Immunoelectron microscopic localization of cell adhesion molecule L1 in developing rat pyramidal tract

Abstract

The glycoprotein L1 is a cell adhesion molecule that has been proposed to function in the peripheral nervous system in axon fasciculation and onset of myelination. In this report we localize L1 during the development of a major central pathway: the pyramidal tract. The (sub)cellular localization of L1 was determined both by pre-embedding staining on Vibratome sections and by immunogold labelling on ultracryosections in developing rat pyramidal tract at the fifth cervical segment. On arrival at the fifth cervical segment, i.e. at postnatal day 1, growth cones of pioneer fibres did not exhibit L1-immunoreactivity. In the contact zone between pyramidal tract growth cones and glial processes no L1-immunoreactivity was observed. A clear L1-immunoreactivity was noted on small unmyelinated other axons situated in the entrance area of the pyramidal tract growth cones. Also on later arriving, i.e. between postnatal days 2 and 10, small unmyelinated fasciculating pyramidal tract axons L1 were present. It is our impression that L1 is localized in an irregular patchy way on the outer side of the axonal membrane. During the onset of myelination, i.e. between postnatal days 10 and 14, L1 could not be detected on axons ensheathed by oligodendrocytic processes. When myelination is largely completed, i.e. at postnatal day 21, the L1 antigen could be localized within the axoplasma of both unmyelinated and myelinated pyramidal tract axons. Furthermore, L1 could be observed occasionally on small unmyelinated pyramidal tract axons. Whereas compact myelin was always L1-negative, L1 was noted periaxonally between the axolemma and compact myelin and at (para)nodal regions at the contact zone between axolemma and oligodendrocytic processes. From these results it may be deduced that: (1) L1 is involved in fasciculation of outgrowing later arriving pyramidal tract fibres; (2) L1 is not involved in the onset of myelination in this central tract; (3) L1 might play an additional adhesive role in myelinated rat pyramidal tract.