Handbook of Food Science and Technology 3

2.1.2.
 Muscle structure 
2.1.2.1.
 Muscle cells 
Muscle tissue is composed of specialized cells called muscle cells or 
muscle fibers (Figure 2.4). These are large cells measuring 1 – 100 
μ
m in 
diameter and 1 mm – 40 cm in length (3 cm maximum in fish). The cell 
membrane or 
sarcolemma
is closely connected to a very fine layer of 
connective tissue called the 
endomysium
, which ensheaths each muscle fiber.
The sarcoplasm (muscle cell cytoplasm) contains a number of organelles 
common to all cells: 
– several 100 nuclei inside the sarcolemma membrane: the muscle cell is 
multinucleated; 

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74 Handbook of Food Science and Technology 3 
– Golgi apparatus: a complex of flattened compartments bound by a 
double membrane. It plays a role in cell metabolism and the process of 
excretion; 
– mitochondria, which play a crucial role in energy processes (electron 
transfer chain and oxidative phosphorylation); 
– small lipid droplets. 
However, it also contains organelles particular to muscle cells: 
– glycogen granules: energy reserves for muscle contraction; 
– lysosomes: small vacuoles containing many enzymes including acid 
proteases such as cathespins; 
– a contractile system of protein filaments running parallel to the 
longitudinal axis of the fiber, giving it its striated appearance. Everything is 
held in place by a sort of internal scaffolding, the cytoskeleton.
Figure 2.4.
Muscle cell structure. For a color version of
this figure, see www.iste.co.uk/jeantet/foodscience.zip 
Myofibrils 
Myofibrils are contractile elements that allow muscle cells to contract. 
They are aligned parallel to the longitudinal axis of the fiber and are 
responsible for the longitudinal striation of the muscle. They are the same 
length as the muscle fiber and have an average diameter of 0.1 
μ
m. They 
have a heterogeneous structure. 

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From Muscle to Meat and Meat Products 75 
Myofibrils repeat along the longitudinal axis of the fiber, approximately 
every 2.5 
μ
m. In skeletal or striated muscles, this repetition forms a cross-
striation pattern (Figure 2.5).
Figure 2.5.
Structure of myofibrils 
This striation is the result of alternating dense, dark or A-band 
(anisotropic) regions measuring 1.0 
μ
m in length, and less dense, light or
I-band (isotropic) regions measuring 1.6 
μ
m in length. I-bands are separated 
by a “partition” known as the Z-line or Z-disc. Z-lines extend from one 
myofibril to the next, eventually connecting to the sarcolemma at the end of 
the muscle fiber. They form an internal skeleton for the cell and play an 
important support role. They also help to create synergy between the various 
contractile units. In the middle of the A-band is a less dense, lighter region 
called the H-zone. At the centre of the H-zone is a darker line called the M-
line.
A complete structural unit between two Z-lines is called a sarcomere. It 
forms the basic contractile unit. In a relaxed muscle, a sarcomere measures 
3–5 
μ
m in length.