Presence of Marchi-positive myelinoid bodies in the spinal cord white matter of some vertebrate species.

Abstract

AbstractThe occurrence of Marchi‐positive myelinoid bodies in the ventrolateral cervical spinal cord white matter was investigated by light and electron microscopy in adult animals of the following species: cat, rabbit, guinea pig, rat, mouse, frog and perch. In addition some observations were made on human specimens. The bodies were found to be present in all the examined species, being most numerous in the rabbit and guinea pig. The lowest incidence was seen in the mouse and frog. The bodies consistently prevailed along the paranodes of large myelinated fibers and generally showed a rounded or ovoid shape. In all animals the size spectrum of the Marchi‐positive myelinoid bodies showed a peak at about 3 μ. The sizes varied from less than 1 μ to over 25 μ. The size range was widest in the perch, the ventrolateral white matter of which contained a great number of very large bodies, and most narrow in the rat. Calculations of the total myelinoid body volume per unit volume of ventrolateral white matter or myelin, showed very high values in the perch (0.03 μ3/μ3 myelin), high values in the cat, rabbit and guinea pig (0.006‐0.007 μ3/μ3 myelin) and low values in the rat, mouse and frog (0.001‐0.002 μ3/μ3 myelin). The white matter specimens from animals with a high total myelinoid body volume contained large fibers with thick myelin sheaths, which were not found in the specimens from animals with a low total myelinoid body volume.Electron‐microscopically, the picture was rather uniform. In all species examined the myelinoid bodies were composed of a lamellated shell of varying thickness that surrounded a central zone of highly electron‐dense material or an empty cavity. In most cases the bodies were linked to myelin sheaths through oligodendroglial cytoplasm. Less frequently free bodies were observed within astrocytes or microglia.The findings are interpreted in support of the view that myelinoid bodies may be regarded as an expression of the turnover of myelin material in thick myelin sheaths.