Early development of the circumferential axonal pathway in mouse and chick spinal cord

Abstract

The early development of the circumferential axonal pathway in the brachial and lumbar spinal cord of mouse and chick embryos was studied by scanning and transmission electron microscopy. The cellular processes which comprise this pathway grow in the transverse plane and along the lateral margin of the marginal zone (i.e., circumferentially oriented), as typified by the early embryonic commissural axons. The first formative event observed was in the ventrolateral margin of the primitive spinal cord ventricular zone. Cellular processes were found near the external limiting membrane that appeared to grow a variable distance either dorsally or ventrally. Later in development, presumptive motor column neurons migrated into the ventrolateral region, distal to these early circumferentially oriented processes. Concurrently, other circumferentially oriented perikarya and processes appeared along the dorsolateral margin. Due to their aligned sites of origin and parallel growth, the circumferential processes formed a more or less continuous line or pathway, which in about 10% of the scanned specimens could be followed along the entire lateral margin of the embryonic spinal cord. Several specimens later in development had two sets of aligned circumferential processes in the ventral region. Large numbers of circumferential axons were then found to follow the preformed pathway by fasciculation, after the primitive motor column had become established. Since the earliest circumferential processes appeared to differentiate into axons and were found nearly 24 hours prior to growth of most circumferential axons, their role in guidance as pioneering axons was suggested.

Expression patterns of engrailed‐like proteins in the chick embryo

“…These cells project axons from the ventrolatral wall of the hindbrain at stages 11-12, approximately 10 hr after AChE activity is observed in their cell bodies (Weikert et al, 1990). In the spinal cord the first axonal projections arise later, also from intersegmental neurons (for recent reports see Holley, 1982;Holley and Silver, 1987;Oppenheim et al, 1988;Schlosser and Tosney, 1988;Yagunima et al, 1990). In the spinal cord of zebrafish embryos En protein is expressed in interneurons (Hatta et al, 1991).…”

Commissural axonal corridors instruct neuronal migration in the mouse spinal cord

“…In a number of higher vertebrates, including mouse and chick embryos, large cohorts of spinal commissural neurons are derived from the dorsal neural tube 37 . Therefore, a reduction of commissural neurons could be achieved by partial ablation of this region.…”

“…5) 38 . We electroporated either control plasmids or a plasmid expressing DTA into chick embryos at around the onset of the first commissural axons, stage HH18, and analysed them at stage HH25 approximately equivalent to E11.5 in mouse embryos 37,39 . Analysis at later stages was not possible, as the only robust commissural axonal marker available in chick Fig.…”

Signals from the notochord and floor plate regulate the region-specific expression of two Pax genes in the developing spinal cord

“…However, a small number of postmitotic cells arise in the dorsolateral cord prior to motor neurons. These cells may be an early population of circumferentially projecting neurons described by Holley (1982).…”