Be. Reese et al., CHRONOTOPIC FIBER REORDERING AND THE DISTRIBUTION OF CELL-ADHESION AND EXTRACELLULAR-MATRIX MOLECULES IN THE OPTIC PATHWAY OF FETAL FERRETS, Journal of comparative neurology, 380(3), 1997, pp. 355-372
We have examined the age-related reordering of optic axons as they pas
s through the chiasmatic region in fetal ferrets. Proportions of young
and old optic axons were determined from electron micrographs taken s
equentially through the prechiasmatic nerve, chasm, and tract. This ''
chronotopic'' reordering of axons was shown to emerge gradually, begin
ning rostral to the fusion of the two optic nerves, but continuing to
develop caudal to the chiasmatic midline. Segregation of young from ol
d optic axons was most pronounced within the optic tract. We then comp
ared the emergence of this fiber reorganization to the distribution of
cell adhesion and extracellular matrix molecules and to the glial arc
hitecture within the pathway. Using immunohistochemistry, the distribu
tions of the cell adhesion molecules L1, NCAM, and TAG-1 and the extra
cellular matrix molecules laminin-l and chondroitin sulfate proteoglyc
ans (CSPGs) were determined. Among these, only the distribution of CSP
Gs was observed to change in a manner that complemented the segregatio
n of young from old optic axons. CSPGs were densest in the deeper part
s of the optic tract, coincident with radial glial fibers that turn to
course within the region of the oldest optic axons. Both the glial ar
chitecture and the CSPG distribution form as a consequence of the inva
sion of the first optic axons, shown by the developmental sequence of
each, and by the fact that these glial and molecular features fail to
form in the absence of optic axons. The data suggest a model in which
the gradient of CSPGs across the depth of the tract contributes to the
formation of the chronotopic fiber reordering by providing a relative
ly unfavorable environment for subsequent axonal growth. The CSPGs may
do so by interfering with adhesion molecules on optic axons that norm
ally promote elongation. (C) 1997 Wiley-Liss, Inc.