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Section I • Histology and Cell Biology
ChapterSummary
•Cartilage like all connective tissue, consists of cells and extracellular matrix (ECM).
•Cartilage formation takes place by differentiation of multipotential mesenchymal cells into chondroid precursor cells, which give rise to chondroblasts, which give rise to chondrocytes. Cartilage can enlarge in 2 ways. In appositional growth, new cells can be added from the outer
perichondrium. In internal or interstitial growth, the chondrocytes embedded deep within the cartilage can continue to produce additional ECM. There are 3 types of cartilage, all containing type II collagen and glycosaminoglycans (GAGs), but sometimes with additional extracellular components, which were produced by the chondrocytes.
•Hyaline cartilage is the type of cartilage that forms a template for bone formation during embryogenesis, as well as comprising the cartilage on the surface of bones at synovial joints and the cartilage of the nose, portions of the larynx, and the cartilage ofthe trachea and bronchi. Elastic cartilage is found in the external ear, the auditory canal, and the epiglottis ofthe larynx. Fibrocartilage is the type of cartilage found in intervertebral disks ofthe vertebral column and the menisci ofthe knee, and may form the attachment of ligaments and tendons to bone.
•
•
Bone is a unique connective tissue in that it not only has cells and ECM called osteoid, including type I collagen and GAGs, but also the matrix is calcified and rigid.
Osteoblasts secrete bone extracellular matrix (osteoid). Osteoblasts are specialized to synthesize and secrete the components of osteoid, type I collagen, and GAGs.
•Osteoclasts are responsible for the breakdown of bone matrix, with release of calcium.
•All bone, regardless offormation, undergoes remodeling throughout life. All 3 processes-bone formation, bone growth, and bone remodeling-have similarities.
•In intramembranous bone formation, primitive mesenchyme can give rise directly to bone. In endochondral bone formation, bone is formed on the template of preexisting hyaline cartilage. Endochondral bone formation occurs in long bones ofthe extremities and vertebrae and bones ofthe pelvis, and starts as a hyaline cartilaginous template.
•An osteon is a Haversian canal with its surrounding lamellae constituents and forms the basic unit of mature lamellar bone. Trabeculae of cancellous bone are similarly remodeled. The process takes place mostly at the surface of the trabeculae, carried out by osteoclasts and osteoblasts from the adjacent marrow.
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Section I • Histology and Cell Biology
Table 1-4-1. Red versus White Skeletal Muscle Fibers
Red Fibers (Type I)
Slow contraction
.J, ATPase activity
i Capacity for aerobic metabolism
iMitochondrial content
iMyoglobin (imparts red color)
Best for slow, posture-maintaining muscles, e.g., back (think chicken drumstick/thigh)
White Fibers (Type II)
Fast contraction i ATPase activity
i Capacity for anaerobic glycolysis
.J, Mitochondrial content
.J, Myoglobin
Best for fast, short-termed, skilled motions, e.g., extraocu lar muscles of eye, sprinter's legs, hands (think chicken breast meat and wings)
In skeletal muscle, striations are visible in the light microscope and consist of dark A bands and the light I bands. In a longitudinal section of an elongated muscle cell a pale H band in the center of the A band may be seen. 'Ihe Z and M lines are usually not visible.
Copyright McGraw-Hill Companies. Used with permission.
Figure 1-4-4. Skeletal muscle cell that consists of sarcomeres with dark A bands in the middle of
the sarcomere (arrow) and light l bands (arrowheads) A peripheral nucleus is on the right side of the cell.
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Table 1-4-2. Structure, Function, and Pharmacology of Muscle
Characteristics
Appearance
Ttubules
Cell junctions
Innervation
Action potential
Upstroke
Plateau
Excitation-contraction coupling
Calcium binding
Skeletal
Striated, unbranched fibers
Z lines
Multinucleated
Form triadic contacts with SR at A-I junction
Absent
Each fiber innervated
Inward Na+ current
No plateau
AP T tubules Ca2+ released from SR
Troponin
Cardiac
Striated, branched fibers
Z lines
Single central nucleus
Form dyadic contacts with SR near Z line
Junctional complexes between fibers (intercalated discs), including gap junc- tions
Electrical syncytium
•Inward Ca2+ current (SA node)
•Inward Na+ current (atria, ventricles, Purkinje fibers)
•No plateau (SA node)
•Plateau present (atria, ventricles, Purkinje fibers)
Inward Ca2+ current during plateau Ca2+ release from SR
Troponin
Abbreviations: AP, action potential; IP3, inositol triphosphate; S R, sarcoplasmic reticulum
Chapter 4 • Muscle Tissue
Smooth
Nonstriated, fusiform fibers
No Z lines; have dense bodies
Single nucleus
Absent; have limited SR
Gap junctions
Electrical syncytium
Inward Na+ current
No plateau
AP opens voltage-gated Ca2+ channels in sarcolemma;
hormones and neurotransmittersopen IP3-gated Ca2+ channels in SR
Calmodulin
Anendomysium surrounds each skeletal muscle cell and provides insulation be tween adjacent cells. A perimysium surrounds a group or fascicle ofmuscle cells and is where capillaries are found. The epimysium surrounds the outside ofthe entire muscle, and is where larger bloodvessels are found. Satellite cells and fibro blasts that have small dark nuclei with tightly condensed chromatin lie adjacent to each muscle cell.
The Z lines mark the ends ofeach sarcomere. The A band is in the center ofthe sarcomere and is the zone including all ofthe myosin filaments. The H zone is the zone with only myosin filaments in the center ofthe A band. The M line is in the center ofthe sarcomere. The I band is the zonewith only actin filaments between the Z line and the start ofthe A zone. When a muscle cell contracts, the A band stays constant in width (the length ofthe myosin molecule). As the overlap ofthe actin and myosin filaments increases, both the I and H bands become smaller. The overlap region is the distance between the I and H bands, and grows as the I and H bands get smaller. During relaxation, the opposite happens.
Note
Desmin intermediate filaments link adjacent myofibrils at adjacent Z lines.
MEDICAL 49
Section I • Histology and Cell Biology
Clinical Correlate
Skeletal muscle has a limited ability to regenerate. Satellite cells are stem cells situated outside the plasma membrane that form new myoblasts.
Muscular dystrophies deplete the pool of satellite cells.
Actin thin filaments in muscle consist of 2 strands of F-actin forming a helix, around which tropomyosin filaments are wrapped. Tropomyosin serves as a site of attachment for troponin, which has 3 component parts: (which attaches to tropomyosin), troponin C (which binds calcium), and troponin I (which inhibits the interaction of actin and myosin). When cytosolic calcium increases, it binds to troponin C, which blocks the inhibition of actin/myosin binding by troponin I. The F-actin filaments have polarity. On each side of the sarcomere, the barbed or plus end inserts into a Z line (composed in part ofdes min and actinin), while the other end faces toward, but does not reach, the M line in the center ofthe sarcomere.
Myosin thick filaments are formed of a bipolar polymer of individual myosin molecules. Each myosin molecule consists of 2 identical heavy chains, which each contribute to the head and tail ofthe molecule, and 2 pairs oflight chains, with one ofeach type bound to each head ofthe molecule. The structure ofthe tail end ofthe heavy chains allows the individual molecules to assemble into fila ments, while the head ends can interact with myosin as well as hydrolyze ATP. When the myosin molecules self-assemble, they form a bipolar thick filament, with myosin heads at each end, and with a central region free ofmyosin heads. The central region is attached to the M line at the center ofthe sarcomere, and the myosin heads extend toward (but do not reach) the Z lines at the end ofthe sarcomere. The polarity ofactin and myosin heads is reversed in the 2 halves of the sarcomere on either side of the M line, so that both halves pull toward the centerwhen they contract.
Nebulin is a scaffolding protein that binds to actin filaments along their entire length and also insert into the Z line. It serves as a template to help maintain constant length ofthe actin filaments.
Titin is a scaffolding molecule that binds to myosin, and which is not only anchored in the M line but also extends beyond the free end ofthe myosin molecule to the Z line. The part bound to myosin again helps to maintain the constant lengthofthe myosin molecule. Thepartthat extends from the end ofthe myosin molecule to the Z line may act as a spring, which is compressed during contrac tion and which helps to restore the sarcomere to its resting length when the sar comere relaxes, at the end ofactin/myosin interaction.
Contractions ofskeletal muscle produce force and movement by the interaction of thin actin (F-actin) and thick myosin (myosin II) filaments. The interaction uses energyderived fromATP and involves repetitive binding, sliding, release and reattachment ofthe head end of the myosin molecules to the adjacent actin fila ments. The contraction is ultimately startedby releasing calcium, which promotes binding and cocking ofthe myosin head, hydrolysis ofATP and movement ofthe myosin head back to its starting position. Relaxation ofskeletal muscle involves a reduction ofcytosolic calcium to halt the interaction ofmyosin and actin.
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Chapter 4 • Muscle Tissue
I A band band
Myofibril
Transverse Sarcoplasmic Terminal
tubules reticulum cisterna
Figure 1-4-5. Striated Muscle Fiber Showing Sarcoplasmic Reticulum
and T-tubule System
T Tubules and the Sarcoplasmic Reticulum
The plasma membrane ofskeletalmuscle cells extendsinfolds into the cell as trans verse tubules (T tubules), allowing rapid spread ofthe action potential throughout the cell. The apparent lumen of T tubules seen in cross section is actually an ex tension of the extracellular space into the interior of the muscle cell. The action potential also opens a voltage-sensitive calcium channel, which allows some extra cellular calcium to enter the cell. This triggers the release of intracellular calcium sequesteredin adjacentsarcoplasmicreticulum (SR) intothe cytosolbyopeningSR membrane-located calcium channels. The bulk ofthe calcium that initiates skeletal muscle contraction comes from the SR rather than the extracellular space. This process is aided by the intimate relationship ofthe T tubules to the SR at the tri ads. Skeletal muscle relaxation results from a reduction of free cytosolic calcium by pumping calcium back into the SR by an energy-dependent calcium pump. In skeletal muscle the T-tubule triads are located near the A-I band junction.
CARDIAC MUSCLE
Cardiac muscle is striated in the same manner as skeletal muscle, but it differs in being composed ofsmaller cells (fibers) with only one or 2 nuclei. The nuclei are located centrally, instead ofperipherally.
Layers ofthe HeartWall
The heart wall is composed of 3 distinct layers: an outer epicardium, a middlemyocardiumandaninnerendocardium.Theepicardium,orviscerallayerof serous pericardium, consists of a simple squamous epithelium (mesothelium) and its underlyingconnective tissue. The connective tissue contains a large num ber of fat cells and the coronary vessels. The muscular wall of the heart is the
MEDICAL 51
Section I • Histology and Cell Biology
myocardium and is composed mainly of cardiac muscle cells. The endocardium, which lines the chambers ofthe heart, is composed ofa simple squamous epithe lium, the endothelium, and a thin layer of connective tissue.
Intercalated Discs
Intercalated discs are special junctional complexes that join myocardial cells. The intercalated discs appear as dark, transverse lines in the light microscope. These disks contain gap junctions and adhering junctions. These junctions per mit the spread of electrical (gap) and mechanical (adhering) effects through the walls ofthe heart, synchronizing activity and for the pumping action ofthe heart chambers. While intercalated discs allow coordinated action of the myocardial cells, the squeezing and twisting movements of the heart chambers (particularly the left ventricle) during systole are due to the disposition of cardiac myocytes.
Cardiac Conduction System
The cardiac conduction system is a specialized group of myocardial cells that initiates the periodic contractions of the heart due to their ability to depolarize at a faster rate than other cardiac myocytes. Electrical activity spreads through the walls of the atria from the SA node and is quickly passed by way of internodal fibers to the atrioventricular node. From the atrioventricular node, activity pass es through the bundle of His and then down the right and left bundle branches in the interventricular septum. The bundle branches reach additional specialized cardiac muscle fibers known as Purkinje fibers in the ventricular walls.
•The Purkinje fibers run in several bundles along the endocardial surface and initiate ventricle activity starting at the apex of the ventricles.
•Purkinje fibers have a large cross section, a cytoplasm with few contrac tile fibrils and a large content of glycogen.
Cardiac muscle has a similar but somewhat less well developed T-tubule system compared to skeletal muscle that is located at the Z-line.
Copyright McGraw-Hill Companies. Used with permission.
Figure 1-4-6. Cardiac muscle cells with centrally placed nuclei and intercalated disks (arrow)
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Chapter If • Muscle Tissue
CopyrightMcGraw-Hill Companies. Used with permission.
Figure 1-4-7. EM of part of an intercalated disk with gap junction (arrow) adjacent to an adhering junction with intracellular density.
SMOOTH MUSCLE
Smooth-muscle cells have a single central nucleus, are spindle-shaped, have a variable diameter and length depending on location, and are generally smaller than the cells of either skeletal or cardiac muscle. Smooth muscle often forms one or more layers ofhollow tube-like structures in the body. Generally, smooth muscle bundles (fascicles} run in various directions in the bladder and ureter, are circumferential in blood vessels, and form at least 2 discrete layers, an inner circular and an outer longitudinal layer, in the gastrointestinal tract.
Copyright McGraw-Hill Companies. Used with permission.
Figure 1-4-8. EM of part of a smooth-muscle cell with dense bodies (arrows) that form attachment sites foractin filaments
MEDICAL 53
Section I • Histology and Cell Biology
Smooth muscle is not striated because it lacks the sarcomeres and Z lines seen in striated muscle. Instead, the actin filaments are oriented in multiple oblique, roughly longitudinal, directions and are anchored to the inner cell membrane at multiple sites, not just at the ends of the cells (the arrangement in striated muscle). This allows more shortening of smooth-muscle cells compared to stri ated muscle. Instead ofaligned Z bands, the actin filaments are anchored in dense bodies scattered within the cell cytoplasm. The dense bodies also anchor the con tractile filaments to the cell surface membrane. In vascular smooth muscle, the dense bodies contain the intermediate filaments desmin and vimentin. In smooth muscle of the gastrointestinal tract, the dense bodies contain desmin but not vi mentin. Smooth muscle lacks a T-tubular system, and the SR has a less ordered relationship to contractile fibers.
ChapterSummary
• Muscles are classified as skeletal, cardiac, or smooth.
Skeletal Muscle
•Skeletal muscle has 3 levels of connective tissue: endomysium, perimysium,
and epimysium. Skeletal muscle is composed of long cylindrical fibers that have dark (A) bands and light (I) bands. A dark transverse line, the Z line, bisects each I band. Skeletal muscle fibers contain myofibrils, which in turn are composed of sarcomeres.
•Sarcomeres have thick and thin filaments. Thick filaments are centrally located in sarcomeres, where they interdigitate with thin filaments. The I band contains thin filaments only, the H band contains thick filaments only, and the A bands contain both thick and thin filaments.
•Thin filaments contain 3 proteins: actin, tropomyosin, and troponin.
•Actin forms a double helix, whereas tropomyosin forms an a-helix. Troponin includes 3 polypeptides: TnT, which binds to tropomyosin; TnC, which binds to calcium ions; and Tnl, which inhibits actin-myosin interaction. Thick filaments are composed of myosin. Myosin has 2 heavy chains with globular head regions. The heads contain actin-binding sites and have ATPase activity. The transverse tubular system (T-tubule system) surrounds each myofibril and facilitates excitation-contraction coupling.
Cardiac Muscle
•Cardiac muscle has an arrangement of sarcomeres similar to that in skeletal muscles, but the fibers are coupled through gap junctions.
•Smooth muscle is found in the walls of blood vessels and hollow viscera.
Gap junctions couple them electrically. Myofilaments of smooth muscles are not arrayed like in skeletal muscles; they are obliquely placed in order to form a latticework. Electrical or chemical signaling via hormones can trigger smooth muscles.
54 M EDICAL
