Organogenesis of the mouse extensor digitorum logus muscle: a quantitative study.

A quantitative analysis of the pattern of development and growth of the fetal extensor digitorum longus muscle of the 129 ReJ mouse was carried out in spaced, serial ultrathin sections with computer-assisted morphometry. Muscle from 12-, 14-, 16-, and 18-day in utero mice and from newborn and 5-day postnatal mice was analyzed to determine age-related changes in such factors as the maximal girth and length of the muscle, the number of myotubes, the "cluster" frequency, and the diameters and lengths of the myotubes and muscle units. A distinct temporal pattern of development was established. It was quantitatively determined that a delay less than or equal to 2 days occurs between the formation of primary myotubes (present at 12 days in utero) and secondary myotubes (present at 16 days in utero). By 16 days in utero, groups of myotubes, consisting of one primary myotube and a variable number of secondary myotubes, form "clusters" surrounded by a common basal lamina. Morphometric analyses of diameter distributions establish that most, if not all, secondary-generation myotubes are formed in association with larger, more mature myotubes. Quantitative data support the hypothesis (Ontell and Kozeka, 1984) that cluster formation and cluster dispersion occur simultaneously, beginning sometime between 16 and 18 days in utero. By 18 days in utero, the adult number of myofibers is present in the developing muscle mass. Analyses of lengths and diameters of the same fibers establish that the pattern of growth of the last-formed myotubes of the developing muscle mass is different from that of myotubes formed earlier in development.