Some have meat and cannot eat,
Some cannot eat that want it:
But we have meat and we can eat,
Sae let the Lord be thankit.

Robert Burns

The fibrous microstructure of meat

The meat we sell in our shops is derived from skeletal muscle, the muscles that pull on bones of the skeleton to produce body movements. But, of course, there are a few of exceptions. The thin flat sheet of cutaneous muscle you see on a side of beef used to twitch the skin to keep flies off, and many major muscles do not pull directly on the skeleton, but achieve the same result by pulling on something else, like a large sheet of connective tissue. However, the characteristic and dominant feature of all this meat is its fibrous structure or texture.

This is a most important point for us in the meat trade. Meat texture is supremely important. Texturized vegetable protein, something that could be quite a commercial threat to us (simulated meat made from plant sources) has, so far, made little impact (apart from the chunks in some canned stews). This is because food technologists so far have been unable to extrude their plant proteins into anything resembling real meat. The taste and colour can be faked quite easily, but the texture cannot. In a way, therefore, it is the texture of meat, and the fact that many of our customers love to eat it, that keeps us all in business. Other topics with a practical importance are aging of meat and electrical stimulation of meat

In live animals, a sliding interaction of microscopic filaments enables a muscle to CONTRACT while, in meat, an ordered arrangement of

creates a characteristic texture that is difficult to imitate with processed plant proteins. These components of meat - fasciculi, fibres, fibrils and filaments - constitute a descending series with respect to size. Fasciculi are the largest units, while filaments are the smallest. The prefix "myo" may be used to create the terms myofibre, myofibril and myofilament, which are identical with muscle fibre, muscle fibril and muscle filament, respectively. If you hail from US, you ain't gonna like all this wierd English spelling of fibres instead of fibers, but up here in Canada we mix 'em up all the time - just to show we like both the Americans and the British.

The complex arrangement of muscle fasciculi is seen when meat is carved, and the same complexity is found in the muscle fibres that are bound into these bundles. Their complex arrangement is related to muscle function in the live animal. It used to be thought that muscles which contracted to a relatively small fraction of their resting length had fibres parallel to the long axis of the muscle, whereas muscles in which the strength of contraction was more important than the distance over which contraction occurred had fibres with an angular arrangement. Thus, it was thought that muscles might gain strength by leverage, but the contraction distance would be reduced. This was a nice idea, which many of us were taught, but really there is little or no evidence to support it, and a pennate (V-shaped fibre arrangement) structure actually may serve to enhance the overall range of muscle excursion.

Muscle fibres

If a small chunk of meat is placed under a dissecting microscope and teased apart with needles, the smallest fasciculi visible without magnification are composed of bundles of muscle fibres. Muscle fibres are the basic cellular units of living muscle and of meat. They are unusual cells because they are multinucleate (with many cell nuclei) and are extremely long (commonly several centimetres) relative to their microscopic diameter (usually less than one tenth of a millimetre).

The image below shows a short part of one muscle fibre, but even this contains many nuclei (the dark blobs).

The muscle fibres found in most commercial cuts of meat seldom run the complete length of the muscle in which they are located. Individual fibres within a fasciculus may terminate at a point along the length of the fasciculus at a tapered ending anchored in the connective tissue on the surface of an adjacent fibre so that tapered endings transmit their force of contraction to the endomysium (the connective tissue around each muscle fibre) or directly to adjacent fibres through fibre to fibre junctions. The image below shows the endomysium in a transverse section of meat (the endomysium is black, the myofibrils are yellow).

Apart from tapered intrafascicular endings, the diameter of a fibre is assumed to be approximately constant along its length. Fibre diameters slowly increase during the growth of a muscle, but they also increase temporarily when a fibre contracts. Thus, when measuring fibre diameters in a growth study, special care must be taken to avoid or to correct for differences in the degree of muscle contraction.

The simplest way to examine individual muscle fibres is to place some meat fragments together with some water in a kitchen blender. After running the blender for a few seconds, the connective tissue holding the muscle fibres together is disrupted to leave a pale red suspension of broken muscle fibres in water. The red colour comes from myoglobin, a soluble red pigment found inside muscle fibres. The essential features of muscle fibre structure may be observed with an ordinary light microscope if a drop of the macerated muscle suspension is mounted on a microscope slide beneath a cover slip, as shown in the image below where, at the bottom of the frame, is an intact muscle fiber and above it is a smashed fiber with all its fibrils visible. The transverse striations of fibres become visible if the iris diaphragm of the substage condenser is almost closed (the gain in contrast is offset by a loss of resolution, which is why the diaphragm is normally open wider). With a high magnification microscope objective, fibrils may be seen if they protrude from the broken end of a fibre or if they have escaped from a broken fibre. Under the surface membranes of muscle fibres may be seen some flattened bubble-like inclusions. These are the nuclei of the muscle fibre, and their DNA may be stained by treating the macerated muscle suspension with dyes such as haematoxylin. On the surfaces of any fibres that have retained some of their surrounding connective tissue may be seen branching capillaries, once part of the vascular bed of the muscle. Red blood cells (erythrocytes) may, or may not remain in the capillaries, depending on the efficiency with which the animal was stuck. If they do remain in the meat, they appear pale yellow in unstained preparations, and are often distorted or piled together like a stack of coins.

When meat animals are slaughtered, normally they are shackled and suspended from their hind limbs and some muscles, such as the filet or psoas muscles ventral to the vertebral column, become stretched. Other muscles, such as those in the posterior part of the hind-limb, are free from skeletal restraint and may contract weakly as the carcass becomes stiff after death (rigor mortis). If samples from stretched and contracted muscles are compared, transverse striations will appear relatively far apart in the stretched muscle and closer together in the contracted muscle.

The distance between the transverse striations is the sarcomere length.

If a drop of saturated sodium chloride solution is mixed with a drop of macerated muscle suspension, the muscle fibre fragments undergo some marked changes. Fibre fragments may slowly swell and disappear, or they may expand so violently that their interiors are extruded from their broken ends. Sarcomere length and the solubility of meat proteins in salt solutions are two commercially important properties of meat: meat with short sarcomeres tends to be tough, and salt-solubilized proteins are used to bind together the meat fragments in many types of processed meat products.

Transverse striations

After prolonged maceration, muscle fibre fragments disintegrate and their fibrils are released into suspension. With an ordinary light microscope at its highest magnification, it is often possible to see the transverse striations on individual fragments of fibrils. The lenses from an old pair of Polaroid sunglasses may be used to make a simple polarizing microscope. One spectacle lens is placed in front of the light source and the other is placed on top of the microscope eyepiece. By rotating one spectacle lens relative to the other, the amount of transmitted light is greatly reduced. The first lens only transmits light waves vibrating in a certain plane (polarized light), but these are unable to get through the second lens whose transmitting plane is now at 90 degrees to the first lens. Thus, the field of view is dark, except for alternate transverse striations on certain fibrils in the field. Alternate striations are able to rotate the polarized light strongly enough for the light to get through the second spectacle lens: these striation are strongly birefringent and appear bright. Rotation of the microscope slide generally allows this property to be observed in any particular fibril.

Striations that appear bright in polarized light are termed anisotropic or A bands while those that appear dark or relatively dim are termed isotropic or I bands.

The transverse striations on muscle fibres are due to the precise alignment of A and I bands on fibrils within the fibre. In most stained preparations for light and electron microscopes, A bands appear darker than I bands (the reverse appearance to that seen with polarized light). This is because the birefringent A bands contain a greater density of protein. A bands also appear darker than I bands when unstained preparations are observed with a phase contrast light microscope (a special type of microscope that shadows differences in glassy properties of the specimen). In the electron micrograph above, the fibrils are running from the top left to the bottom right, so that the transverse striations are from the top right to the bottom left.

Other features of the fibril are detectable by light microscopy under optimum conditions, but these details are seen more clearly by electron microscopy. A thin Z line or disc occurs at the middle of the I band.

The repeating unit of a regular series of transverse striations is termed the sarcomere, and it is usually considered to be the structural unit from Z-line to Z-line.

The Z-line resembles a woven disk, like the bottom of a wicker basket, and it extends as a partition across the fibril.

The transmission electronmigrograph above is not the only way to look at transverse striation. The image below is a scanning electron micrograph (showing the surface of a specimen rather than looking through a thin slice of it), and the fibrils run horizontally across the image, showing transverse striations vertically. Only, of course, they do not look the same as they did in the previous image. In the scanning micrograph we see the sarcomeres because of the raised region of the Z-line attached to the cytoskeleton (desmin).


At the ultrastructural level, the transverse striations of fibrils are caused by the regular longitudinal arrangement of sets of thick filaments (10 to 12 nm in diameter) and thin filaments (5 to 7 nm in diameter). In a transverse section cut through overlapping thick and thin filaments, each thick filament is surrounded by six thin filaments, although this hexagonal lattice may change to a tetragonal lattice when sarcomeres are stretched. When a muscle fibre contracts, the thick filaments slide between the thin filaments so that the I band gets shorter. The length of the A band remains constant. This is called the sliding filament theory of muscle contraction, and was put forward in 1972. If a muscle is at its resting length, the gap between opposing thin filaments at the mid-length of the sarcomere causes a pale H zone in the A band. Although the sliding filament theory now is widely accepted, there remain many unsolved problems in the mechanism of the system.

Contraction is an active process requiring energy, which is provided by the hydrolysis of phosphate from adenosine triphosphate (ATP), although the transduction from chemical to mechanical energy may be delayed until the resulting adenosine diphosphate (ADP) and inorganic phosphate are released by myosin when it recombines with actin. Contraction by filament sliding may be achieved by the rowing action of numerous cross bridges that protrude from the thick filaments and are formed from the heads of myosin molecules whose backbones are bound into the thick filament. However, the conformational change that causes the cross bridge movement does not seem to be a simple angular change of the cross bridge as was originally supposed, and the movement probably originates elsewhere in the molecule, whose structure was first established three-dimensionally in 1993.

Filament sliding and muscle contraction come from the rowing of very large numbers of myosin molecules. Each individual stroke by a myosin molecule head takes about 1 millisecond and produces a 12 nm movement. Although this is a very small distance, many thousands of sarcomeres are arranged in a series, and in a very short time the sum of all these small distances may be measured in centimetres. The myosin head only releases its grip on the actin, and swings back for another power stroke with another actin, if it is recharged with another ATP molecule. Thus, when muscles are converted to meat and no more ATP is available, thick and thin filaments lock together wherever they overlap. This prevents any further filament sliding and the muscle becomes almost inextensible: this condition is called rigor mortis.

The arrangement of actin molecules in a thin filament is like two strings of pearls twisted around each other: a cross section would reveal two pearls, one from each string, separated by grooves. The two grooves are important because, located in them, are other strings of proteins. One of these proteins, troponin, responds to the presence of calcium ions and causes the other protein, tropomyosin, to change its depth in the groove. This is the switch that allows myosin heads to row against actin molecules: it is the calcium-activated trigger mechanism for muscle contraction.