ABSTRACT
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.
ABSTRACT
An electron microscope study was conducted on the origin of the dorsal longitudinal muscles of a Nematocerous Diptera (Chironomus). These imaginal muscles arise from three pairs of slender larval muscles that are characterized by the presence of myoblasts located beneath the basal lamina and adhering to the sarcoplasmic membrane. During the last larval instar the myoblasts increase in number, each of the associated muscle fibers loses its contractile material and splits longitudinally into two to form six columns of sarcoplasm. Differentiation of the fibrillar material begins in each of the six muscle rudiments after the adhering myoblasts have become incorporated. There are several possible origins for these myoblasts: they may be embryonic cells that persist in association with the larval muscle fibers; or --as in the case of Cyclorrhaphous Diptera-- they may migrate from elsewhere to invest these fibers.
