Changes in axon arrangement in the retinofungal pathway of mouse embryos: Confocal microscopy study using single- and double-dye label

The changes in quadrant-specific fiber order in the retinofugal pathway of the C57-pigmented mouse aged embryonic day 15 were investigated by using si ngle- (1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate; DiI) and double- (N-4-4-didecylaminostyryl-N-methylpyridinium iodide; 4Di- 10ASP in addition to DiI) labeling techniques. At this earliest stage of de velopment, before any fibers arrive at their targets, retinal axons display a distinct quadrant-specific order at the optic stalk close to the eye. Th is order gradually disappears along the stalk and is virtually lost at the chiasm, as shown in single-label preparations. The double-label preparation s, in which the population peaks of fibers from two retinal quadrants are s hown simultaneously in an image, show a fiber arrangement at the chiasm tha t is different from the pattern seen in the single-label preparations. A di stinct and consistent preferential distribution of fibers from different re tinal quadrants is shown in the chiasm. Before the midline, the central par t of the cross section of the chiasm is dominated by dorsal fibers, whereas the rostral and caudal parts of the chiasm are dominated by ventral nasal and ventral temporal fibers, respectively. Moreover, the double-label prepa rations demonstrate a major reshuffling of fiber position after the fibers cross the midline. Fibers from ventral retina are shifted gradually to a ro stral position at the threshold of the optic tract, whereas fibers from dor sal retina are shifted caudally. These changes in fiber position indicate a postmidline location in the chiasm, where fibers are re-sorted in accordan ce with their origins in the dorsal ventral axis of the retina, and suggest a change in axon response to guidance signals when the fibers cross the mi dline of the chiasm. These changes in fiber order may also be related to th e re-sorting of fibers according to their ages at the postmidline chiasm. J . Comp. Neurol. 406:251-262, 1999. (C) 1999 Wiley-Liss, Inc.