Ionizing irradiation on development of the muscle spindle

Abstract

Intrafusal (IF) fibers in leg muscles of the rat increase in number from two at birth, to three at day 3, and to the adult four fibers by the sixth day. The question of whether this multiplication and the attainment of the adult complement of fiber nuclei involves mitotic division was approached by subjecting one leg of rat pups to x-irradiation in single doses up to 1200 R, and comparing treated and control plantaris muscles on complete histological serial sections taken at 1 month of age. The mitotic process is particularly susceptible to ionizing irradiation and since periods for multiplication of IF fibers are fairly precisely scheduled, irradiation even in a single dose might be expected to disrupt future development. However, no effects were detected on number of spindles per muscle, on spindle complement of nuclear bag and chain fibers, or configuration of the nucleated region. Irradiation of gravid rats in the latter half of pregnancy with a single dose that was near lethal for the fetuses was also without obvious effect. Muscle spindles in several rat pups given single doses of colchicine were unaffected. Failure of these procedures to disturb spindle development supports the likelihood that the multiplication of IF fibers and allotment of nuclei that takes place in late natal and early postnatal periods is an amitotic process.