Myotome and early neurogenesis in chick embryos

Abstract

The present study was undertaken in order to verify the identification of profiles of presumptive growth cones in vivo. The developing spinal nerves of chick embryos were studied by light and electron microscopy. We traced the onset of efferent and afferent innervation of the myotome in 2- to 4-day-old chick embryos in order to be sure that we were examining the growing tips of axons. In the process of studying these growing axons, we were able to observe some unique relationships of neural tube, myotome, and differentiating spinal nerves. The neural tube tightly abuts the myotome in Hamburger and Hamilton's (HH) stage 14 chick embryos and cytoplasmic projections from the myotome directly abut the neural tube. The first ventral roots could be identified in HH stage 15 embryos and dorsal roots in HH stage 16 embryos, both under 2 1/2 days of age. The advancing spinal nerve courses toward the anterior or cranial half of the myotome, and growth cones directly contact the medial wall of the myotome. The spinal nerves continue to abut tightly the myotome during the succeeding day of embryonic life, and growth cones enter the substance of the myotome by 3 days, or HH stage 19 embryos. These dorsolaterally directed axons will form the dorsal ramus of the spinal nerves and the ventral ramus continues to be contiguous with the myotome. Invasion of the myotome by axons (putative innervation), and thus innervation of myotomal cells in the 3-day chick embryos, was a totally unexpected finding. The myotome and its potential derivatives thus have extensive neural contact by 3 days of embryonic life in the chick. These findings document a parallel differentiation of afferent and efferent elements of the nervous system and confirm previous accounts identifying growth cones in an intact organism. These findings suggest that afferent as well as efferent nerves may have critical roles in the differentiation of the mesodermal as well as ectodermal derivatives.

Differentiation of the anterior body wall and truncal epidermis and associated co‐migration of cutaneous nerves and mesenchyme

“…As noted in the discussion, this phenomenon has been observed frequently as axons initially approach the epidermis during early development. The axonal profile in Figure 8 is a portion of a growth cone as identified previously in vivo in developing skin (Dell and Munger, 1986;Moore and Munger, 1989;King and Munger, 1989).…”

Regulation of myosin expression during myotome formation

“…This delay prompted an investigation into the developmental events known to occur immediately prior to the stages of development at which sMyHC2 expression was first seen (~HH 16). Previously published data suggest that, at HH 14-16, nerve fibers first extend from the neural tube toward the myotome (AudaBoucher et al, 1997; Bo et al, 2000;Hollyday, 1995;Kil and Bronner-Fraser, 1996;King and Munger, 1990;Meiniel and Bourgeois, 1982). It has been demonstrated that myotubes formed in vitro from myoblasts from slow-MyHC-expressing muscle will express slow MyHC 2 only if innervated (DiMario and Stockdale, 1997).…”

Highly reproducible spatiotemporal patterns of mammalian embryonic movements at the developmental stage of the earliest spontaneous motility

“…In non-mammalian vertebrates, e.g. ®sh, amphibians and chick, spontaneous movements are ®rst observed shortly after the initial formation of neuromuscular contact (Hamburger & Balaban, 1963;Muntz, 1975;Myers et al, 1986;King & Munger, 1990). However, the initial spontaneous motility in rat or mouse embryos in in vivo experiments was seen 2±3 days after the initial neuromuscular contact was formed (Angulo Y Gonzalez, 1932;Walker, 1969;Dennis et al, 1981;Carry et al, 1983;Kodama & Sekiguchi, 1984).…”