Development of muscle fibre nuclei in holometabolous insect larvae

The growth and development of the nuclear population from intersegmental dorsal longitudinal muscles (IDLM) of Bombyx mori and Pieris brassicae during larval development were studied. Using whole muscle mounts, examined with an image analysis system, we observed increases in muscle size but a constant number of fibres and concomitant increases in the number and size of fibre nuclei. The gradual growth of the nuclei throughout larval life, corresponding very probably to an increase in their DNA content, is unequal and results in heterogeneity of the nuclear population. This suggests periodic incorporation of myoblasts into the larval muscles; the nuclei arising from these myoblasts would therefore undergo endomitotic processes and grow polyploid. The possibility of somatic reduction divisions of the polyploid muscle nuclei is discussed.