Development of the amyelinated lesion in the ventral root of the dystrophic mouse: Ultrastructural, quantitative and autoradiographic study

Abstract

Postnatal development of the L5 ventral spinal root was studied in dystrophic (Bar Harbor 129 ReJ dy/dy) and normal mice by ultrastructural, quantitative and autoradiographic methods. Ultrastructural studies of normal roots showed that at birth most axons were invested with Schwann cell cytoplasm, while in dystrophic roots there were large groups of amyelinated fibres. By the 4th day, the normal animal had no naked axons, while large groups of amyelinated fibres remained in the dystrophic animals. Further abnormalities present in the ventral roots of dystrophic mice, both at birth and on the 4th day, included the presence of Schwann cells not committed to any axons, and deficiency of the Schwann cell basement membrane. Quantitative studies showed that at birth the number of myelinated fibres in dystrophic roots was 34% of normal, and remained constant throughout life, while in normals there was a 10-fold increase within 15 days. The number of dystrophic Schwann cell nuclei per whole L5 root transverse section, when corrected for nuclear and root length, was 76% of normal at birth, 20% 15 days later and 50% in adults. The rate of proliferation of dystrophic Schwann cells (labelling index) was normal at birth (dystrophic 8%, control 8.4%), fell to 2.2% in dystrophics and 4.7% in controls at 7 days, and in adults was 0.4% in dystrophics and < 0.1% in controls. It was calculated that even at the observed slower rate of proliferation, the Schwann cell population of the dystrophic L5 ventral root could have risen to that of the normal root by about the 12th postnatal day. It is suggested that in the dystrophic L5 ventral root from birth to 3 weeks of age the Schwann cell production is balanced by an equivalent cell loss.