ORGANIZATION OF PRIMARY AFFERENT AXONS IN THE TRIGEMINAL SENSORY ROOTAND TRACT OF THE RAT

A combination of immunocytochemical and electron microscopic methods w ere employed to assess the organization of the trigeminal (V) spinal t ract in adult rats. Immunostaining was employed at the light microscop ic level to selectively label large myelinated (by using antibodies ag ainst neurofilament protein) and small unmyelinated (by using antibodi es against calcitonin gene-related peptide) primary afferents. In addi tion, the plant lectin Bandeiraea simplicifolia-I was employed to hist ochemically label small unmyelinated primary afferents. Results from t hese experiments indicated that larger myelinated axons were distribut ed throughout the cross-sectional extent of the V spinal tract (TrV), whereas smaller fibers were most numerous just below the pial surface. These results were confirmed with quantitative electron microscopy wh ich demonstrated that the central portion of the V sensory root and Tr V were composed primarily of larger myelinated fibers, whereas the per iphery of the root and the portion of TrV just below the pial surface contained a higher percentage of smaller myelinated and unmyelinated a xons. When considered together with results regarding the birthdates o f neurochemically defined classes of V ganglion cells (White et al. [1 994] J. Comp. Neurol. 350:397-411), these results suggest that TrV is laid down in a chronotopic fashion with the first axons forming its de eper portion and later arriving axons being added more superficially. (C) 1996 Wiley-Liss, Inc.