An evaluation of myelomeres and segmentation of the chick embryo spinal cord

We have investigated whether the neuromeres of the developing chick spinal cord (myelomeres) are manifestations of intrinsic segmentation of the CNS by studying the patterns of cell proliferation and neuronal differentiation. Treatment of 2-day embryos with colchicine does produce exaggerated myelomeres, in confirmation of Kallen (Z. Anat. Entwickl.-Gesch. 123, 309–319, 1962). However, this does not imply that myelomeres are segmental proliferation centres: the undulations caused by colchicine are irregular alongside the unsegmented mesoderm, and another mitotic inhibitor, bromodeoxyuridine, has no such effects. In contrast to lower vertebrate embryos, there is no evidence for segmental groups of primary motor neurons in the chick: the earliest motor neurons express cholinesterase, and project their axons into the adjacent sclerotome, at random positions in relation to the somite boundaries. The population of motor neurons projecting HRP-labelled axons into a single somite lies out of phase with both myelomere and somite, and is placed symmetrically about the anterior half-sclerotome. The earliest intrinsic spinal cord neurons, as stained with zinc iodide-osmium tetroxide or anti-68 × Mr neurofilament antibody, show no segmental patterns of differentiation. We conclude that, in contrast to the rhombomeres of the developing hindbrain, myelomeres are not matched by segmental groupings of differentiating nerve cells, and result from mechanical moulding of the neuroepithelium by the neighbouring somites.

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Slow posttranslational modification of a neurofilament protein.

The Journal of Cell Biology ◽

10.1083/jcb.100.5.1799 ◽

1985 ◽

Vol 100 (5) ◽

pp. 1799-1804 ◽

Cited By ~ 65

Author(s):

G S Bennett ◽

C DiLullo

Keyword(s):

Spinal Cord ◽

Motor Neurons ◽

Immunoblot Analysis ◽

Apparent Size ◽

Cytoskeletal Proteins ◽

Neurofilament Protein ◽

Two Dimensional Gel Electrophoresis ◽

Spinal Cord Neurons ◽

Kinetics Of ◽

Chick Spinal Cord

The synthesis and subsequent modification of neurofilament (NF) polypeptides has been examined in pulse-chase experiments, using cultured chick spinal cord neurons. Fluorography of the [35S]methionine-labeled cytoskeletal proteins, after separation by two-dimensional gel electrophoresis, revealed that (a) the mid-size chicken NF protein, NF-M160, is synthesized as a smaller and more basic precursor, NF-M130; (b) beginning approximately 8 h after translation, NF-M130 slowly and continuously becomes larger and more acidic, attaining the size and charge of NF-M160 16 or more h later, and undergoing no further change in mobility for many days thereafter; and (c) in contrast, the low molecular weight NF protein, NF-L, is synthesized as such, and undergoes no subsequent change in apparent size or charge. Additional experiments provided evidence that the conversion of NF-M130 to NF-M160 is due, at least in part, to phosphorylation: (a) Incubation of similar cultures in 32PO4 resulted in incorporation into NF-M160 and transitional forms, but not into NF-M130. (b) An antiserum to NF-M160 was found by immunoblot analysis to bind strongly to untreated NF-M160, but poorly to phosphatase-treated NF-M160, and not at all to NF-M130. It has already been demonstrated (Bennett, G. S., S. J. Tapscott, C. DiLullo, and H. Holtzer, 1984, Brain Res., 304:291-302) that this anti-NF-M160 fails to stain the soma of motor neurons in sections of chick spinal cord, but detects an increasing gradient of immunoreactivity in the proximal axons. These results, together with the known kinetics of axoplasmic transport of NF, suggest that the mid-size chicken NF protein is synthesized as NF-M130 and is extensively modified, at least in part by phosphorylation, to become NF-M160 during transport along proximal neurites. Once maximally modified, NF-M160 undergoes no further net change during transport along distal neurites.

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