Regeneration of skeletal muscle tissue. Muscle as organ. Changes of muscles with aging and depending on life-style
General morphofunctional characteristic of muscle tissues. Morphofunctional and histogenetic classification of muscle tissues. Muscle tissues of epidermal and neural origin.
Muscle tissues.
• are group of tissues having different origin and structure, but united on functional sign – contractility.
• They provide moving of whole organism in space and moving of different its parts and of organs inside of organism (heart, tongue, intestine).
For all groups of muscle tissues are typical:
1) elongated shaped of their structural components;
2) presence of longitudinally located myofibrils – special organelles providing contractility;
3) location of mitochondria near contractile elements;
4) presence of inclusions of glycogen, lipids and myoglobin.
Classification of muscle tissues:
There are 2 classifications of muscle tissues:
1) morphofunctional
2) histogenetic.
Morphofunctional classification is based on structure of organelles of contraction:
1.Striated muscle tissues, in which contractile organelles (myofibrils) have transversal striations. To this type of tissue skeletal and cardiac muscle tissues are related.
2.Smooth (not striated) muscle tissues, in which contractile organelles (myofibrils) do not have transversal striation.
Histogenetic classification is based on source of development of muscle tissues:
1.mesenchyme muscle tissues which are derived from mesenchyme;
2.epidermal – are derived from skin ectoderm;
3.neural – are derived from nerve tube;
4.celomic – are derived from myoepicardail lamina of splanchnotom (ventral mesoderm);
5.somatic – are derived from dorsal mesoderm.
Muscle tissues
• Smooth muscle tissue:
1. mesenchyme
2. epidermal
3. neural
• Striated muscle tissue:
1. celomic
2. somatic
Smooth muscle tissues
There are 3 groups of smooth muscle tissues:
• mesenchyme
• epidermal
• neural
Smooth muscle tissue of epidermal origin (ectodermal)
• It is represented by myoepithelial cells (2) of sweat, mammary, salivary, lacrimal glands.
• These cells have stellate shape.
• At the central part of cell nucleus and organelles of common significance are located, in the islets contractile apparatus is located.
• It is organized as in smooth myocytes of mesenchyme origin.
• Contraction of processes leads to excretion of secretion from gland.
Muscle tissue of neural origin
• Myocytes of this tissue are derived from cells of neural rudiment in structure of the wall of eye goblet.
• Myocytes form 2 muscles of eye iris – dilator pupil and sphincter pupil.
Regeneration of smooth muscle
• Physiological regeneration of smooth muscle tissue appears in conditions of high functional exertions.
• It is sharply seen in muscle layer of the uterus during pregnancy.
• Myocytes grow, synthetic processes in cytoplasm are activated, number of myofilaments is increased.
• Some cells are proliferated.
Skeletal muscle tissue. Sources of development. Muscle fibre as structural unit of tissue. Structure of muscle fibre. Sarcomer as structural unit of myofibril. Mechanism of muscle contraction.
Striated muscle tissues are
skeletal and cardiac
• Skeletal muscle tissue is derived from myotomes of sommites.
• It is related to somatic type.
• Skeletal muscle tissue has symplastic type of structure.
• The structural unit of skeletal muscle tissue is muscle fiber.
Muscle fiber consists of myosymplast and myosattelites (3).
• Myosymplast has number of oblong nuclei located at the periphery, immediately under the sarcolemma.
• Number of nuclei of myosymplast can reach a few dozen thousands.
• The basic part of myosymplast myofibrils compose.
Myosattelite
• Myosatellites are slightly differentiated one nuclear cells.
• They do not contain special organelles.
• Myosatelites are source of regeneration of muscle tissue, its cambial elements.
• Myosatellites are located between basement membrane and plasmalemma of muscle fibre.
Striated muscle tissue of tongue
• Myofibrils are located at the center.
• Each myofibril has striations and consists of thick myofilaments and thin myofilaments.
• Thick myofilaments contain protein myosin.
• Thin myofilaments contain protein actin, troponin.
• Thick myofilaments make up dark anisotropic disk A, characterized by double light-refraction.
• Dark disk A consists of myosin and partly actin myofilaments.
• Through the middle of disk A M - band – mesofragm passes.
• Thin myofilaments form light isotropic disk I without double light-refraction.
• Through the middle of I disk Z- band - thelogfragm passes.
• Light disk I consists of actin myofilaments.
• During contraction I-disk is decreased or dissappears.
Sarcomere
• The parcel between two thelofragms (Z-bands) is called sarcomere.
• Sarcomere is structural-functional unit of myofibril.
• Z-bands of sarcomers provide connection between actin threads of neighbor sarcomers.
• Formula of sarcomere is
½ of I disk + A-disk + ½ of I-disk
• Tubular invaginations of the sarcolemma penetrate the myofibers in a transverse direction.
• These finger-like invaginations of the sarcolemma form a complex of anastomosing network of tubules are known as the T-tubules (transverse tubules).
• They are found at the area of overlap between the A and I bands of myofibrils
• Each sarcomere has two of these tubules.
• The sarcoplasmic reticulum is a network of sarcotubules surrounding each myofibril.
• Adjacent to opposite sides of each T-tubule are expanded swollen terminal cisternae or sacs of the sarcoplasmic reticulum.
• This specialized complex, consisting of a T-tubule with 2 terminal cisterns of sarcoplasmic reticulum, is known as the triad.
Endomysium, perimysium, epimysium
• Between muscle fibers thin layers of loose fibrous connective tissue are located. It is endomysium.
• More thick layers of loose fibrous connective tissue surround bundle of muscle fibers forms perimysium.
• Connective tissue surrounding whole muscle is called epimysium.
Histophysiology of contraction
• Muscle contraction occurs because myosin heads attach to and “walk” along the thin myofilaments at both ends of a sarcomere, progressively pulling the thin myofilaments toward the M line.
• As a result, the thin myofilaments slide inward and meet at the center of a sarcomere.
• They may even move so far inwards that their ends overlap as the thin myofilaments slide inward, the Z lines come closer together, and the sarcomere shortens.
• However, the lengths of the individual thick and thin myofilaments do not change.
• Shortening of the sarcomeres causes shortening of the whole muscle fiber, which in turn leads to shortening of the entire muscle.
• At the onset of contraction, the sarcoplasmic reticulum releases calcium ions (Ca2+) into the cytosol.
• There, they bind to troponin and cause the troponin-tropomyosin complexes to move away from the myosin-binding sites on action.
• Once the binding sites are “free”, the contraction cycle begins.
The contraction cycle consists of four steps:
• 1. ATP hydrolysis.
• 2. Attachment of myosin to actin to form cross-bridges.
• 3. Power stroke.
• 4. Detachment of myosin from actin.
Classification of muscle fibers
Muscle fibers are classified into three main categories:
• 1.slow oxidative fibers (type I),
• 2.fast glycolytic fibers (type IIa) and
• 3.intermediate fast oxidative-glycolytic fibers (type IIb).
Regeneration of skeletal muscle tissue. Muscle as organ. Changes of muscles with aging and depending on life-style.
Regeneration of skeletal muscle
• The source of regeneration of skeletal muscle is myosatellite.
• While organism grows myosatellites are divided and daughter cells are built into ends of symplasts.
• After finishing of growth division of myosatellites is stopped.
• Regeneration of skeletal muscle is realized at the expense of 2 mechanisms: compensative hypertrophy of myosymplast and proliferation of myosatellites.
Age-related changes of skeletal muscle
• With aging, humans undergo a slow, progressive loss of skeletal muscle mass that is replaced largely by fibrous connective tissue and adipose tissue.
• In fibers regularity of arrangement of mitochondria is damaged, mitochondria can hypertrophy and are degenerated or giant forms appear.
• Volume of sarcoplasmic net is increased.
• Some myofibrils loose transversal striation, are fragmented and myofilaments are disorganized.
• In myofibres brown pigment lipofuscin is accumulated.
• As a result of growth of connective tissue resiliency and elasticity are decreased.
• All these changes make muscles quickly tired.
Cardiac muscle tissue. Sources of development. Classification of cardiomyocytes. Features of structure of contractile and conducting cardiomyocytes. Possibilities of regeneration of cardiac muscle tissue.
Cardiac muscle tissue
• Cardiac muscle tissue is related to celomic type.
• It is derived from visceral layer of ventral mesoderm (myoepicardial lamina).
• The structural-functional unit of cardiac muscle tissue is cardiac muscle cell – cardiomyocyte.
Cardiomyocytes
• Cardiomyocytes are joined with the help of intercalated disks forming functional fibres.
• They are elongated with centrally located nucleus (1 or 2).
• Contractile apparatus is represented by myosin and actin myofilaments.
Cardiac muscle tissue
• During histogenesis 5 types of cardiomyocytes appear.
• These are:
1.working (contractile)
2.sinus
3.transitional
4.conducting
5.secretory
Possibilities of regeneration of cardiac muscle tissue
• During long work (f.i. in conditions of constant arterial pressure of blood) working hypertrophy of cardiomyocytes occurs.
• Stem cells or cells-predecessors of cardiomyocytes are absent in muscle tissue, so dead cardiomyocytes (infarct of myocardium) are not regenerated.