Immunocytochemical localization of neurofilaments in the fibre systems of the developing rat spinal cord white matter.

The appearance and localization of the protein subunits of neurofilaments in the ascending and descending fibre systems of the developing rat spinal cord white matter were studied. The monoclonal antibody NF-90 (specific for the phosphorylated NF-L, NF-M and NF-H subunits) was used as neurofilament marker in fresh cryostat and Bouin fixed paraffin sections. The results were compared with Nissl and Bodian stained sections. Within the white matter, phosphorylated neurofilament proteins were expressed with regional variations. At embryonal day 12 (E12), the first positive fibres were found in the lateral funiculus. During further development, the peripheral region of the lateral funiculus showed an intense neurofilament staining, due to the presence of a higher number of fibres. From postnatal day 12 (P12) on, an increased amount of neurofilaments was found in the region close to the periphery, probably due to the presence of large calibre fibres. The dorsolateral part of the lateral funiculus filled in with fibres after birth, which indicated the extended development of the rubrospinal tract. At E13, positive fibres were present in the ventral commissure and the ventral funiculus. At E14, an increased amount of neurofilaments was detected in the periphery of the ventral funiculus. At maturity, an intense staining in the subsurface region could be found, due to the presence of large calibre fibres of the fasciculus longitudinalis medialis. At E13, the first neurofilament positive fibres were present in the dorsal funiculus. At this day, a concentration of fibres was found in the dorsal part of the dorsal root bifurcation zone and three days later, more fibres were detected in the medial part of the dorsal funiculus. At E18, a higher number of fibres was present in the dorsal region of the fasciculus gracilis. The mature fasciculus cuneatus showed an intense neurofilament staining, which was mainly present in large calibre fibres. The ventral part of the dorsal funiculus filled in with neurofilament positive fibres after birth. This indicated the relative late arrival of the corticospinal tract.