HUMAN ANATOMY – VOLUME 1

tolemm forms a large number of villi. The cytoplasm is rich in RNA, rough and smooth endoplasmic reticulum, Golgi complex, mitochondria, lysosomes and granules of glycogen. The nucleus is rich in chromatin and can have one or two nucleoli.

C h o n d r o c y t e s are mature large cells in oval, rounded or polygonal in shape, cytoplasmic projections and well-developed organelles. They are situated in cavities called lacunae, which are surrounded by extracellular matrix. If a lacuna contains only one cell, it is called primary. More often cells lie in a secondary lacuna in isogenic groups (2–3 cells). Walls of the lacunae are made up of collagen fibers, and aggregates of proteoglycans.

The structural and functional unit of cartilage is a chondron, which includes a cell or group of isogenic cells, extracellular matrix and a lacuna capsule.

Depending on its structure, cartilage is divided into three types: hyaline, fibrocartilage and elastic.

H y a l i n e c a r t i l a g e has a bluish color. It consists of ground substance and fibers of collagen. Hyaline cartilage forms joint and costal cartilage and most cartilage plates in the larynx.

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F i b r o c a r t i l a g e contains a large number of collagen fibers in its ground substance and is highly durable. Elongated cells, which are found between collagen fibers, contain oblong rod-shaped nuclei and have thin bands of basophilic cytoplasm. This type of cartilage forms fibrous rings in vertebral disks, articular disks and menisci, covers the articular surfaces of the temporomandibular and sternoclavicular joints.

E l a s t i c c a r t i l a g e is resilient and flexible. Besides collagen, its matrix contains a large number of complexly interwoven elastic fibers. Elastic cartilage forms the epiglottis, the cuneiform and corniculate cartilages of the larynx, processus vocalis of the arytenoid cartilage, the outer ear and the cartilaginous part of the auditory tube.

Bone tissue

Bone tissue is characterized by special mechanical properties. It consists of bone cells, bricked up in bone ground substance, which contains collagen fibers and is saturated with inorganic compounds. There are two types of bone cells: osteoblasts and osteocytes (Fig.16). Another category of cells found in bones is osteoclasts, which are not bone cells. They have a monocyte origin and pertain to the system of macrophages.

Fig. 16. Osteal cells (acc. to V.G Eliseev and others).

A — structure of the osteoblast: 1 — nucleus; 2 — cytoplasma; 3 — osteocyte; 4 — rough endoplasmic reticulum (is evident). B — structure of osteocyte: 1 — processes of osteocytes; 2 — endoplasmic reticulum; 3 — nucleus; 4 — reticular apparatus; 5 — mitochondrion; 6 — osteoid osteal substance located on margins of osteocyte lacuna.

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Osteoblasts are young process bearing cells polygonal in shape. They contain rough endoplasmic reticulum, ribosomes, well-developed Golgi complex and highly basophilic cytoplasm. Osteoblasts are situated in superficial layers of bones. They have a round or oval nucleus, which contains one large nucleolus usually in the peripheral zone. There are microfibrils surrounding the osteoblasts. These cells produce and secrete substances, which form walls of the lacunae, which contain the osteocytes. Spaces between fibers are filled by an amorphous substance, which is an osteoid tissue, or pre-bone, which is later calcified. The organic matrix of bone tissue contains crystals of hydroxyapatite and amorphous calcium phosphate, elements of which pass into bone from the blood through tissue fluid.

O s t e o c y t e s are mature fusiform cells with a large round nucleus, which contains a clearly visible nucleolus, and many cytoplasmic processes. They lie in lacunae, where they are surrounded by a thin layer of so-called bone fluid (tissue fluid). Osteocytes are not in direct contact with the calcified matrix. Their long cytoplasmic processes lie inside bone canaliculi. They are also separated from the calcified matrix by space containing tissue fluid, which feeds the osteocytes. The distance between an osteocyte and the nearest capillary does not exceed 100 – 200 mm.

O s t e o c l a s t s are large multinuclear cells of size 190 mm. The have a monocyte origin and are able to destroy bone and cartilage and reabsorb bone tissue during its physiological and reparative regeneration. Their cytoplasm contains a large number of mitochondria, elements of granular endoplasmic reticulum and Golgi complex, free ribosomes and lysosomes.

O s t e o c l a s t s have numerous cytoplasmic projections, usually found on the surface, that is, facing the deteriorating bone. This brush border increases the surface area of the osteoclast in contact with the bone. The projections are covered with microvilli, spaces between which contain hydroxyapatite crystals. These crystals can be found inside osteoclasts in phagolysosomes, where they are dissolved.

Bone tissue can be subdivided into two varieties: reticulofibrous and lamellar. R e t i c u l o f i b r o u s bone tissue is found in zones where tendons attach to bones and in sutures of the skull after their fusion. This type of bone contains thick bundles of collagen, spaces between which are filled with an amorphous substance. Reticulofibrous tissue is covered on the outside by periosteum.

L a m e l l a r b o n e t i s s u e is formed by bone lamellae 4 – 20 mm thick, which consist of osteocytes and fibrous ground substance. In each lamella collagen fibers lie parallel to each other. Fibers in neighboring lamella are oriented differently, which provides for high durability of the bone.

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Questions for revision and examination

1.What is blood plasma? What are its functions?

2.Name the cell elements of blood and describe the morpho-functional characteristics of each.

3.Describe the structure and functions of cartilage tissue.

4.Describe the structural characteristics of hyaline, fibrocartilage and elastic cartilage.

5.Recall and describe the structural characteristics and functions of osteocytes, osteoblasts and osteoclasts.

6.What are the differences between lamellar and reticulofibrous bone tissue?

MUSCLE TISSUE

Muscle tissue includes skeletal, smooth and cardiac muscles, all of which have different embryological derivation and structure. A common characteristic of these tissues is their ability to contract or change their length. The human organism also contains muscle tissue of ectodermal derivation (myoepithelial cells of glands and myocytes of the iris).

Skeletal (striated) muscle tissue is made up of muscle fibers 4 cm or more in length and 0.1 mm thick. Each fiber consists of a myosymplast and myosatellitocytes, covered with a common membrane called sarcolemma (Fig. 17).

The sarcolemma consists of a basement membrane with interwoven collagen and reticular fibers. The myosymplast, which lies beneath the sarcolemma is also called the sarcoplasm. It contains a large number of ellipsoid nuclei (up to 100), myofibrils and cytoplasm. The sarcoplasm is

Fig. 17. Striated (skeletal) muscle tissue.

A — structure of muscle fibre; B — muscle fibers; 1 — myofiber; 2 — sarcolemma; 3 — myofibrils; 4 — nucleus.

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rich in granular endoplasmic reticulum. A considerable part of the muscle fiber is occupied by myofibrils. Between the muscle fibers there are many granules of glycogen and mitochondria with welldeveloped cristae. The sarcoplasm contains a lot of the protein myoglobin, which can bind with oxygen like hemoglobin. Depending on thickness and myoglobin content, striated muscle fibers are divided into red and white types. Red fibers are rich in sarcoplasm, myoglobin and mitochondria but contain fewer myofibrils. These fibers contract slowly and can remain in a contracted /working/ condition for a long time. White muscle fibers contain little sarcoplasm and myoglobin and few mitochondria, but have a lot of myofibrils. White fibers contract faster than red fibers, but tire quickly. Combinations of slow («red») and fast («white») muscle fibers provides quickness of muscle reaction (contraction) and long lasting work capability.

The main part of the sarcoplasm is made up of myofibrils (Fig. 18). Each myofibril consists of interchanging segments of dark anisotropic A bands and light isotropic I bands. In the middle of each band there is a light strip called the H zone. The middle of the H zone is marked by the M line, or mesophragm, and in the

middle of the I band is the Z line, or the so-called telophragm. The alternating of the light and dark bands in neighboring myofibrils creates the

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striated appearance. The dark bands are formed by thick myosin strands 10–15 nm in diameter. These strands are made up of a high-molecular protein myosin. The light bands consist of thin actin filaments 5–8 nm in diameter and about 1 mm in length. These thin filaments are formed by a low-molecular protein actin, and two other low-molecular proteins troponin and tropomyosin.

The section between two telophragms (Z lines) is called the sarcomere and is considered to be the functional unit of the myofibril. A sarcomere is about 2.5 m long and includes a dark A band and the

adjoining half of a light I band Fig. 19. Structure of the smooth nonstriated

at each side. Thin actin fila- muscle tissue (acc. to I.V. Almasov and L.S. Sutulov).

ments are situated in the intervals between the heavy myosin filaments. During contractions of the muscle the actin

and myosin filaments slide towards each other; during muscle relaxation they move in opposite directions.

On borders between A and I bands the sarcolemma of the muscle fiber invaginates, forming transverse tubules (T tubules). These play an important role in fast conduction of the action potential to each myofibril. The action potential spreads along T tubules, passes onto the sarcoplasmic reticulum and between myofibrils.

Myosatellitocytes are situated directly beneath the sarcolemma. These are flattened cells with large nuclei. Each myosatellitocyte has a centrosome and a small number of organelles and lacks myofibrils. Myosatellitocytes are precursor cells of striated (skeletal) muscle tissue. They are able to synthesize DNA and undergo mitosis.

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Smooth (non-striated) muscle tissue consists of muscle cells called myocytes, which are found in the walls of circulatory and lymph vessels and hollow organs, forming their contractile apparatus. Smooth myocytes are fusiform cells 20–500 m long and 5–15 m thick (Fig 19). There is no striation in these cells. Myocytes are situated in groups with their pointed ends in between two neighboring cells. Each smooth myocyte has a basement membrane, which is absent at sites of cell junctions. These cells contain elongated rod-shaped nuclei, which can reach 10–25 m in length and during contraction acquire a corkscrew shape. Adjoining the plasma membrane from within are dense (adhesive) corpuscles.

Smooth myocytes contain thin and thick myofilaments. During contraction actin and myosin filaments move towards each other and the myocyte shortens. Nerve impulses are passed from one myocyte to another through cell junctions at a speed of approximately 8–10 cm/s. Smooth myocytes contract considerably slower (by 100–1000 times) than striated muscle fibers (Fig.19). Smooth muscles carry out prolonged tonic contractions and relatively slow movements.

Cardiac muscle tissue differs by structure and functions from skeletal muscles. It consists of cardiac myocytes (cardiomyocytes), which form interconnected complexes. Cardiac muscle resembles skeletal muscle tissue by its microscopic structure (transverse striation), but it contracts involuntarily like smooth muscle tissue (Fig.20). Cardiomyocytes have an

Fig. 20. Structure of cardiomyocyte (acc. to V.G.Eliseev).

1 — basal membrane; 2 — ending of myofibrils on cardiomyocyte cell membrane; 3 — intercallated disc (between cardiomyocytes); 4 — sarcoplasmic reticulum; 5 — mitochondrion (sarcosome); 6 — myofibrils; 7-A — band (anisotropic); 8- I — band (isotropic); 9 — sarcoplasma.

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irregular cylindrical shape; cells are 100–150 mm long and 10–20 mm in diameter. Each cardiomyocyte contains 1 or 2 elongated oval nuclei, which lie in the center of the cell and are surrounded by longitudinally positioned myofibrils. Between neighboring cardiomyocytes there are specialized contacts in the form of intercalated disks, which actively take part in passing excitation from one cell to another. The structure of myofibrils in cardiomyocytes is analogous to that in skeletal muscles. Beneath the cytolemma and between mitochondria there are granules of glycogen and elements of smooth endoplasmic reticulum. Cardiomyocytes contain many large mitochondria with well-developed cristae, which are situated in groups between myofibrils. The cytolemma of cardiomyocytes also forms T tubules, near which there are accumulations of smooth endoplasmic reticulum cisterns.

Questions for revision and examination

1.Describe the structure of skeletal (striated) muscle tissue.

2.What are the red and white muscle fibers? What are their morpho-functional differences?

3.Describe the structure of a sarcomere and its location in a muscle fiber.

4.What is smooth /non-striated/ muscle tissue? Describe its structure and functions.

5.Describe the structure and functions of cardiomyocytes /cardiac muscle tissue/.

NERVOUS TISSUE

Nervous tissue is the main structural element of organs of the nervous system, namely, the brains and spinal chord, ganglia, nerves and nerve endings. Nervous tissue consists of nerve cells (neurocytes or neurons) and accessory neuroglial cells, which are anatomically and functionally associated with them.

N e u r o c y t e s (n e u r o n s) and their processes are able to perceive excitation, become excited, produce, store and pass information encoded in electrical or chemical signals (nerve impulses).

Each neuron has a cell body and processes with endings (Fig. 21). Nerve cells are covered with a cytolemma, which is able to conduct excitation and provide for cell metabolism. The body of a nerve cell contains a nucleus and the cytoplasm surrounding it (perikaryon). The cytoplasm of neurons contains certain characteristic structures—neurofibrils and a chromatophilic substance (Nissl substance) the presence of which (granular endoplasmic reticulum) indicates high levels of protein synthesis. Neurofibrils are bundles of microtubules and neurofilaments, which participate in transport of substances. The diameter of neuron cells bodies rang-

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es from 4–5 to 135 mm. The shape of cell bodies also varies — it may be round, oval or pyramidal. Nerve cells have different length cytoplasmic processes, which are divided into two types. There are one or several branched out processes called dendrites, which carry nerve impulses to the neuron body. In majority of cells they are approximately 0.2 mm long. The cytoplasm in dendrites contains elongated mitochondria and a small number of cisterns of smooth endoplasmic reticulum. There is only one process, usually it is long, which carries nerve impulses away from the cell body — the axon, or neurite.

The axon starts from the neuron body and ends with a bunch of terminal branches, which form synapses. The surface of the axon cytolemme (axolemma) is smooth. Axons contain thin elongated mitochondria, a large amount of neurotubules and neurofilaments, vesicles and tubes of smooth endoplasmic reticulum. Nerve cells are dynamically polarized, meaning they are able to conduct nerve impulses only in one direction—from dendrites to axon.

Fig. 21. Ultramicroscopic structure of nerve cell.

1 — axodendritic synapse; 2 — axosomatic synapse; 3 — presynaptic vesicules; 4 — presynaptic membrane; 5 — synaptic gap; 6 — postsynaptic membrane; 7 — endoplasmic reticulum; 8 — mitochondrion; 9 — golgi complex; 10 — neurofibrils; 11 — nucleus; 12 — nucleolus.

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tissue fluid, which participates in conduction of nerve impulses.

M y e l i n a t e d n e r v e f i b e r s are about 20 mm in thickness (Fig.23). They contain the relatively thick axon as the axial cylinder. Around the axon there is a sheath, which consists of two layers. The inside layer is the thicker myelin sheath. On the outside is a thin layer formed by Schwann cells. Dendrites do not have myelin sheaths. Each Schwann cell wraps only a small area of the axon. The underlying myelin layer, which is made up of lipids, is also discontinuous. Thus, every 0.3–1.5 mm there are gaps called neurofibral nodes (nodes of Ranvier), in which there is no myelin layer (Fig.24). In these areas neighboring Schwann cells approach the axon directly with their edges. The basement membrane, which covers the Schwann

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