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Effect of growth and plane of nutrition on apparent muscle fiber numbers in the pig

Abstract

A histological sampling technique was used to estimate the number of muscle fibers appearing in transverse sections of whole sartorius muscles. Transverse sections were taken at the midlength of sartorius muscles from pigs growing at a rate of approximately 0.7 kg/day. Between 56 and 168 days (postnatal), the apparent number of muscle fibers increased at an approximate rate of 300 fibers/day. No evidence of the formation of new muscle fibers by myoblast fusion or muscle fiber fission was detected by light microscopy. Animals placed on a maintenance diet which prevented growth for 28-day periods prior to slaughter showed a fixation or decrease in apparent numbers of muscle fibers. These data support the hypothesis that the arrangement of muscle fibers during growth of the porcine sartorius muscle is a dynamic rather than a fixed system.
... The apparent number of muscle fibres at the midlength of the Sartorius, increased (P < 0.005) through post-natal growth (Fig. 8) during the commercial growing period (0.7 kg per day). When pigs were placed on a restricted diet at various times to stop growth in live weight, the increase in apparent fibre numbers stopped [8]. Tapered fibres easily escape detection in transverse sections unless the endomysium is stained with silver ( Figure 9). ...
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This review gives credit to Theodor Schwann who discovered myotubes and secondary fibres in foetal pigs in 1839, and to John MacCallum in 1898 who discovered how the longitudinal growth of fibres could change the number of fibres in a cross section of muscle. My personal data collected over a period of fifty years amplify their discoveries and confirm the importance of the nervous system - all the way from the epigenetics of muscle development, through to reflex activity in the abattoir affecting pork quality.
... While the virtually unmeasurable real number of myofibers may or may not be constant during postnatal development, it is evident that muscle mass is more closely related to apparent number of myofibers multiplied by myofiber volume. It is now known that, in some muscles at least (Bendall and Voyle, 1967; Swatland, 1976b), the apparent number of myofibers may decrease or increase during postnatal development. As regards myofiber volume, the previously held contrast of radial growth (continuous) to longitudinal growth (discontinuous ) must, to a large degree, be reversed since radial growth has been shown to be discontinuous (Chrystall et al., 1969) during a period at which longitudinal growth is still continuous (figure 7). ...
... ,-~ o.o. Goldspink, 1973; Swatland, 1976 ). Thus, factors which would limit or reduce fiber number during this critical prenatal period of cell development would apparently not be corrected by hyperplasia during post-natal development . ...
Chapter
Traditionally, growth of the whole body or of individual organs has been measured by weight gain alone (Donaldson, 1924; Dunn et al., 1947). This measurement, however, does not take into account the contribution made by various cellular and noncellular components. More recently, the finding that deoxyribonucleic acid (DNA) is located almost entirely within the nucleus and that the amount of DNA is constant within the diploid nucleus of any species (Boivin et al., 1948; Mirsky and Ris, 1949; Thomson et al., 1953) has enabled Enesco and LeBlond (1962) to reexamine postnatal growth.
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