Distribution of mitotic nuclei in the biceps brachii of the mouse during post‐natal growth

Distribution of mitotic nuclei in the biceps brachii of five day old mice has been studied after subcutaneous injection of 1 μCi/gm body weight 3H‐thymidine. Serial transverse sections at 5 μm thickness were cut and the number of labeled nuclei per hundred fibers were determined on every fortieth section starting from the proximal end of the muscle. The proximal region of the muscle has significantly more labeled nuclei than either the belly region (P < 0.005) or the distal region (P < 0.001) of the muscle. The distal region of the muscle and the belly of the muscle did not differ significantly. Thus, most of the fiber elongation seems to take place at the proximal region of the muscle. As far as the transverse growth is concerned it appears that either every fiber at this stage is increasing in diameter or else the fibers which are undergoing an increase in diameter are randomly distributed throughout the body of the muscle, as no significant differences have been found in the percentage labeled nuclei in the four transverse areas of the muscle in the belly region.