Livestock Production Science 66 2000 177–188 www.elsevier.com locate livprodsci
Myogenesis and postnatal skeletal muscle cell growth as influenced by selection
C. Rehfeldt , I. Fiedler, G. Dietl, K. Ender
Divisions of Muscle Biology and Growth and Genetics and Biometry , Research Institute for the Biology of Farm Animals,
Wilhelm-Stahl-Allee 2, D-18196 Dummerstorf, Germany
Abstract
The major component of a given muscle is the constituent muscle fibres. Lean growth and ultimate muscle mass are therefore largely determined by the number of muscle fibres and the size of those fibres. During myogenesis, myoblasts
develop from mesenchymal precursor cells by proliferation and myogenic commitment. Myoblasts subsequently fuse to form multinucleated myofibres. Postnatal growth of skeletal muscle is mainly realised through increases in length and girth of the
muscle fibres, but not by increases in muscle fibre number. Postnatal fibre hypertrophy, associated with accumulation of myonuclei satellite cell proliferation and muscle-specific proteins, is inversely correlated with the number of prenatally
formed muscle fibres. On the other hand, both fibre number and fibre thickness are positively correlated with muscle mass and lean meat percentage. Both fibre number and fibre size are influenced by selection as shown by differences between
breeds and correlated responses to lean growth selection. Increases in muscle mass solely by fibre hypertrophy, as observed particularly in meat-type pigs and chickens, may be associated with problems in stress adaptability and ultimate meat quality.
Genetic variability and heritability of muscle fibre number and size are sufficiently high to include these traits in farm animal selection in addition to commonly used selection criteria for improving lean meat content and meat quality.
2000
Elsevier Science B.V. All rights reserved.
Keywords : Muscle fibre; Growth; Meat; Selection; Heritability; Genetic correlation
1. Introduction muscle fibres and the size of those fibres. Current
research suggests that animals with greater numbers The understanding of the growth and development
of muscle fibres of moderate size produce more meat of skeletal muscle is one of the most important goals
of better quality. During myogenesis, the extent of in animal science. The major component of a given
muscle cell multiplication largely determines how muscle is the constituent muscle fibres. Muscle mass
many muscle fibres are formed. Therefore, the is therefore largely determined by the number of
number of muscle fibres is mainly determined by genetic factors and those environmental factors
which are
capable of
influencing prenatal
Corresponding author. Tel.: 1 49-38208-68853; fax: 1 49-
myogenesis. The aim of this paper is to describe the
38208-68852. E-mail address
: rehfeldtfbn-dummerstorf.de C. Rehfeldt.
principles of skeletal muscle growth, to highlight the
0301-6226 00 – see front matter
2000 Elsevier Science B.V. All rights reserved. P I I : S 0 3 0 1 - 6 2 2 6 0 0 0 0 2 2 5 - 6
178 C
. Rehfeldt et al. Livestock Production Science 66 2000 177 –188
importance of muscle cell growth in animal per- Myonuclei themselves remain mitotically quiescent.
formance and to explain how it is influenced by The importance and relations of the different
selection. myogenic lineages primary and secondary are not
yet clarified. It seems, however, that the lineage’s are not related to fibre type composition Hughes and
2. Principles of skeletal muscle growth Blau, 1992.
2.1. Prenatal development 2.2. Postnatal growth
During embryonic development, myoblasts de- During postnatal growth, the increase in skeletal
velop from myogenic precursor cells which are of muscle mass is mainly due to an increase in muscle
mesodermal origin Fig. 1. These cells are de- fibre size hypertrophy. This process is accom-
termined to enter the myogenic lineage and are able panied by the proliferative activity of satellite cells
to proliferate and divide to establish a pool of which are the source of new nuclei incorporated into
myoblasts. Special signals cause the myoblasts to the muscle fibres. After birth, total muscle fibre
exit the cell cycle, to stop dividing and to differen- number has been reported to remain unchanged in
tiate. They begin to express muscle cell-specific mammals and birds by most authors. No significant
proteins and finally fuse to form multinucleated changes in postnatal fibre number have been found in
myotubes. mice e.g., Rowe and Goldspink, 1969; Nimmo and
During myogenesis, muscle fibres develop from Snow, 1983, rat e.g., Brown, 1987; Rosenblatt and
two distinct populations. Fibres which form during Woods, 1992; Schadereit et al., 1995, pig e.g.,
the initial stages of myoblast fusion are primary Fiedler, 1983, cattle e.g., Wegner et al., 2000,
myofibres which provide a framework for the larger chicken e.g., Smith, 1963 and quail e.g., Fowler et
population of smaller secondary fibres e.g., Beer- al., 1980.
mann et al., 1978; Miller et al., 1993. These are Some reports have indicated increases in muscle
formed from foetal myoblasts during a second wave fibre number shortly after birth in rodents e.g.,
of differentiation. Another population of myoblasts Rehfeldt and Fiedler, 1984; Summers and Medrano,
does not form fibres but stays close to the myofibres; 1994 and pigs Swatland, 1975; Fiedler et al.,
these are termed satellite cells and they are able to 1998. In these studies, fibre counts were done on
divide and serve as the source of new myonuclei histological transverse sections. It is possible that the
during postnatal growth Moss and Leblond, 1971; increase in fibre number during the first days of
Schultz, 1974. They contribute to growth of the postnatal life in rodents is a result of maturation and
fibres and also participate in regeneration processes. elongation of the existing myotubes rather than due
Fig. 1. Basic events of myogenesis.
C . Rehfeldt et al. Livestock Production Science 66 2000 177 –188
179
to production of new fibres Ontell and Kozeka, 1984. This may be true also for pig muscle, since
fibre formation is known to be finished at about day 70 of gestation Swatland, 1973.
Fig. 2 depicts the postnatal development of muscle fibre thickness and muscle fibre number in different
muscles of the mouse and pig. Muscle fibres grow in size towards a plateau, whereas fibre number remains
constant after initial increases shortly after birth. Decreases in fibre number with ageing e.g., Faul-
Fig. 3. Relationships by linear phenotypic correlation coefficients
kner et al., 1972 are possibly related to a reduction
between muscle cross-sectional area, muscle fibre size diameter
in physical exercise. Activity stimuli are able to
or cross-sectional area and muscle fibre number per cross-section
induce increases whereas disuse of muscles may be
Staun, 1968, 1972; Osterc, 1974; Rehfeldt and Fiedler, 1984;
followed by decreases in muscle fibre number see
Locniskar et al., 1980; Rehfeldt et al., 1988, 1989; Fiedler et al., 1997; Larzul et al., 1997.
Rehfeldt et al., 1999. At this point, an interesting phenomenon of mus-
cle growth should be emphasised. Postnatal muscle fibre hypertrophy depends on the total number of
clear antagonism between fibre thickness and fibre muscle fibres within a muscle. The postnatal growth
number would be that nutritional energy is distribut- rate of the individual muscle fibre is lower when
ed evenly among all fibres. However, the correlation there are high numbers of fibres and higher when
coefficient is not 2 1.0 which means that some there are low numbers of fibres. This can be con-
animals exhibit fast-growing fibres despite high fibre cluded from the fact that muscle fibre number is
numbers. inversely correlated with muscle fibre thickness at
the end of the intensive growth period. Negative correlation coefficients were estimated when the
3. Importance of muscle fibre number and size
