Journal of Neuroscience, Vol 10, 3118-3134, Copyright © 1990 by Society for Neuroscience
The avian tectobulbar tract: development, explant culture, and effects of antibodies on the pattern of neurite outgrowth
S Kroger and U Schwarz
Max-Planck-Institut fur Entwicklungsbiologie, Tubingen, Federal Republic of Germany.
The tectobulbar tract is the first long-distance projecting fiber pathway
to appear during the development of the avian optic tectum (dorsal half of
the mesencephalon). Immunologically stained wholemounts of the E3
mesencephalon reveal that the early tectobulbar axons course in a
dorsal-to-ventral direction and abruptly turn in a caudal direction shortly
before reaching the ventral midline. During subsequent development, more
tectobulbar axons are generated that form a parallel array of thick
fascicles coursing ventrally within the mesencephalon. At this later stage
of development, the tectobulbar tract bifurcates into an ipsilateral and
contralateral pathway, both growing in a caudal direction near the
mesencephalic ventral midline. Bifurcation and change in direction of
growth is accompanied by a complete loss of the fasciculated growth
pattern. Each tectobulbar axon is thus divided into a proximal fasciculated
and a distal unfasciculated segment. Tectobulbar fascicles occupy the most
superficial surface layer of the mesencephalon at early stages and are
displaced deeper into the tissue beginning at embryonic day 5. In both of
these locations, tectobulbar axons express molecules involved in axon-axon
and axon-substrate interactions like the G4 antigen, neural cell adhesion
molecule (N-CAM), neurofascin, and T61 antigen as revealed by
immunohistochemistry and immunoblotting. Stripes of the mesencephalon
explanted onto a basal lamina substratum show vigorous outgrowth of
neurites. These processes grow in fascicles at a growth rate of 40
microns/h. Staining of the neurites with specific antibodies, as well as
the position of the retrogradely labeled cell bodies, is in agreement with
these processes being tectobulbar axons. This in vitro explant system was
used to investigate the expression and possible functional involvement of
N-CAM, neurofascin, G4 protein, and T61 antigen in the growth of these
axons. The presence of antigen- binding fragments of polyclonal anti-G4
antibodies completely blocks fasciculation of the neurites but has no
influence on their rate of elongation. Antibodies against N-CAM and
neurofascin have no detectable effects. The number and length of the in
vitro growing axons are reduced by the monoclonal T61 antibody. This effect
is reversible. The elucidation of the exact course in vivo and the
accessibility to the axons growing in vitro make the tectobulbar tract an
excellent model system for the investigation of the role of these and other
proteins in axonal growth and guidance during the development of the CNS.
Previous Article | Next Article
