PEASE DC: DEGENERATIVE CHANGES IN RAT DORSAL ROOTS DURING WALLERIAN DEGENERATION

Electron-microscopic structure of denervated skeletal muscle

(1) An electron-microscopic study was made of normal and denervated muscle fibres in the rat diaphragm. (2) Early after denervation muscle fibres become hypertrophic. The cross-sectional area of the fibres and the number of myofibrils within them are increased. Since filament spacing is not significantly altered, it is concluded that during hypertrophy the number of actin and myosin filaments is increased. (3) A few weeks after denervation the muscle fibres are greatly reduced in size. This atrophy is mainly a consequence of two processes: fragmentation of the muscle fibre, with subsequent degeneration of the fragments; and disintegration of myofilaments.

Cytological studies of organotypic cultures of rat dorsal root ganglia following X-irradiation in vitro. II. Changes in Schwann cells, myelin sheaths, and nerve fibers

Under suitable conditions rat dorsal root ganglia differentiate and myelinate in culture, providing an organotypic model of the ganglion (8). Mature cultures of this type were irradiated with a 40 kR dose of 184 kvp X-rays and, after daily observation in the living state, were fixed for light and electron microscopy. Within 24 hr after irradiation, numerous Schwann cells investing unmyelinated axons acutely degenerate. The axons thus denuded display little change. Conversely, few ultrastructural changes develop in Schwann cells investing myelinated axons until after the 4th day. During the 4-14 day period, these Schwann cells and their related myelin sheaths undergo progressive deterioration. Associated axons decrease in diameter but are usually maintained. Myelin deterioration begins as a nodal lengthening and then progresses along two different routes. In intact Schwann cells, fragmentation of myelin begins in a pattern reminiscent of Wallerian degeneration, but its slow breakdown thereafter suggests metabolic disturbances in these Schwann cells. The second pattern of myelin deterioration, occurring after complete degeneration of the related Schwann cell, involves unusual configurational changes in the myelin lamellae. Atypical repeating periods are formed by systematic splitting of lamellae at each major dense line with further splitting at the intraperiod line (Type I) or by splitting in the region of every other intraperiod line (Type II); some sheaths display a compact, wavy, inner zone and an abnormally widened lamellar spacing peripherally (Type III). Extensive blebbing of myelin remnants characterizes the final stages of this extracellular myelin degradation. These observations provide the first description of ultrastructural changes produced by ionizing radiation in nerve fascicles in vitro.

Pulsatile and myelin-forming activities of Schwann cells in vitro

1. Sympathetic and dorsal root ganglia from young mice and new-born rats were cultured in vitro. The activity of the outgrowing Schwann cells was studied with time-lapse photography.2. Schwann cells exhibited a pulsatile activity with a contraction rate varying from 1 to 12 min. The occurrence of pulsations depended on the mode of attachment of cells to their surroundings.3. Fibres of various materials were introduced into the cultures in an attempt to simulate conditions obtaining in vivo. Schwann cells attached themselves to these fibres and rotated about them depositing myelin. Polarized light and histochemical studies showed that myelin was formed as early as four days in vitro.4. The possible significance of the behaviour of Schwann cells in vitro is discussed.