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
animals were slaughtered at the same age mouse: for animal performance
Rehfeldt et al., 1988, 1989; chicken: Locniskar et al., 1980; pig: Fiedler et al., 1997, at almost the same
3.1. Lean growth weight pig: Staun, 1968, 1972; Larzul et al., 1997
or at a different age and weight cattle: Osterc, From the principles of skeletal muscle growth, it
1974. On the other hand, both fibre number and becomes clear that lean growth depends on the
fibre thickness are positively correlated with muscle number of the prenatally formed fibres and on the
cross-sectional area Fig. 3. An explanation for this degree of their postnatal hypertrophy. This has been
Fig. 2. Postnatal development of muscle fibre thickness cross-sectional area or diameter and total muscle fibre number per muscle cross-section in: a rectus femoris muscle of laboratory mice Rehfeldt and Fiedler, 1984; b semitendinosus muscle of German Landrace
pigs Fiedler, 1983; Rehfeldt et al., 1993.
180 C
. Rehfeldt et al. Livestock Production Science 66 2000 177 –188
confirmed by significant positive correlation coeffi- Wegner et al., 2000. This increase in muscle fibre
cients of muscle mass or lean meat percentage with number is associated with increases in muscle mass
both fibre number and size Otto and Wegner, 1976; of 20 Shahin and Berg, 1985; Wegner et al.,
Fiedler and Otto, 1982; Klosowska et al., 1985; 2000. Another example of muscular hypertrophy is
Larzul et al., 1997; Henckel et al., 1997. On the the callipyge condition in sheep which is associated
other hand, it seems to be important to which extent with selective fibre hypertrophy Carpenter et al.,
each of the two fibre characteristics contribute to 1996.
lean growth. The potential for lean growth of an animal largely depends on the number of the prenat-
3.2. Meat quality stress susceptibility ally formed muscle fibres, because the postnatal
increase in muscle fibre size is limited by genetic and It was discussed above that low muscle fibre
physiological reasons. The latter is supported by the number correlates with fibres which exhibit greater
following results. A 15-week treatment with porcine hypertrophy. However, strong fibre hypertrophy
somatotrophin pST, which repartitions nutrients to seems to reduce the capacity of the fibres to adapt to
muscle, was able to accelerate fibre growth in activity-induced demands which in turn may be
Landrace pigs, but the maximum fibre size did not associated with stress susceptibility and poor meat
exceed that of the control pigs attained 5 weeks later quality in modern meat-type pig breeds Cassens et
´ Rehfeldt et al., 1996. In Pietrains, representing the
al., 1975; Fiedler et al., 1993, 1999; Wicke et al., pig breed with the largest muscle fibres, exogenous
1991; Lengerken et al., 1997. Possibly energy and pST was not capable at all of increasing fibre size
oxygen supply are limited with increasing fibre size Rehfeldt and Ender, 1995 and, similarly, Sørensen
due to reduced capillary density Cassens and et al. 1996 suggested that pig genotypes with larger
Cooper, 1971; Fiedler et al., 1993 and nuclear muscle fibres are less responsive to pST treatment
control of cellular processes may be impaired in than genotypes with smaller fibres. On the other
large fibres which often exhibit a low nuclear:cytop- hand, pigs are mostly slaughtered before their po-
lasm ratio Cheek et al., 1970; Rehfeldt and Ender, tential for lean growth is exhausted, so that in most
1995. Particularly in modern meat-type pigs and cases the rate of lean growth, depending on the rate
chickens, larger fibres tend to have lower numbers of of muscle fibre hypertrophy e.g., Larzul et al.,
mitochondria, and they belong to the white, fast- 1997, seems to be of greater interest than the
contracting type. In pigs, higher white fibre per- potential of lean growth. Although within breed
centages have been shown to correlate with the PSE phenotypic correlation coefficients are mostly in-
pale, soft, exudative meat condition Linke, 1972; significant e.g., Fiedler and Otto, 1982; Larzul et al.,
Larzul et al., 1997; Fiedler et al., 1999 and with 1997, there is some evidence that pigs with more
stress susceptibility Nelson and Schochet, 1982; muscle
fibres exhibit
less fat
Stickland and
Fiedler et al., 1993, 1999. These fibres produce Goldspink, 1975; Kuhn et al., 1998. This may be
energy for contraction mainly by the glycolytic related to the fact that the plateau of muscle fibre
pathway and, under energy-demanding conditions growth is achieved earlier at lower fibre numbers,
e.g., before slaughter, their metabolism contributes and that afterwards available nutrients are preferen-
to a very fast pH decline by over-production of tially used for fat deposition. In part, this may
lactate which cannot be removed. This in turn is explain the fact that the highest lean meat percentage
related to the PSE condition after slaughter. cannot be expected at the maximum fibre size
Lengerken et al. 1997 investigated the relation- Lengerken et al., 1997.
ship between muscle fibre number and pH value of The relationship between muscle fibre number and
pig longissimus muscle and meat percentage after lean growth becomes very obvious by the example of
slaughter. They were able to demonstrate that there double-muscled cattle which exhibit almost double
is a range of optimum muscle fibre number which the number of muscle fibres compared with other
guarantees both high meat percentage and good meat cattle breeds e.g., Holmes and Ashmore, 1972;
quality at a moderate fibre size. As shown in Table 1,
C . Rehfeldt et al. Livestock Production Science 66 2000 177 –188
181 Table 1
a
´ Meat characteristics dependent on the total muscle fibre number of longissimus muscle in Pietrain pigs mean6S.D.
3
Class of muscle fibre number 3 10 Low
Middle High
800–1000 .
1000–1200 .
1200–1600 Number of animals
9 9
8
3
Total fibre number 3 10 908656
1112657 13256110
Fibre diameter mm 86.068.2
77.568.6 67.165.4
Lean meat 60.062.3
59.862.4 59.163.4
2
Loin muscle area cm 54.962.0
57.266.0 58.267.5
pH 45 min p.m. 5.9560.36
6.0160.44 6.2060.39
Reflectance 17 h p.m. 4863
4964 4663
Drip loss 4.0162.21
4.3962.85 2.9161.45
a
Halothane status as the number of homozygous negative homozygous positive pigs: low, 3 6; middle, 4 5; high, 2 6.
´ Pietrain pigs with the highest number of low-size
Larzul et al. 1997, Klont et al. 1998 and Karlsson fibres in the longissimus muscle tended to exhibit the
et al. 1999. best meat quality without significant differences in
lean meat percentage and loin muscle area. From data reported by Maltin et al. 1997, it may be
4. Influence of selection on muscle fibre size and
