Pattern of development of ascending and descending fibers in embryonic spinal cord of chick: I. Role of position information

Abstract

Previous studies on the rigid time and position pattern of embryonic spinal cord development are extended to include the pattern of growth of axonal processes. This study was designed to analyze the position of axons in the marginal zone as they course (fasciculate) in the ascending and descending direction from a single spinal segment. Injections of 3H-proline were made into L1 or T6 of chick embryos of stages 21--29 (3.5--6 days of incubation). The location of radioactive label in the marginal zone was analyzed autoradiographically to show the location of labeled axons originating from the injected segment. In the early stages of development (stage 21), the labeled descending axons were located in the most ventromedial aspect of the marginal zone. The labeled ascending axons were located more laterally and to the perimeter of this zone. In stages 27--29, the labeled axons near the site of injection were located in the inner two-thirds of the marginal zone, and in the progressively more rostral levels the label shifted laterally and to the perimeter in a laminar pattern. It was concluded that the labeled ascending axons from an individual spinal segment layer upon the axons which originate from the more rostral segments. How this rigid time and position pattern of axonal growth can impart a topographic order into fiber tracts of the spinal cord was discussed.

Formation of alternating tiers in the optic chiasm of the chick embryo

“…A chronotopic organization is also suggested for the chiasm of lower vertebrates and for mammals (Scholes, 1981;Guillery, 1982;Walsh, 1986;Reese, 1987;Taylor, 1987;Wilson et al, 1988). Even in other systems fibers seem to grow following the same rules described for the spinal cord (His, 1904;Nornes et al, 1980;Holley, 1982;Nordlander and Singer, 1982;Taylor and Roberts, 1983) and for the rhombencephalon (Kevetter and Lasek, 1982). Thus, the chronotopic organization of fiber pathways seems to be a more general phenomenon in the nervous system (Silver at al., 1987) and is perhaps based on similar mechanisms in different systems (Bernhardt and Easter, 1986).…”

Organization of the optic chiasm in the hatched chick

“…The pattern of alternating tiers is widely distributed and homologous organizations can be found in various inframammalian species. The arrangement of axons into distinct layers is not confined to the optic chiasm; it can also be seen in some other pathways of the central nervous system as in the spinal cord (His, 1904;Nornes et al, 1980;Holley, 1982;Nordlander and Singer, 1982;Taylor and Roberts, 1983) or in the rhombencephalon (Krevetter and Lasek, 1982). This indicates that with the organization of the chiasm into alternating tiers, a more general ordering principle for fiber pathways is realized.…”

The axonal projections of the Hofmann nuclei in the spinal cord of the late stage chicken embryo

“…All commissural spinal interneurons decussate through the ventral commissure, typically within the same segment as the parent soma. Once the commissural axon reaches the other side, the initial longitudinal trajectory, if present, is typically in the ventral white matter (Nornes et al 1980). In the case of ascending axons, the trajectory is typically oblique and shifts from a medial to a lateral position over the first several proximal segments (Nornes et al 1980).…”