The meaning of the word satellite cells in medical terms. Where are the satellite cells of skeletal muscle tissue located? Satellite cells

Muscle tissue carries out the motor functions of the body. Some of the histological elements of muscle tissue have contractile units - sarcomeres (see Fig. 6-3). This circumstance allows us to distinguish between two types of muscle tissue. One of them - striped(skeletal and cardiac) and the second - smooth. It functions in all contractile elements of muscle tissue (striated skeletal muscle fiber, cardiomyocytes, smooth muscle cells - SMC), as well as in non-muscle contractile cells. actomyosin chemomechanical transducer. Contractile function of skeletal muscle tissue (voluntary muscles) controlled by the nervous system (somatic motor innervation). Involuntary muscles (cardiac and smooth) have autonomic motor innervation, as well as a developed humoral control system. SMCs are characterized by pronounced physiological and reparative regeneration. Skeletal muscle fibers contain stem cells (satellite cells), so skeletal muscle tissue is potentially capable of regeneration. Cardiomyocytes are in the G 0 phase of the cell cycle, and there are no stem cells in cardiac muscle tissue. For this reason, dead cardiomyocytes are replaced by connective tissue.

Skeletal muscle tissue

Humans have more than 600 skeletal muscles (about 40% of body weight). Skeletal muscle tissue provides conscious and conscious voluntary movements of the body and its parts. The main histological elements: skeletal muscle fibers (contraction function) and satellite cells (cambial reserve).

Sources of development histological elements of skeletal muscle tissue - myotomes and neural crest.

Myogenic cell type consists of the following stages: myotome cells (migration) → mitotic myoblasts (proliferation) → postmitotic myoblasts (fusion) → myoblasts

cervical tubules (synthesis of contractile proteins, formation of sarcomeres) → muscle fibers (contraction function).

Muscular tube. After a series of mitotic divisions, myoblasts acquire an elongated shape, line up in parallel chains and begin to merge, forming myotubes (myotubes). In myotubes, contractile proteins are synthesized and myofibrils are assembled - contractile structures with characteristic transverse striations. The final differentiation of the muscular tube occurs only after its innervation.

Muscle fiber. The movement of symplast nuclei to the periphery completes the formation of striated muscle fiber.

Satellite cells- G 1 myoblasts that have separated during myogenesis and are located between the basement membrane and the plasmalemma of muscle fibers. The nuclei of these cells make up 30% in newborns, 4% in adults and 2% in the elderly of the total number of nuclei of skeletal muscle fiber. Satellite cells are the cambial reserve of skeletal muscle tissue. They retain the ability for myogenic differentiation, which ensures the growth of muscle fibers in length in the postnatal period. Satellite cells are also involved in the reparative regeneration of skeletal muscle tissue.

SKELETAL MUSCLE FIBER

The structural and functional unit of skeletal muscle - symplast - skeletal muscle fiber (Fig. 7-1, Fig. 7-7), has the shape of an extended cylinder with pointed ends. This cylinder reaches a length of 40 mm with a diameter of up to 0.1 mm. The term "fiber sheath" (syarcolemma) designate two structures: the plasmolemma of the symplast and its basement membrane. Between the plasma membrane and the basement membrane are located satellite cells with oval kernels. The rod-shaped nuclei of the muscle fiber lie in the cytoplasm (sarcoplasm) under the plasmalemma. The contractile apparatus is located in the sarcoplasm of the symplast - myofibrils, Ca 2+ depot - sarcoplasmic reticulum(smooth endoplasmic reticulum), as well as mitochondria and glycogen granules. From the surface of the muscle fiber to the expanded areas of the sarcoplasmic reticulum, tube-shaped invaginations of the sarcolemma - transverse tubules - are directed(T-tubes). Loose fibrous connective tissue between individual muscle fibers(endomysium) contains blood and lymphatic vessels, nerve fibers. Groups of muscle fibers and the fibrous connective tissue surrounding them in the form of a sheath(perimysium) form bundles. Their combination forms a muscle, the dense connective tissue cover of which is called epimysium

(Figure 7-2).

The transverse striation of the skeletal muscle fiber is determined by the regular alternation of different refractive indexes in the myofibrils.

Rice. 7-1. Skeletal muscle consists of striated muscle fibers.

A significant volume of muscle fiber is occupied by myofibrils. The arrangement of light and dark disks in myofibrils parallel to each other coincides, which leads to the appearance of transverse striations. The structural unit of myofibrils is the sarcomere, formed from thick (myosin) and thin (actin) filaments. The arrangement of thin and thick filaments in the sarcomere is shown on the right and below. G-actin is globular, F-actin is fibrillar actin.

Rice. 7-2. Skeletal muscle in longitudinal and cross section. A- lengthwise cut; B- cross section; IN- cross section of an individual muscle fiber.

areas (disks) that contain polarized light - isotropic and anisotropic: light (Isotropic, I-disks) and dark (Anisotropic, A-disks) disks. The different light refraction of the discs is determined by the ordered arrangement of thin and thick filaments along the length of the sarcomere; thick threads are found only in dark disks; light disks do not contain thick threads. Each light disk is crossed by a Z-line. The area of the myofibril between adjacent Z-lines is defined as a sarcomere. Sarcomere. The structural and functional unit of the myofibril, located between adjacent Z-lines (Fig. 7-3). The sarcomere is formed by thin (actin) and thick (myosin) filaments located parallel to each other. The I-disc contains only thin filaments. In the middle of the I-disc there is a Z-line. One end of the thin filament is attached to the Z-line, and the other end is directed towards the middle of the sarcomere. Thick filaments occupy the central part of the sarcomere - the A-disc. Thin threads partially fit between thick ones. The section of the sarcomere containing only thick filaments is the H-zone. In the middle of the H-zone there is an M-line. The I-disk is part of two sarcomeres. Consequently, each sarcomere contains one A-disc (dark) and two halves of the I-disc (light), the sarcomere formula is 1/2 I + A + 1/2 I.

Rice. 7-3. Sarcomere contains one A-disc (dark) and two halves of the I-disc (light). Thick myosin filaments occupy the central part of the sarcomere. Titin connects the free ends of myosin filaments to the Z-line. Thin actin filaments are attached to the Z-line at one end, and the other is directed to the middle of the sarmeter and partially inserts between the thick filaments.

Thick thread. Each myosin filament consists of 300-400 molecules of myosin and C protein. Half of the myosin molecules face their heads towards one end of the filament, and the other half – towards the other. The giant protein titin binds the free ends of the thick filaments to the Z-line.

Thin thread consists of actin, tropomyosin and troponins (Fig. 7-6).

Rice. 7-5. Thick thread. Myosin molecules are capable of self-assembly and form a spindle-shaped aggregate with a diameter of 15 nm and a length of 1.5 μm. Fibrillar tails molecules form the core of a thick filament, the myosin heads are arranged in spirals and protrude above the surface of the thick filament.

Rice. 7-6. Thin thread- two spirally twisted filaments of F-actin. In the grooves of the helical chain lies the double helix of tropomyosin, along which troponin molecules are located.

Sarcoplasmic reticulum

Each myofibril is surrounded by regularly repeating elements of the sarcoplasmic reticulum - anastomosing membrane tubes ending in terminal cisterns (Fig. 7-7). At the border between the dark and light disks, two adjacent terminal cisterns contact T-tubules, forming so-called triads. The sarcoplasmic reticulum is a modified smooth endoplasmic reticulum that functions as a calcium depot.

Pairing of excitation and contraction

The sarcolemma of the muscle fiber forms many narrow invaginations - transverse tubules (T-tubules). They penetrate into the muscle fiber and, lying between the two terminal cisterns of the sarcoplasmic reticulum, together with the latter form triads. In triads, excitation is transmitted in the form of an action potential from the plasma membrane of the muscle fiber to the membrane of the terminal cisterns, i.e. the process of pairing excitation and contraction.

INNERVATION OF SKELETAL MUSCLES

Skeletal muscles are divided into extrafusal and intrafusal muscle fibers.

Extrafusal muscle fibers performing the function of muscle contraction, has direct motor innervation - a neuromuscular synapse formed by the terminal branching of the α-motoneuron axon and a specialized section of the muscle fiber plasmalemma (end plate, postsynaptic membrane, see Fig. 8-29).

Intrafusal muscle fibers are part of the sensitive nerve endings of skeletal muscle - muscle spindles. Intrafusal muscle

Rice. 7-7. Fragment of skeletal muscle fiber. Sarcoplasmic reticulum cisterns surround each myofibril. T-tubules approach myofibrils at the level of the boundaries between dark and light disks and, together with the terminal cisterns of the sarcoplasmic reticulum, form triads. Mitochondria lie between the myofibrils.

These fibers form neuromuscular synapses with the efferent fibers of γ-motoneurons and sensory endings with the fibers of pseudounipolar neurons of the spinal ganglia (Fig. 7-9, Fig. 8-27). Motor somatic innervation skeletal muscles (muscle fibers) is carried out by α- and γ-motoneurons of the anterior horns of the spinal

Rice. 7-9. Innervation of extrafusal and intrafusal muscle fibers. Extrafusal muscle fibers of the skeletal muscles of the trunk and limbs receive motor innervation from α-motoneurons of the anterior horns of the spinal cord.

Intrafusal muscle fibers as part of muscle spindles have both motor innervation from γ-motoneurons and sensory innervation (afferent fibers of types Ia and II of sensory neurons of the spinal ganglion). brain and motor nuclei of the cranial nerves, and sensory somatic innervation - pseudounipolar neurons of the sensory spinal ganglia and neurons of the sensory nuclei of the cranial nerves. Autonomic innervation

no muscle fibers were detected, but the SMC walls of the blood vessels of skeletal muscles have sympathetic adrenergic innervation.

CONTRACTION AND RELAXATION

Contraction of the muscle fiber occurs when a wave of excitation in the form of nerve impulses arrives along the axons of motor neurons to the neuromuscular synapses (see Fig. 8-29) and the release of the neurotransmitter acetylcholine from the terminal branches of the axon. Further events unfold as follows: depolarization of the postsynaptic membrane → propagation of the action potential along the plasma membrane → signal transmission through triads to the sarcoplasmic reticulum → release of Ca 2 + ions from the sarcoplasmic reticulum

cellular network → interaction of thin and thick filaments, resulting in shortening of the sarcomere and contraction of the muscle fiber → relaxation.

TYPES OF MUSCLE FIBERS Skeletal muscles and the muscle fibers that form them differ in many ways. Traditionally distinguished red, white And intermediate, and slow and fast

muscles and fibers.(oxidative) muscle fibers are small in diameter, surrounded by a mass of capillaries, and contain a lot of myoglobin. Their numerous mitochondria have high levels of oxidative enzyme activity (for example, succinate dehydrogenase).

White(glycolytic) muscle fibers have a larger diameter, the sarcoplasm contains a significant amount of glycogen, and mitochondria are few. They are characterized by low activity of oxidative enzymes and high activity of glycolytic enzymes.

Intermediate(oxidative-glycolytic) fibers have moderate succinate dehydrogenase activity.

Fast muscle fibers have high myosin ATPase activity.

Slow fibers have low myosin ATPase activity. In reality, muscle fibers contain combinations of different characteristics. Therefore, in practice, three types of muscle fibers are distinguished - fast twitch red, fast twitch white red, white slow-twitch intermediates.

MUSCLE REGENERATION AND TRANSPLANTATION

Physiological regeneration. In skeletal muscle, physiological regeneration constantly occurs - renewal of muscle fibers. In this case, satellite cells enter cycles of proliferation followed by differentiation into myoblasts and their inclusion in pre-existing muscle fibers.

Reparative regeneration. After the death of the muscle fiber under the preserved basement membrane, activated satellite cells differentiate into myoblasts. The postmitotic myoblasts then fuse to form myotubes. The synthesis of contractile proteins begins in myoblasts, and in myotubes the assembly of myofibrils and the formation of sarcomeres occur. Migration of nuclei to the periphery and formation of the neuromuscular synapse complete the formation of mature muscle fibers. Thus, during reparative regeneration, the events of embryonic myogenesis are repeated.

Transplantation. When transferring muscles, a flap from the latissimus dorsi muscle is used. Removed from the box along with his own

Using blood vessels and nerves, the flap is transplanted to the site of the muscle tissue defect. Cambial cell transfer is also beginning to be used. Thus, in hereditary muscular dystrophies, myoblasts normal for this characteristic are injected into muscles defective in the dystrophin gene. With this approach, they rely on the gradual renewal of defective muscle fibers with normal ones.

Cardiac muscle tissue

Cardiac-type striated muscle tissue forms the muscular lining of the heart wall (myocardium). The main histological element is the cardiomyocyte.

Cardiomyogenesis. Myoblasts originate from the cells of the splanchnic mesoderm surrounding the endocardial tube. After a series of mitotic divisions, Gj-myoblasts begin the synthesis of contractile and auxiliary proteins and, through the G0-myoblast stage, differentiate into cardiomyocytes, acquiring an elongated shape. Unlike striated muscle tissue of the skeletal type, in cardiomyogenesis there is no separation of the cambial reserve, and all cardiomyocytes are irreversibly in the G 0 phase of the cell cycle.

CARDIOMYOCYTES

The cells (Fig. 7-21) are located between the elements of loose fibrous connective tissue containing numerous blood capillaries of the coronary vessel basin and terminal branches of the motor axons of nerve cells of the autonomic division of the nervous system.

Rice. 7-21. Heart muscle in the longitudinal (A) and transverse (B) section.

systems.

Each myocyte has a sarcolemma (basal membrane + plasmalemma). There are working, atypical and secretory cardiomyocytes.

Working cardiomyocytes

Working cardiomyocytes - morpho-functional units of cardiac muscle tissue, have a cylindrical branching shape with a diameter of about 15 microns (Fig. 7-22). With the help of intercellular contacts (intercalated discs), working cardiomyocytes are united into so-called cardiac muscle fibers - a functional syncytium - a collection of cardiomyocytes within each chamber of the heart. The cells contain centrally located, elongated along the axis, one or two nuclei, myofibrils and associated cisterns of the sarcoplasmic reticulum (Ca 2+ depot). Numerous mitochondria lie in parallel rows between myofibrils. Their denser clusters are observed at the level of I-disks and nuclei. Glycogen granules are concentrated at both poles of the nucleus. T-tubules in cardiomyocytes - unlike skeletal muscle fibers - pass at the level of the Z-lines. In this regard, the T-tubule contacts only one terminal tank. As a result, instead of skeletal muscle fiber triads, dyads are formed. Contractile apparatus.

The organization of myofibrils and sarcomeres in cardiomyocytes is the same as in skeletal muscle fiber. The mechanism of interaction between thin and thick filaments during contraction is also the same. At the ends of contacting cardiomyocytes there are interdigitations (finger-like protrusions and depressions). The growth of one cell fits tightly into the recess of another. At the end of such a protrusion (the transverse section of the intercalary disk), contacts of two types are concentrated: desmosomes and intermediate ones. On the side surface of the protrusion (longitudinal section of the insert disk) there are many slot contacts (nexus, nexus), transmitting excitation from cardiomyocyte to cardiomyocyte.

Atrial and ventricular cardiomyocytes. Atrial and ventricular cardiomyocytes belong to different populations of working cardiomyocytes. Atrial cardiomyocytes are relatively small, 10 µm in diameter and 20 µm in length. The system of T-tubules is less developed in them, but in the area of intercalary discs there are significantly more gap junctions. Ventricular cardiomyocytes are larger (25 µm in diameter and up to 140 µm in length), they have a well-developed T-tubule system. The contractile apparatus of atrial and ventricular myocytes includes different isoforms of myosin, actin and other contractile proteins.

Rice. 7-22. Working cardiomyocyte- an elongated cell. The nucleus is located centrally, near the nucleus there are the Golgi complex and glycogen granules. Numerous mitochondria lie between the myofibrils. Intercalated discs (inset) serve to hold cardiomyocytes together and synchronize their contraction.

Secretory cardiomyocytes. In some of the cardiomyocytes of the atria (especially the right one), at the poles of the nuclei there are a well-defined Golgi complex and secretory granules containing atriopeptin, a hormone that regulates blood pressure (BP). With an increase in blood pressure, the atrium wall is greatly stretched, which stimulates atrial cardiomyocytes to synthesize and secrete atriopeptin, which causes a decrease in blood pressure.

Atypical cardiomyocytes

This obsolete term refers to the myocytes that form the conduction system of the heart (see Fig. 10-14). Among them, pacemakers and conductive myocytes are distinguished.

Pacemakers(pacemaker cells, pacemakers, Fig. 7-24) - a collection of specialized cardiomyocytes in the form of thin fibers surrounded by loose connective tissue. Compared to working cardiomyocytes, they are smaller in size. The sarcoplasm contains relatively little glycogen and a small number of myofibrils, located mainly at the periphery of the cells. These cells have rich vascularization and motor autonomic innervation. The main property of pacemakers is spontaneous depolarization of the plasma membrane. When a critical value is reached, an action potential arises, propagating through electrical synapses (gap junctions) along the fibers of the conduction system of the heart and reaching working cardiomyocytes. Conducting cardiomyocytes

- specialized cells of the atrioventricular bundle of His and Purkinje fibers form long fibers that perform the function of conducting excitation from pacemakers. Atrioventricular bundle.

Cardiomyocytes of this bundle conduct excitation from pacemakers to Purkinje fibers and contain relatively long myofibrils with a spiral course; small mitochondria and a small amount of glycogen. Rice. 7-24. Atypical cardiomyocytes. A B- pacemaker of the sinoatrial node;

- conducting cardiomyocyte of the atrioventricular bundle. Purkinje fibers.

Conducting cardiomyocytes of Purkinje fibers are the largest cells of the myocardium. They contain a rare disordered network of myofibrils, numerous small mitochondria, and a large amount of glycogen. Cardiomyocytes of Purkinje fibers do not have T-tubules and do not form intercalary discs. They are connected by desmosomes and gap junctions. The latter occupy a significant area of contacting cells, which ensures a high speed of impulse transmission along the Purkinje fibers.

Parasympathetic innervation is carried out by the vagus nerve, and sympathetic innervation is carried out by adrenergic neurons of the cervical superior, cervical middle and stellate (cervicothoracic) ganglia. The terminal sections of axons near cardiomyocytes have varicosities (see Fig. 7-29), regularly located along the length of the axon at a distance of 5-15 µm from each other. Autonomic neurons do not form neuromuscular synapses characteristic of skeletal muscle. Varicose veins contain neurotransmitters, from where their secretion occurs. The distance from varicosities to cardiomyocytes is on average about 1 µm. Neurotransmitter molecules are released into the intercellular space and, through diffusion, reach their receptors in the plasmalemma of cardiomyocytes. Parasympathetic innervation of the heart. Preganglionic fibers running as part of the vagus nerve end on the neurons of the cardiac plexus and in the wall of the atria. Postganglionic fibers predominantly innervate the sinoatrial node, atrioventricular node and atrial cardiomyocytes. Parasympathetic influence causes a decrease in the frequency of impulse generation by pacemakers (negative chronotropic effect), a decrease in the speed of impulse transmission through the atrioventricular node (negative dromotropic effect) in Purkinje fibers, and a decrease in the force of contraction of working atrial cardiomyocytes (negative inotropic effect). Sympathetic innervation of the heart. Preganglionic fibers of neurons in the intermediolateral columns of the gray matter of the spinal cord form synapses with neurons of the paravertebral ganglia. Postganglionic fibers of neurons of the middle cervical and stellate ganglia innervate the sinoatrial node, atrioventricular node, atrial and ventricular cardiomyocytes. Activation of sympathetic nerves causes an increase in the frequency of spontaneous depolarization of pacemaker membranes (positive chronotropic effect), facilitating impulse conduction through the atrioventricular node (positive

telial dromotropic effect) in Purkinje fibers, increasing the force of contraction of atrial and ventricular cardiomyocytes (positive inotropic effect).

Smooth muscle tissue

The main histological element of smooth muscle tissue is the smooth muscle cell (SMC), capable of hypertrophy and regeneration, as well as the synthesis and secretion of intercellular matrix molecules. SMCs as part of smooth muscles form the muscular wall of hollow and tubular organs, controlling their motility and lumen size. The contractile activity of SMCs is regulated by motor autonomic innervation and many humoral factors. Development. Cambial cells of the embryo and fetus (splanchnomesoderm, mesenchyme, neuroectoderm) at the sites of smooth muscle formation differentiate into myoblasts, and then into mature SMCs, acquiring an elongated shape; their contractile and accessory proteins form myofilaments. SMCs within smooth muscles are in the G 1 phase of the cell cycle and are capable of proliferation.

SMOOTH MUSCLE CELL

The morpho-functional unit of smooth muscle tissue is the SMC. With their pointed ends, SMCs wedge between neighboring cells and form muscle bundles, which in turn form layers of smooth muscle (Fig. 7-26). In the fibrous connective tissue, nerves, blood and lymphatic vessels pass between the myocytes and muscle bundles. Single SMCs are also found, for example, in the subendothelial layer of blood vessels. MMC shape - extended

Rice. 7-26. Smooth muscle in longitudinal (A) and transverse (B) sections. In a cross section, myofilaments are visible as dots in the cytoplasm of smooth muscle cells.

nut fusiform, often processed (Fig. 7-27). The length of SMCs is from 20 µm to 1 mm (for example, SMCs of the uterus during pregnancy). The oval nucleus is localized centrally. In the sarcoplasm at the poles of the nucleus there is a well-defined Golgi complex, numerous mitochondria, free ribosomes, and the sarcoplasmic reticulum. Myofilaments are oriented along the longitudinal axis of the cell. The basement membrane surrounding SMCs contains proteoglycans, collagen types III and V. The components of the basement membrane and elastin of the intercellular substance of smooth muscles are synthesized both by the SMCs themselves and by connective tissue fibroblasts.

Contractile apparatus

In SMCs, actin and myosin filaments do not form myofibrils, characteristic of striated muscle tissue. Molecules

Rice. 7-27. Smooth muscle cell. The central position in the MMC is occupied by a large core. At the poles of the nucleus are mitochondria, the endoplasmic reticulum and the Golgi complex. Actin myofilaments, oriented along the longitudinal axis of the cell, are attached to dense bodies. Myocytes form gap junctions among themselves.

smooth muscle actin forms stable actin filaments, attached to dense bodies and oriented predominantly along the longitudinal axis of the SMC. Myosin filaments form between stable actin myofilaments only during SMC contraction. The assembly of thick (myosin) filaments and the interaction of actin and myosin filaments are activated by calcium ions coming from the Ca 2 + store. The essential components of the contractile apparatus are calmodulin (Ca 2+-binding protein), kinase and light chain phosphatase of smooth muscle myosin.

Ca 2+ depot- a collection of long narrow tubes (sarcoplasmic reticulum) and numerous small vesicles (caveolae) located under the sarcolemma. Ca 2 + -ATPase constantly pumps Ca 2 + from the cytoplasm of SMCs into the cisterns of the sarcoplasmic reticulum. Through Ca 2+ channels of calcium stores, Ca 2+ ions enter the cytoplasm of SMCs. Activation of Ca 2+ channels occurs when the membrane potential changes and with the help of ryanodine and inositol triphosphate receptors. Dense bodies(Figure 7-28). In the sarcoplasm and on the inner side of the plasmalemma there are dense bodies - an analogue of the Z-lines transversely

Rice. 7-28. Contractile apparatus of smooth muscle cells. Dense bodies contain α-actinin, these are analogues of the Z-lines of striated muscle. In the sarcoplasm they are connected by a network of intermediate filaments; vinculin is present at the sites of their attachment to the plasma membrane. Actin filaments are attached to dense bodies, myosin myofilaments are formed during contraction.

but-striated muscle tissue. Dense bodies contain α-actinin and serve to attach thin (actin) filaments. Slot contacts connect neighboring SMCs and are necessary for conducting excitation (ionic current) that triggers contraction of SMCs.

Reduction

In SMC, as in other muscle tissues, the actomyosin chemomechanical converter operates, but the ATPase activity of myosin in smooth muscle tissue is approximately an order of magnitude lower than the activity of the myosin ATPase of striated muscle. Slow formation and destruction of actin-myosin bridges require less ATP. From here, as well as from the fact of the lability of myosin filaments (their constant assembly and disassembly during contraction and relaxation, respectively), an important circumstance follows - in the SMC, contraction develops slowly and is maintained for a long time. When a signal arrives to the SMC, cell contraction is triggered by calcium ions coming from calcium stores. The Ca 2+ receptor is calmodulin.

Relaxation

Ligands (atriopeptin, bradykinin, histamine, VIP) bind to their receptors and activate the G protein (G s), which in turn activates adenylate cyclase, which catalyzes the formation of cAMP. The latter activates the work of calcium pumps that pump Ca 2+ from the sarcoplasm into the cavity of the sarcoplasmic reticulum. At a low concentration of Ca 2 + in the sarcoplasm, myosin light chain phosphatase dephosphorylates the myosin light chain, which leads to inactivation of the myosin molecule. Dephosphorylated myosin loses its affinity for actin, which prevents the formation of cross bridges. Relaxation of the SMC ends with the disassembly of myosin filaments.

INNERVATION

Sympathetic (adrenergic) and partly parasympathetic (cholinergic) nerve fibers innervate the SMC. Neurotransmitters diffuse from varicose terminal nerve fibers into the intercellular space. The subsequent interaction of neurotransmitters with their receptors in the plasmalemma causes contraction or relaxation of the SMC. It is significant that in many smooth muscles, as a rule, not all SMCs are innervated (more precisely, located next to varicose axon terminals). Excitation of SMCs that do not have innervation occurs in two ways: to a lesser extent - with slow diffusion of neurotransmitters, to a greater extent - through gap junctions between SMCs.

HUMORAL REGULATION

Receptors of the plasmalemma of the SMC are numerous. Receptors for acetylcholine, histamine, atriopeptin, angiotensin, adrenaline, norepinephrine, vasopressin and many others are built into the SMC membrane. Agonists, connecting with their re-

receptors in the SMC membrane, cause contraction or relaxation of the SMC. SMCs of different organs react differently (by contraction or relaxation) to the same ligands. This circumstance is explained by the fact that there are different subtypes of specific receptors with a characteristic distribution in different organs.

TYPES OF MYOCYTES

The classification of SMCs is based on differences in their origin, localization, innervation, functional and biochemical properties. According to the nature of innervation, smooth muscles are divided into single and multiple innervated (Fig. 7-29). Single innervated smooth muscles. Smooth muscles of the gastrointestinal tract, uterus, ureter, and bladder are composed of SMCs that form numerous gap junctions with each other, forming large functional units for synchronizing contraction. In this case, only individual SMCs of the functional syncytium receive direct motor innervation.

Rice. 7-29. Innervation of smooth muscle tissue. A. Multiple innervated smooth muscle. Each SMC receives motor innervation; there are no gap junctions between SMCs. B. Single innervated smooth muscle. In-

only individual SMCs are nervous. Adjacent cells are connected by numerous gap junctions, forming electrical synapses.

Multiple innervated smooth muscles. Each SMC muscle of the iris (which dilates and constricts the pupil) and the vas deferens receives motor innervation, which allows fine regulation of muscle contraction.

Visceral SMCs originate from mesenchymal cells of the splanchnic mesoderm and are present in the wall of the hollow organs of the digestive, respiratory, excretory and reproductive systems. Numerous gap junctions compensate for the relatively poor innervation of visceral SMCs, ensuring the involvement of all SMCs in the contraction process. The contraction of the SMC is slow and wave-like. Intermediate filaments are formed by desmin.

SMC of blood vessels develop from the mesenchyme of blood islands. SMCs form single innervated smooth muscle, but the functional units are not as large as those in visceral muscle. The contraction of SMCs of the vascular wall is mediated by innervation and humoral factors. Intermediate filaments contain vimentin.

REGENERATION

It is likely that among mature SMCs there are undifferentiated precursors capable of proliferation and differentiation into definitive SMCs. Moreover, definitive SMCs are potentially capable of proliferation. New SMCs arise during reparative and physiological regeneration. Thus, during pregnancy, not only hypertrophy of SMCs occurs in the myometrium, but their total number also increases significantly.

Non-muscle contracting cellsMyoepithelial cells

Myoepithelial cells are of ectodermal origin and express proteins characteristic of both ectodermal epithelium (cytokeratins 5, 14, 17) and SMCs (smooth muscle actin, α-actinin). Myoepithelial cells surround the secretory sections and excretory ducts of the salivary, lacrimal, sweat, and mammary glands, attaching to the basement membrane using hemidesmosomes. Processes extend from the cell body, covering the epithelial cells of the glands (Fig. 7-30). Stable actin myofilaments attached to dense bodies and unstable myosin myofilaments formed during contraction are the contractile apparatus of myoepithelial cells. By contracting, myoepithelial cells promote the movement of secretions from the terminal sections along the excretory ducts of the glands. Acetyl-

Rice. 7-30. Myoepithelial cell. The basket-shaped cell surrounds the secretory sections and excretory ducts of the glands. The cell is capable of contraction and ensures the removal of secretions from the terminal section.

choline stimulates the contraction of myoepithelial cells of the lacrimal and sweat glands, norepinephrine - salivary glands, oxytocin - lactating mammary glands.

Myofibroblasts

Myofibroblasts exhibit the properties of fibroblasts and SMCs. They are found in various organs (for example, in the intestinal mucosa, these cells are known as “pericryptal fibroblasts”). During wound healing, some fibroblasts begin to synthesize smooth muscle actins and myosins and thereby contribute to the rapprochement of wound surfaces.

SATELLITES(lat. satellites - bodyguards, satellites). 1. S. cells (syn. amphicytes, perineuronal cells, Trabantenzel-len), the name given by Ramon y Cajal to special cells located in the nerve nodes of the cerebrospinal system between the capsule of the ganglion cell and its body. They usually have a flattened body with long, sometimes branching processes, but they can increase in volume and become rounded or multifaceted, resembling epithelium. This takes place between the bends of the nerve process, in the so-called. glomerulus, and ch. arr. in fenestrated spaces that form along the periphery of the ganglion cell in old age. S.'s cells are currently recognized as non-voglial; they form a direct continuation of the Schwann cells that form the nerve fiber sheaths. S. is also called glial cells, which are sometimes adjacent to the nerve cells of the brain. It is assumed that S. cells serve to nourish nerve elements, but in addition they have, like other glial cells, the ability to phagocytose: they penetrate the body of the nerve cell and destroy it, first forming pits on its surface (neuronophagy; Marinesco, Le -vaditi, Mechnikov). At Pat. processes, for example during inflammation, C proliferation phenomena are often observed, which, with parallel degeneration of ganglion cells, leads to the formation of peculiar cellular nodules in place of the latter (for example, in rabies). 2. Veins C, venae satellites arteriarum, s. comites, - deep veins of the extremities accompanying the cognate artery (Hyrtl). 3. In the science of city planning, satellites mean a system of small satellite cities surrounding a particular large city. On the development of cities-S. one of the city planning systems (Unwin) was founded (see. Layout).

See also:

SATYRIAZ, satyriasis, a special type of sexual hyperesthesia in men, is expressed in a constant desire for sexual gratification. Should be distinguished from priapism (see).
SATURATION(Saturatio), a dosage form, now almost out of use, representing an aqueous solution of drugs saturated with carbon dioxide. To prepare S. in a pharmacy, you need to add some kind of...
SAPHENAE VENAE, saphenous veins of the lower extremity (from the Greek saphenus - clear, visible; designation of a part instead of a whole - veins are visible over a short distance). The large saphenous vein runs from the inner ankle to the upper anterior part of the thigh, the small one from the outer...
SAFRANIN(sometimes Shafranik), coloring substances belonging to the group of azo dyes, of a basic nature, usually in the form of hydrochloric acid salts. Pheno-C has the simplest formula; the composition of tolu-C, containing methyl groups, is more complex. Sales brands S.: T, ...
SUGAR, a sweet-tasting carbohydrate with widespread nutritional and flavor properties. Of the various types of S., the greatest nutritional value is: cane (sucrose, beet), grape (glucose, dextrose), fruit (fructose, levulose), ...

SATELLITE CELLS

see Mantle gliocytes.

Medical terms. 2012

47.2.Sources of development

Division of cells into neurons and glia.

Nervous tissue was the last to arise in embryogenesis. It is formed at the 3rd week of embrygenesis, when the neural plate is formed, which turns into the neural groove, then into the neural tube. Ventricular stem cells proliferate in the wall of the neural tube, from which neuroblasts are formed - from which nerve cells are formed. Neuroblasts give rise to a huge number of neurons (10-12), but soon after birth they lose the ability to divide.

and glioblasts - from which glial cells are formed - these are astrocytes, oligodendrocytes and ependymocytes. Thus, nervous tissue includes nerve and glial cells.

Glioblasts, maintaining proliferative activity for a long time, differentiate into gliocytes (some of which are also capable of division).

At the same time, i.e. in the embryonic period, a significant part (up to 40-80%) of the resulting nerve cells die by apoptosis. It is believed that these are, firstly, cells with serious damage to chromosomes (including chromosomal DNA) and, secondly, cells whose processes could not establish a connection with the corresponding structures (target cells, sensory organs, etc.). d.)

47.3.Localization of various types of glial cells

Glia of the central nervous system:

macroglia - comes from glioblasts; these include oligodendroglia, astroglia and ependymal glia;

microglia - comes from promonocytes.

Glia of the peripheral nervous system (often considered a type of oligodendroglia): mantle gliocytes (satellite cells, or ganglion gliocytes),

neurolemmocytes (Schwann cells).

47.4.Structure of various types of glial cells

Briefly:

Details:Astroglia- represented by astrocytes, the largest of the glial cells that are found in all parts of the nervous system. Astrocytes are characterized by a light oval nucleus, cytoplasm with moderately developed essential organelles, numerous glycogen granules and intermediate filaments. The last cells from the body penetrate into the processes and contain a special glial fibrillary acidic protein (GFAP), which serves as a marker of astrocytes. At the ends of the processes there are lamellar extensions (“legs”), which, connecting to each other, surround vessels or neurons in the form of membranes. Astrocytes form gap junctions among themselves, as well as with oligodendrocytes and ependymal glia.

Astrocytes are divided into two groups:

Protoplasmic (plasmatic) astrocytes are found predominantly in the gray matter of the central nervous system; they are characterized by the presence of numerous branched short relatively thick processes and a low content of GFCB.

Fibrous (fibrous) astrocytes are located mainly in the white matter of the central nervous system. Long, thin, slightly branched processes extend from their bodies. Characterized by a high content of GFCB.

Functions of astroglia

supporting formation of the supporting frame of the central nervous system, within which other cells and fibers are located; During embryonic development, they serve as supporting and guiding elements along which the migration of developing neurons occurs.

The guiding function is also associated with the secretion of growth factors and the production of certain components of the intercellular substance, recognized by embryonic neurons and their processes.

demarcation, transport and barrier (aimed at ensuring an optimal microenvironment of neurons):

metabolic and regulatory is considered one of the most important functions of astrocytes, which is aimed at maintaining certain concentrations of K + ions and mediators in the microenvironment of neurons.

Astrocytes, together with oligodendroglial cells, take part in the metabolism of mediators (catecholamines, GABA, peptides). protective (phagocytic, immune and reparative) participation in various protective reactions when nerve tissue is damaged. Astrocytes, like microglial cells, are characterized by pronounced phagocytic activity. Like the latter, they also have the characteristics of APCs: they express MHC class II molecules on their surface, are able to capture, process and present antigens, and also produce cytokines. At the final stages of inflammatory reactions in the central nervous system, astrocytes proliferate and form a glial scar at the site of damaged tissue. Ependymal glia

, or

Since ependymal glia cells form layers in which their lateral surfaces are connected by intercellular connections, according to their morphofunctional properties they are classified as epithelia (ependymoglial type according to N.G. Khlopin). The basement membrane, according to some authors, is not present everywhere. In certain areas, ependymocytes have characteristic structural and functional features; Such cells, in particular, include choroid ependymocytes and tanycytes.

Choroid ependymocytes- ependymocytes in the area of the choroid plexus where CSF is formed. They have a cubic shape and cover protrusions of the pia mater, protruding into the lumen of the ventricles of the brain (roof of the III and IV ventricles, sections of the wall of the lateral ventricles). On their convex apical surface there are numerous microvilli, the lateral surfaces are connected by complexes of compounds, and the basal surfaces form protrusions (pedicles), which intertwine with each other, forming the basal labyrinth. The layer of ependymocytes is located on the basement membrane, separating it from the underlying loose connective tissue of the pia mater, which contains a network of fenestrated capillaries that are highly permeable due to numerous pores in the cytoplasm of the endothelial cells. Ependimopitis of the choroid plexus is part of the hematocerebrospinal fluid barrier (barrier between blood and CSF), through which ultrafiltration of blood occurs with the formation of CSF (about 500 ml/day).

Tanycytes- specialized ependymal cells in the lateral areas of the wall of the third ventricle, infundibular recess, and median eminence. They have a cubic or prismatic shape, their apical surface is covered with microvilli and individual cilia, and a long process extends from the basal surface, ending in a lamellar extension on the blood capillary. Tanycytes absorb substances from the CSF and transport them along their process into the lumen of blood vessels, thereby providing communication between the CSF in the lumen of the ventricles of the brain and the blood.

Functions of ependymal glia:

supporting (due to the basal processes);

formation of barriers:

neurocerebrospinal fluid (with high permeability),
hematocerebrospinal fluid

ultrafiltration of CSF components

Oligodendroglia(from the Greek oligo few, dendron tree and glia glue, i.e. glia with a small number of processes) a large group of various small cells (oligodendrocytes) with short, few processes that surround the bodies of neurons, are part of nerve fibers and nerve endings. Found in the central nervous system (gray and white matter) and PNS; characterized by a dark nucleus, dense cytoplasm with a well-developed synthetic apparatus, high content of mitochondria, lysosomes and glycogen granules.

Satellite cells(mantle cells) envelop the cell bodies of neurons in the spinal, cranial, and autonomic ganglia. They have a flattened shape, a small round or oval core. They provide a barrier function, regulate neuronal metabolism, and capture neurotransmitters.

Lemmocytes(Schwann cells) in the PNS and oligodendrocytes in the CNS participate in the formation of nerve fibers, isolating the processes of neurons. They have the ability to produce myelin sheath.

Microglia- a collection of small elongated stellate cells (microgliocytes) with dense cytoplasm and relatively short branching processes, located mainly along the capillaries in the central nervous system. Unlike macroglial cells, they are of mesenchymal origin, developing directly from monocytes (or perivascular macrophages of the brain) and belong to the macrophage-monopitary system. They are characterized by nuclei with a predominance of heterochrome! ina and high content of lysosomes in the cytoplasm.

The function of microglia is protective (including immune). Microglial cells are traditionally considered as specialized macrophages of the central nervous system - they have significant mobility, becoming activated and increasing in number during inflammatory and degenerative diseases of the nervous system, when they lose processes, become rounded and phagocytose the remains of dead cells. Activated microglial cells express MHC class I and II molecules and the CD4 receptor, perform the function of dendritic APCs in the central nervous system, and secrete a number of cytokines. These cells play a very important role in the development of nervous system lesions in AIDS. They are credited with the role of a “Trojan horse”, carrying (together with hematogenous monocytes and macrophages) HIV throughout the central nervous system. Increased activity of microglial cells, which release significant amounts of cytokines and toxic radicals, is also associated with increased death of neurons in AIDS by the mechanism of apoptosis, which is induced in them due to disruption of the normal balance of cytokines.

Restoration of damaged muscle tissue occurs thanks to satellite cells. And they cannot function without a special protein, scientists have found.

Muscles have a remarkable ability to heal themselves. With the help of training, you can restore them after injury, and age-related atrophy can be overcome with an active lifestyle. When a muscle is sprained, it hurts, but the pain usually goes away after a few days.

The muscles owe this ability to satellite cells - special cells of muscle tissue that are adjacent to myocytes, or muscle fibers. The muscle fibers themselves - the main structural and functional elements of the muscle - are long multinucleated cells that have the property of contraction, since they contain contractile protein filaments - myofibrils.

Satellite cells are, in fact, stem cells of muscle tissue. When muscle fibers are damaged, which occurs due to injury or with age, satellite cells rapidly divide.

They repair damage by fusing together to form new multinucleated muscle fibers.

With age, the number of satellite cells in muscle tissue decreases, and accordingly, the ability of muscles to recover, as well as muscle strength, decreases.

Scientists from the Max Planck Institute for Heart and Lung Research (Germany) have elucidated the molecular mechanics of muscle self-healing using satellite cells, which until now was not thoroughly known. They wrote about the results in the journal Cell Stem Cell.

Their discovery, according to scientists, will help create a muscle restoration technique that could someday be transferred from the laboratory to the clinic for the treatment of muscular dystrophy. Or maybe muscle aging.

Researchers have identified a key factor, a protein called Pax7, which plays a major role in muscle regeneration.

Actually, this protein in satellite cells has been known for a long time, but experts believed that the protein plays the main role immediately after birth. But it turned out that it is indispensable at all stages of the body’s life.

To pinpoint its role, biologists created genetically altered mice in which the Pax7 protein in satellite cells did not work. This led to a radical reduction in the satellite cells themselves in muscle tissue. The scientists then caused damage to the mouse muscles by injecting the toxin. In normal animals, the muscles began to regenerate intensively, and the damage healed. But in genetically altered mice without the Pax7 protein, muscle regeneration became almost impossible. As a result, biologists observed large numbers of dead and damaged muscle fibers in their muscles.

Scientists regarded this as evidence of the leading role of the Pax7 protein in muscle regeneration.

The muscle tissue of the mice was examined under an electron microscope. In mice without the Pax7 protein, biologists found very few surviving satellite cells, which were very different in structure from normal stem cells. Damage to organelles was noted in the cells, and the state of chromatin—DNA combined with proteins, which is normally structured in a certain way—was disrupted.

Interestingly, similar changes appeared in satellite cells that were cultured for a long time in the laboratory in an isolated state, without their “hosts” - myocytes. The cells degraded in the same way as in the body of genetically modified mice. And scientists found in these degraded cells signs of deactivation of the Pax7 protein, which was observed in mutant mice. Further - more: isolated satellite cells stopped dividing after some time, that is, stem cells ceased to be stem cells.

If, on the contrary, the activity of the Pax7 protein in satellite cells is increased, they begin to divide more intensively. Everything points to the key role of the Pax7 protein in the regenerative function of satellite cells. All that remains is to figure out how to use it in potential cell therapy for muscle tissue.

“When muscles deteriorate, such as in muscular dystrophy, implanting muscle stem cells will stimulate regeneration,” explains Thomas Brown, director of the institute.

Understanding how Pax7 works will help modify satellite cells to make them as active as possible.

This could lead to a revolution in the treatment of muscular dystrophy and may help maintain muscle strength in old age."

And healthy muscles and physical activity in old age are the best way to delay age-related diseases.

The meaning of the word satellite cells in medical terms. Where are the satellite cells of skeletal muscle tissue located? Satellite cells

See also interpretations, synonyms, meanings of the word and what SATELLITE CELLS are in Russian in dictionaries, encyclopedias and reference books:

47.2.Sources of development

47.3.Localization of various types of glial cells

47.4.Structure of various types of glial cells