The 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 p. The sizes varied from less than 1 p to over 25 p . 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 p 3 / p 3 myelin), high values in the cat, rabbit and guinea pig (0.006-0.007 p 3 / p 3 myelin) and low values in the rat, mouse and frog (0.001-0.002 p 3 / p 3 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 a s an expression of the turnover of myelin material in thick myelin sheaths.Since its original description (Marchi, 1886), the Marchi reaction has been widely used as a method for staining degenerating myelin in pathological or experimental conditions (Duncan, '31; Swank and Davenport, '34, '35a, Adams, '65). Bodies with a positive Marchi-staining, however, are also present in normal white matter (Singer and Munzer, 1888; Elzholz, 1898; Myers, '01; Spielmeyer, '03; Jacob, '12; Allen, '231, and this feature has been claimed to represent an artifact inherent to the Marchi method (Swank and J. MORPH., 153: 1-22. Davenport, '35b; Smith, '56a,b; Giolly and Scully, '68; Strich, '68; Fraser, '72).Lamellated bodies with a n ultrastructural appearance resembling that of myelin, and which appeared to be preferentially associated with the paranodes of large myelinated fibers, were previously found to be abundant in mature feline spinal cord white matter (Hildebrand, '71). Light-microscopically, the bodies were Marchi-positive and OTAN-black, and seemed to have some associated acid phosphatase activ...
