recognizes an epitope on 3 protein species of 150–160, 100– 110, and 80 kDa, present in mouse and rat brain during the fetal period. Vital immunostaining of dissociated cultures of fetal forebrain indicates that the antigen is localized largely on the external plasma membrane of a subpopulation of neurites. Immunocytochemistry reveals that the distribution of the antigen in vivo is restricted to the nervous system. Immunoreactivity is concentrated primarily in the pathways of a limited set of CNS and PNS axon systems during early stages of their development, as delineated by staining with the neurofilament antibody, C2. Depending on the particular axon system, immunoreactivity with 4D7 persists only for one to several days of prenatal or perinatal development. In the spinal cord, stage-specific- neurite-associated proteins (SNAP) expression occurs first along motor axon pathways on embryonic day (E) 10 and then within the nerve trunks of dorsal root ganglia and the commissural fiber system on E11. Immunoreactivity is detectable among most cranial nerves starting in the interval from E11 to E13. Within the brain, the onset of SNAP expression within several discrete axon tracts occurs in the interval E14–16, including the lateral olfactory tract, anterior commissure, corpus callosum, fasciculus retroflexus, and fornix. Immunoreactivity within the embryonic intermediate zones of some structures matches the location of certain other axon systems. Sites of 4D7 staining which do not correspond to the location of axon populations include the internal portion of the external granular layer of the postnatal cerebellum and the cortex of the reeler mutant mouse. The predominant localization of the 4D7 antigen among axon systems and its precisely regulated spatio- temporal pattern of expression are consistent with the possibility that the SNAP antigens play a significant role in the early stages of growth of axonal tracts in vivo.
