A comparison between the synthesis of contractile proteins and the accumulation of their translatable mRNAs during calf myoblast differentiation

The morphogenesis of muscle spindles in the mouse extensor digitorum longus was studied in closely spaced, serial, ultrathin transverse sections, permitting evaluation of the developing spindles' three-dimensional cytoarchitecture. Afferent nerve terminals are identifiable as early as 15 days in utero, and the adult number of spindles is present at 17 days in utero. By establishing continuity between the clearly identifiable equatorial regions and the polar regions, which are morphologically indistinguishable from surrounding extrafusal fibers, it could be determined that fusimotor innervation was present by the 19th day of the in utero development. In all spindles examined, the efferent innervation occurred extracapsularly. At birth, the adult number (four or five) of intrafusal fibers are found in each spindle, and fusimotor innervation is frequently intracapsular. Evidence is presented supporting the hypothesis that successive generations of intrafusal myotubes are formed by the fusion of mononucleated cells. Newly formed myotubes are preferentially distributed in close relationship to the sensory-innervated region of the primary intrafusal myotube. Analyses of growth parameters indicate that the difference in length and diameter between intrafusal and extrafusal fibers is principally a postnatal phenomenon.