Astrocytes and guidance of outgrowing corticospinal tract axons in the rat. An immunocytochemical study using anti-vimentin and anti-glial fibrillary acidic protein

Abstract

In the present investigation the role of astrocytes and their precursors in guidance of outgrowing corticospinal tract axons in the rat is studied. Antibodies against glial fibrillary acidic protein and vimentin are used to analyse immunogen expression of glial cells, whereas the postnatal outgrowth of corticospinal tract axons through the spinal cord was studied using anterogradely transported horseradish peroxidase. The first, leading corticospinal tract axons, being the objective of the present study, are characterized by dilatations at their distal ends, the growth cones. Growth cones of pioneer corticospinal tract axons are randomly distributed in the presumptive corticospinal tract area of the ventral most part of the dorsal funiculus. A dramatic change in glial cell labelling is found from the majority being vimentin immunoreactive and glial fibrillary acidic protein-negative at birth to almost all being the reverse at the end of the fourth postnatal week. From double labelling experiments it can be concluded that the vimentin-glial fibrillary acidic protein transition occurs within astrocyte precursor cells. The absence of glial fibrillary acidic protein-immunoreactive glial cells during the outgrowth period of pioneer corticospinal tract axons indicates that they cannot play a role in the guidance of outgrowing corticospinal tract pioneer axons. Vimentin-immunoreactive glial cells are present throughout the presumptive corticospinal tract area at the time of arrival of the leading corticospinal tract fibres. The vimentin-immunoreactive glial cells, which themselves are orientated perpendicular to the outgrowing corticospinal tract axons, are mainly arranged in longitudinal tiers parallel to the rostrocaudal axis. Electron microscopically, growth cones of pioneer corticospinal tract axons frequently exhibit protrusions into vimentin-immunoreactive glial cell processes, suggesting an adhesive type of contact. Therefore, in addition to a positional role, vimentinimmunoreactive glial cells probably play a chemical role in guidance of pioneer corticospinal tract axons. A prominent vimentin-immunoreactive glial septum was noted during corticospinal tract outgrowth in the midline raphe of the medulla oblongata and spinal cord whereas it is absent in the decussation area of corticospinal tract fibres. After the first postnatal week the major vimentin-immunoreactive glial barrier either completely disappears (medullary levels) or gradually reduces to a minor glial fibnllary acidic protein-immunoreactive one (spinal cord levels). This glial septum is suggested to play a physical role in guidance of outgrowing corticospinal tract axons in preventing them from decussation. © 1989.