10 Myofibrils

10.1 Introduction

We have seen how multinucleated myofibres develop in our meat animals, and how myofibre development is related to meat yield and quality.  But we have not yet looked inside the myofibre to see its myofibrils.  Myofibrils are contractile organelles (because they are intracellular - inside the cell).

It is important to distinguish between these F words!
Fasciculi are bundles of fibres (muscle fibre = myofibre).
Fibres are multinucleate cells.
Fibrils (muscle fibril = myofibril) are contractile organelles inside a fibre.
Filaments (muscle filament = myofilament) are protein filaments within fibrils.

Fasciculi are visible when meat is carved.
Fibres can be seen with a light microscope.
Fibrils can just be seen with a light microscope but an electron microscope is needed to see their internal structure.
Filaments can only be seen with an electron microscope.

10.2 Myofibres

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 myofibres. Myofibres 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 myofibres 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 myofibre so that tapered endings transmit their force of contraction to the endomysium (the connective tissue around each myofibre).

A tapered myofibre terminating within a fasciculus is called an intrafascicularly terminating myofibre. An intrafascicularly terminating myofibre is labeled ift in the image above. It is tapering from left to right and finishes near the middle of the field. You can also see some myofibres with a constant diameter, some myofibre nuclei and some capillaries.

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 myofibre is assumed to be approximately constant along its length. Myofibre diameters slowly increase during the growth of a muscle (radial hypertrophy), but they also increase temporarily when a myofibre contracts. Thus, when measuring myofibre diameters in a growth study, special care must be taken to avoid or to correct for differences in the degree of muscle contraction.

10.3 Myofibrils

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 myofibres together is disrupted to leave a pale red suspension of broken myofibres in water. The red colour comes from myoglobin, the soluble red pigment from  inside the myofibres. Place a drop of the macerated muscle suspension  on a microscope slide beneath a cover slip, as shown in the image below where, at the bottom of the frame, is an intact myofibre and above it is a smashed myofibre with all its myofibrils visible.

The transverse striations of myofibres become visible if the iris diaphragm of the sub stage 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, myofibrils may be seen if they protrude from the broken end of a myofibre or if they have escaped from a broken myofibre. Under the surface membranes of myofibres may be seen some flattened bubble-like inclusions. These are the nuclei of the myofibre, and their DNA may be stained by treating the macerated muscle suspension with dyes such as haematoxylin. On the surfaces of any myofibres retaining some of their surrounding connective tissue may be seen branching capillaries, once part of the vascular bed of the muscle. Red blood cells (erythrocytes) are rarely seen in the capillaries.

When meat animals are slaughtered, they are shackled and suspended from their hindlimbs 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 hindlimb, 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 myofibre fragments undergo some marked changes. Myofibre fragments may slowly swell and disappear, or they may expand so violently their interiors are extruded from their broken ends. Solubility of meat proteins in salt solutions is commercially important. S salt-solubilized proteins are used to bind together the meat fragments in many types of processed meat products.

10.4 Transverse striations

With an ordinary light microscope at its highest magnification, it is often possible to see the transverse striations on individual fragments of myofibrils. 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 myofibrils 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 shadowing differences in glassy properties of the specimen). In the electron micrograph below, the fibrils are running from the top left to the bottom right. The clear space at the top right is outside the myofibre.

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. Also marked are some other features - a mitochondrion (m) and the start of a transverse tubule (t) which we will consider later. If the clear spaces at top right is outside the myofibre, then you can see the membrane around the myofibre, and you can see the transverse tubule is a finger-like inpushing connecting with the space outside the myofibre.

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 myofibril.

The transmission electronmigrograph above is not the only way to look at transverse striations. 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 myofibrils 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).


Yet another way of looking at myofibrils is to examine a transverse section by transmission electron microscopy, as seen below. We are looking inside one myofibre of pork and once myofibril is centred in the field.  From the bar scale at the bottom left you can see the myofibril is a little over one micrometre in diameter. The space around the myofibril is greatly enlarged because the myofibril is loosing fluid - more about this multimillion dollar problem later in the course! The arrow shows part of the cytoskeleton resisting the shrinkage of the myofibrils and the loss of fluid from the myofibril.

10.5 Myofilaments

At the ultrastructural level, the transverse striations of myofibrils are caused by the regular longitudinal arrangement of sets of thick myofilaments (10 to 12 nm in diameter) and thin myofilaments (5 to 7 nm in diameter). In a transverse section cut through overlapping thick and thin myofilaments, each thick myofilament is surrounded by six thin myofilaments, although this hexagonal lattice may change to a tetragonal lattice when sarcomeres are stretched.


When a myofibre contracts, the thick filaments slide between the thin filaments so the I band gets shorter. The length of the A band remains constant. This is called the sliding filament theory of muscle contraction proposed  in 1972. If a muscle is at its resting length, the gap between opposing thin myofilaments at the mid-length of the sarcomere causes a pale H zone in the A band. Although the sliding myofilament 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. Myosin is the dominant protein of thick myofilaments.  Actin is the dominant protein of thin myofilaments. Contraction by myofilament sliding may be achieved by the rowing action of numerous cross bridges  protruding from the thick myofilaments, Cross bridges are formed from the heads of myosin molecules whose backbones are bound into the thick myofilament (more details later). However, the conformational change causing 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.

Myoilament sliding and muscle contraction come from the rowing action 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. Rigor mortis is very important in the meat industry - it is taken as the point at which muscle becomes meat. This important topic will be considered in more detail later.

 Further information

Structure and Development of Meat Animals and Poultry. Chapter 5.