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STRIATED MYOCYTE

Striated skeletal muscle cells (SSMC) form the voluntary contraction muscles, which are usually attached to bones by tendons. However, there are SSMC not attached to muscles. SSMC are actually syncytia, that is, a cytoplasm containing many nuclei. They are very long cells characterized by a highly well-developed cytoskeleton. The cytoskeleton is responsible for the shortening of the cell length, which in turn produces the muscle contraction, and therefore the movement of the organism.

1. Morphology

SSMC are really long cells ranging from several millimeters to more than one meter. That is why they are also known as muscle fibers. In transverse section, they are 10 to 100 µm in thickness. These dimensions are achieved by the fusion of several hundreds of undifferentiated muscle cells or myoblasts and a posterior period of elongation. SSMC contain many nuclei located in the peripheral cytoplasm, just below the plasma membrane or sarcolemma. During differentiation, nuclei of myoblasts are lined up in the interior of the cell and then move to the periphery where they immobilized by the cytoskeleton. Many other organelles are also displaced to the cell periphery. These movements leave much room in the inner part of the cell, which can be occupied by the cell contraction machinery (Figures 1 and 2). The plasma membrane of SSMC show many deep invaginations that form the T-tubules (transverse tubules). T-tubules are found at the level of the Z lines (see below) and are involved in cell contraction.

Skeletal muscle
Figure 1. Striated skeletal muscles of the rat tongue. The image on the right shows the direct adhesions to the hyoid bone. No tendon is formed.
Skeletal muscle
Figure 2. Striated skeletal muscles in longitudinal view (top) and transverse view (bottom).

The SSMC content is mostly cytoskeleton, including actin filaments and myosin II filaments. Both types of filaments associated to form bundles known as myofibrils. Myofibrils can be visualized at light microscopy in transverse sections (Figure 2). Each myofibril is surrounded by membranes of the smooth endoplasmic reticulum, referred in SSMC as sarcoplasmic reticulum (Figure 3). The sarcoplasmic reticulum is organized as a net of tubules around the myofibrils, and this net is also tightly associated with T tubules. Interspersed among the myofibrils, there are mitochondria and inclusions of glycogen. Most organelles are found between the myofibril bundles and the plasma membrane.

skeletal muscle cells
Figure 3. Relation between T tubules and endoplasmic reticulum (sarcoplasmic reticulum) that wraps myofibrils.

When longitudinally oriented SSMC are observed at light microscopy in a longitudinal view, a pattern of dark and light striations is visualized. Striations are perpendicularly oriented to the long cellular axis (Figure 4). This striated pattern is a consequence of the organization of the cytoskeleton filaments that form myofibrils. Dark striations are called A bands (A stands for anisotropy, which can be observed with the polarization microscope) and light striations or I bands (I stands for isotropy). Inside each A and I bands a division line can be observed, known as Z and H discs, respectively. Z discs has a zigzag organization when observed at electron microscopy. H discs have an additional inner darker line called M line. The portion of myofibril between two consecutive Z discs are known as sarcomere.

Sarcomere
Figure 4. Sarcomere organization.

2. Sarcomere organization

The organization of SSMC sarcomeres is the result of the arrangement of a wide variety of proteins (Figure 5). Let's see some of them and their functions.

Sarcómero
Figura 5. Sarcomere organization and its interaction with the plasma membrane (adapted from Clarck et al., 2002).

Thin filaments

Actin. Actin molecules are the main component of thin filaments in the sarcomere. Each thin filament is composed of two coiled alpha-helix of actin proteins associated to tropomyosins and troponins (Figure 6). Mature thin filaments are 1 µm long, and this length is extraordinary constant in SSMCs thanks to the influence of many associated proteins (see below). Although there are several actin isoforms showing a similar structure in the body, they are expressed in both different cell types and developmental stages. Two of them are specifically expressed in the SSMC and cardiac muscle, and two can be expressed in SSMCs and in other tissues.

Topomyosin, troponin
Figure 6. Organization of tropomyosin, troponin, and actin filaments.

Tropomyosin. It consists of two amino acid helical alpha-chains associated with actin filaments. There are several tropomyosin isoforms derive from alternative splicing of 4 genes. Tropomyosin chains are as long as the actin filaments and are placed at the grooves left by the actin proteins. The main role of tropomyosin, together with troponin, is regulate the interactions between actin filaments and myosin II heads (which form the thick filaments). Tropomyosin may change its spatial conformation, and either may allow or inhibit the physical contact between actin filaments and myosin II heads. During muscle relax, tropomyosin does not permit the actin-myosin contact. The increase in calcium concentration after the electrical stimuli (motoneuron innervation) makes possible the actin-myosin interaction, and therefore the effective muscle contraction. It is interesting that the precise position of tropomyosin over the actin filament may chenge slightly depending on the tropomyosin isoform expressed.

Troponin. There are three types of troponin associated with actin filaments: C, I and T. The three molecules form a complex that cooperate with tropomyosin to let actin filament and myosin filament interact between each other. There are several isoforms of troponin types, so that cell expression of different isoform regulates the muscle contraction in a slightly different way. Troponin mechanism in muscle cells is not fully known, but 25 % of hypertrophic cardiomyopathies are caused by mutations in troponin genes.

CapZ and tropomodulin. They are proteins attached to the ends of actin filaments that control the length and shortening of actin filaments. CapZ joins the plus end and influences the nucleation and stability of actin filament. CapZ is a heterodimer formed of alpha and beta units that in SSMCs is found in the Z line, where actin filament is attached. Tropomodulin is linked to the minus end, the free end of the actin filament. Its ability to join and stabilize the minus end depends on the presence of tropomyosin molecules. Tropomodulin is essential to establish the total length and stability of the actin filaments, which determines the contractile properties of the muscle cell. For example, tropomodulin defects may cause longer actin filaments leading to slower contraction rhythms.

Thick filaments

Myosin. Thick filaments of the sarcomere is made up of myosin II and several associated proteins (Figure 7). There are hundreds of myosin II proteins in each thick filament. Myosin molecule consists of a globular domain or head that generates the movement, and a lineal domain that is linked to other lineal domains of other myosins molecules. All the lineal domains together form the stem of the thick filament. There are several myosin II isoforms that may affect the activity of the globular domain and that are expressed selectively in different types of muscles. The overlapping between the thick and the thin filaments form the A band of the sarcomere, the darker ones. The segment of the thick filament non overlapped with the actin filaments forms the H discs. The globular domains of myosin contact with the actin filaments to drag them and produces the shortening of the sarcomere. Other proteins associated with the thick filament are the C and H proteins, titin and AMP-desaminase.

Thick filament
Figure 7. Thick filament organization.

M line is the area where the thick filaments are anchored. Myomesin is a protein found in M line that binds myosin and titin, and might develop a relevant structural role. In cardiomyocytes and fast contraction SSMCs, M line contains the M protein. MURF-1 is another protein of M line that binds titin.

Other proteins

La itin can be regarded as a third filament of the sarcomere. After actin and myosin, titin is the third most abundant protein in muscles and is the longest protein found so far (38138 amino acids). It is anchored to the Z line by the amino terminal end, and to the M line by the carboxyl terminal end. Both ends of the titin of one sarcomere overlap with titins of adjoining sarcomeres forming continuous scaffold. The segment of the protein found in the I Band is elastic and may counteract the sarcomere stretching, as well as limiting the maximal length of the sarcomere.

Nebulin is another giant protein, extending from the Z line untill the free end of the actin filament. It is a protein that cannot be stretched and this property makes nebulin a good candidate to control the exact length of the actin filament. It also may modulate the interactions between actin and myosin filaments. Curiously, nebulin has not been found in muscle cells, although other similar proteins may perform its function.

There are many intermediate filaments associated with the Z line that probably help with the lateral connections of myofibrils. One of these intermediate filaments is desmin, which is also associated with costameres (see below). Desmin interlocks Z disc and establishes connections between Z line and the plasma membrane, nuclei, mitochondria, and probably microtubules. There are also proteins working as transducers of signals that may exchange their position between the Z line and other cell locations. Alpha-actinin is a protein linking actin filaments between one another, MLP helps to keep the sacomere integrity and connects myofibrils and nuclei. Other proteins are FATZ, myopaladins, filamins, telotonins, and many others.

Costameres

Myofibrils of SSMCs must be anchored to the plasma membrane. Costameres are protein complexes found in the plasma membrane that are anchoring points for Z an M lines. SSMCs are wrapped by a special extracellular matrix called basal lamina that provides structural integrity to SSMCs. Costameres are the intermediary structures that link myofibrils with the basal lamina. Integrins are among the proteins mediating this anchoring that, in addition, work as mechanical sensors of cell stretching. A central protein of costameres is dystrophin, which binds the laminins of the basal lamina. Dystrophin mutations cause muscle dystrophies. Spectrin is another protein found in costameres that collaborate in the connection between costameres and SSMC cytoskeleton.

Plectins are proteins that work as intermediaries between intermediate filaments and myofibrils. Ankyrins facilitates the association of costameres to the plasma membrane, but also influence the distribution of channels and pumps of the plasma membrane into specific domains.

Some proteins associated to the sarcomere have been found in the nucleus too. These proteins can cross the nuclear envelope and are thought to be messengers carrying information to the nucleus about the state of the sarcomeres.

Mechanism

SSMCs get shorter by the activity of their cytoskeleton. The contraction unit is called sarcomere, which is the segment of the myofibril between two consecutive Z discs. This is about 4 µm in length in a relaxed muscle. During contraction, Z discs get closer at a distance of about 1 µm. This happens because I bands are shortened when actin filaments are dragged over the myosin II filaments. The A band keeps the same length.

Shortening of I band can be explained as follows. Thick filaments are organized in a way that the myosin II molecules of one half of the filament show antiparallel orientation when compared with the myosin II molecules of the other half (see Figure 7). So, the traction forces in the two halves are toward the center of the thick filament, and each half is dragging actin filaments from different I bands.

3. Innervation

SSMCs are innervated by axons of motoneurons, neurons that can be located in the brain or in the spinal cord. Each motoneuron is able to contact several SSMCs, but a SSMC receives innervation from only one motoneuron. A motor unit is a motoneuron and all SSMCs it innervates. Motoneuron axons make special synaptic contacts onto SSMCs known as motor plates or neuromuscular junctions. The neurotransmitter released in motor plates is the acetylcholine. Some SSMCs are also innervated by sensory terminals forming the encapsulated receptors and tendinous receptors, both carrying information about the contraction and position states of SSMCs.

4. Types

The function of SSMCs is muscle contraction that result in animal movement. Histochemical techniques are able to distinguish 3 types of SSCM: red, intermediate, and white. The three types are present in each muscle but with different proportions according to the type of muscle. They differ in the amount of myoglobine they contain. Myoglobine is a protein of SSMCs that is able to transport oxygen. Red SSMCs are short cells with a high concentration of myoglobin and many mitochondria, that allow a slow and sustained contraction. They are abundant in those muscle responsible for keep the body posture. White SSMCs contain a lower amount of myoglobin and less mitochondria, and are longer cells. They are able to make fast contraction, but they fatigue quickly, and participate in fast and precise movements. Intermediate fibers show average properties. La función de los miocitos es la contracción y como resultado la producción de movimiento. Mediante técnicas histoquímicas se pueden detectar tres tipos de células musculares: rojas, claras e intermedias. Los tres tipos están presentes en todos los músculos pero su proporción varía según la función particular de dicho músculo. La diferencia entre ellos es la presencia de más o menos mioglobina, que es un proteína presente en el interior de la célula y con capacidad para transportar oxígeno. Así, las fibras rojas son células de pequeño tamaño con una gran concentración de mioglobina en su interior y con muchas mitocondrias, y esto les permite una contracción lenta y sostenida, por ejemplo en aquellas que permiten mantener la postura. Las fibras blancas poseen menos mioglobina y menos mitocondrias, y tienen un tamaño celular mayor. Son células con capacidad para una contracción rápida, pero se fatigan fácilmente. Estos músculos participan en los movimientos precisos y rápidos. Las fibras intermedias tienen propiedades mixtas.

Bibliography

Clark KA, McElhinny AS, Beckerle MC, Gregorio CC. 2002. Striated muscle cytoarchitecture: an intricate web of form and function. Annual review of cell and development biology. 18: 637-706.

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