Indiana University, Bloomington, Indiana, USA*
Abstract
Locally applied X-rays drastically inhibited the otherwise vigorous regeneration of mammalian skeketal muscle tissue.
Regeneration of mammalian skeletal muscle was first observed microscopically in human tissues from deaths during the Berlin typhoid epidemic of 1864, and the process has been the object of investigation ever since (1). In the microscope the cardinal signs of regeneration -- newly differentiating, multinucleated myotubes with centrally situated rows nuclei -- are numerous, conspicuous and readily identifiable from about four days after injury (2, 3); within a month the new muscle fibers have acquired all the morphological and histochemical attributes of mature muscle fibers.
X-irradiation is known to have a devastating effect on limb regeneration in amphibians (4). Indeed, Butler demonstrated the local origin of regenerating limb blastema cells by locally inflicted X-irradiation (op. cit.)
The objective of these experiments was to see if regeneration of mammalian skeletal muscle, likewise, would show sensitivity to locally delivered X-irradiation.
The left leg of one group of animals received 588 r while that of a second group received 2940 r. Six days later, three 1 x 1 x 1 mm wounds were inflicted in the belly of the anterior tibialis of the left exposed leg and, moments thereafter, in the right unexposed leg of each animal. (The wounds of the unexposed, and thus unirradiated, right limbs were the controls.) The wounds were fixed 6 days later in Zenker-formalin fluid. Processed for paraffin sectioning, the tissues were stained with hematoxylin and eosin.
Regeneration was severely depressed in wounds from the exposed left legs, at both doses (fig. 2). The results for both doses of irradiation were very similar to the effects of colchicine, when the alkaloid is applied during the second-third days after wounding (see Fig. 7 in ref. 3), except for the presence in the latter of numerous, arrested mitotic figures.
Careful examination of the exposed wounds did reveal an occasional syncytium: lone multinucleated myotube whose features suggested a stage of development earlier the 6 days (1, 2); this was true for both dosages (fig. 3). Whether the latter would have proceeded to produce contractile proteins has not been determined. Uncertainties notwithstanding, the occasional myotube found in irradiated wounds demonstrate that X-irradiation did not merely "fry" the tissues but, instead, had exerted a severe block on the production of myogenic cells.
These findings indicate that, true of development in general and of regeneration in particular, myogenesis in wounded mammalian skeletal muscle is vulnerable to the localized effects of X-irradiation.
