Neonatal muscle growth: A quantitative study

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Abstract
Various growth parameters of the rat extensor digitorum longus muscle were studied during the first week of neonatal growth, including changes in weight, length, girth and fiber number. These quantitative studies were supplemented by ultrastructural analysis of fiber diameter and morphology in an attempt to determine the source of “new” muscle fibers in the neonate. In the 2‐day‐old extensor digitorum longus many of the myofibers were enclosed with other filamented cells (a satellite myofiber and/or myotube) in a common basement membrane, forming a cluster. Clusters accounted for 40% of the total independent filamented cells and clusters in 2‐day‐old muscles, for 2% in the 5‐day‐old, and for less than 1% in the 8‐day‐old. The decrease in the percentage of clusters occurred concomitantly with an increase in the number of independent filamented cells. In the 2‐day‐old there were 2,932 ± 130 independent cells and clusters; this number increased by 90% in the 8‐day‐old muscle. Mean diameters of filamented cells (whether found independently or as part of a cluster) increased from 6.5 μm to 7.4 μm and to 9.5 μm on days 2, 5, and 8 respectively. Increased filamented cell diameter between days 2 and 5 was a result of growth of the smallest filamented cells into the medium‐size compartment. Between the fifth and eighth day postpartum, the increase was due to the growth of both small and medium‐size filamented cells. The largest fibers did not increase in diameter during the first week of postnatal development, maximal fiber diameter in the 2‐day‐old being 15.9 μm and in the 8‐day‐old, 16.1 μm. At two‐days the area through the widest girth of the muscle occupied by filamented cells was 35% of the total cross‐sectional area of the extensor digitorum longus. At eight days muscle fibers occupied 59% of the cross‐sectional area. To accommodate the myofibers the muscle belly increased its girth, and there was a decrease in the percentage of the area occupied by interstitial tissue, nerves, and blood vessels.

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