Locomotor System: Muscle Tissue Types

• The material provided has been created for the academic year 2024-2025.
• This is for year 1, semester 2.
• The module is Locomotor System (LCS) 105.
• Revision created by Professor Safinaz Salaheldin, from the Histology department.

Muscle Tissue Types

• Muscle tissue includes skeletal, cardiac, and smooth types.
• Skeletal muscle fibers are cylindrical, unbranched, have multiple peripheral nuclei, clear striations, and numerous myofibrils.
• Cardiac muscle fibers are cylindrical, branched, have a single central nucleus, less clear striations, and fewer myofibrils.
• Smooth muscle fibers are fusiform, have a single central nucleus, are non-striated, and have an irregular arrangement of myofibrils.
• Skeletal muscle fibers have a thick sarcolemma and a large diameter of 10-100μm; each fiber is a single cell.
• Cardiac muscle fibers have a very thin sarcolemma and a medium size of 15-30μm; fibers are formed of multiple cells.
• Smooth muscle fibers have a thin sarcolemma and a small diameter of 8 µm; each fiber is a single cell.
• Skeletal muscle has regular sarcomeres and a triad tubular system, and lacks intercalated discs.
• Cardiac muscle has irregular sarcomeres, a diad tubular system, and intercalated discs.
• Smooth muscle has no sarcomeres or T-tubule system, contains caveolae, and lacks intercalated discs.
• Skeletal muscle lacks junctions but can regenerate from satellite cells.
• Cardiac muscle has gap junctions, fascia adherens, and desmosomes, but does not regenerate.
• Smooth muscle has gap junctions and regenerates through mitosis and from pericytes.
• Skeletal muscles are part of the triad tubular system with two dilated sarcoplasmic reticulum tubules at the A-I junction.
• Cardiac muscles are part of the diad tubular system with one dilated sarcoplasmic reticulum tubule at the Z line.

Key Muscle Components

• Actin filaments are anchored to the Z disc by alpha-actinin.
• Titin supports and stabilizes thick myofilaments, anchors them in the Z line, and prevents sarcomere overstretching.
• Dystrophin, a large actin-binding protein, is located inside the sarcolemma and is involved in the functional organization of myofibrils.
• Duchenne muscular dystrophy results from mutations in the dystrophin gene, leading to defective linkages between the cytoskeleton and the extracellular matrix (ECM).
• Muscle contractions disrupt weak linkages, leading to atrophy of muscle fibers.

Muscle Types Comparison

• Red muscles oxidative and slow twitch contain red color, rich myoglobin, large and numerous mitochondria, rich capillaries, and less glycogen.
• Red muscles oxidative and slow twitch are small, has a slow and prolonged contraction, an aerobic energy source, and is exemplified by postural muscles of the back.
• Fast oxidative-glycolytic muscles are red, rich in myoglobin, numerous in mitochondria, rich in capillaries, and contain intermediate glycogen.
• Fast oxidative-glycolytic muscles are Intermediate in size, contracts fast and intermediates, uses an oxidative-glycolytic energy source, and includes major muscles of the legs.
• White muscles glycolytic and fast twitch are white, poor in myoglobin, less numerous in mitochondria, poor in capillaries, and high in glycogen.
• White muscles glycolytic and fast twitch are large, contracts fast and are short/easy, utilizes energy through glycolysis anaerobic, and includes extraocular muscles.

Intercalated Discs

• Intercalated discs are junctions between the sarcolemma of two adjacent cardiac myocytes forming muscle fiber with a steplike appearance.
• They appear as densely stained clear transverse lines at irregular intervals.
• Intercalated discs are formed of transverse and lateral regions exhibiting steplike appearance.
• Transverse components cross the cardiac fiber at a right angle and include desmosomes and fascia adherens junctions.
• Lateral components lie parallel to the myofiber with gap junctions.

Purkinje Fibers

• Purkinje muscle fibers are specialized cardiac muscle fibers that conduct impulses 4-5 times faster via gap junctions
• They form the A-V bundle and its branches within moderator band.
• Purkinje fibers are larger and paler than ordinary cardiac muscle and have eccentric nuclei.
• These fibers exhibit no striations due to few myofibrils and have sarcoplasm that appears pale vacuolated due to high amounts of glycogen.
• Purkinje fibers lack intercalated discs.
• Dense bodies are cytoplasmic and plasmalemma associated.
• Dense bodies include alpha-actinin.
• Dense bodies correspond to the Z line of striated muscle, intermediate filaments made of desmin also attach to the dense bodies.

Cartilage Composition

• Cartilage consists of cells, fibers, and matrix.
• Cells include chondroblasts and chondrocytes.
• Fibers include collagen and elastic fibers.
• The matrix is abundant and firm.

Cartilage Types

• There are three types of cartilage: hyaline, elastic, and white fibrocartilage.
• Hyaline cartilage has a present perichondrium except at the articular surface and epiphyseal plate.
• Elastic and white fibrocartilage have a present, and an absent perichondrium respectively.
• Hyaline cartilage has collagen type II fibers.
• Elastic cartilage contains elastic fibers, and white fibrocartilage contains collagen type I fibers.
• Hyaline cartilage sites include fetal skeleton, epiphyseal plate, articular and costal cartilage, and the respiratory passage.
• Elastic cartilage sites include the ear pinna, Eustachian tube, epiglottis, and external auditory canal.
• White fibrocartilage sites include intervertebral discs, the mandibular joint, symphysis pubis, sternoclavicular joint, lips of acetabulum & glenoid cavity, and semilunar cartilages.
• The matrix of hyaline cartilage is homogenous, rubbery, deeply basophilic, and transparent.
• The matrix of elastic cartilage is heterogenous and flexible, while the matrix of white fibrocartilage is scanty, strong and tough.

Hyaline Cartilage

• The perichondrium possesses an outer fibrous layer made of fibroblasts and collagen type I containing blood vessels and nerves.
• The inner chondrogenic layer cells is rich in chondroblasts.
• The function of the perichondrium is to: provide attachment for muscle fibers, nutrition, and formation of new cartilage.
• Chondroblasts (can divide) originate from UMC and are flat and oval in shape, they have a dark basophilic cytoplasm and an oval pale nucleus.
• The site of the chondroblasts is the inner aspect of perichondrium with ECM displaying features of protein forming cells.
• Its function includes the secretion of matrix and collagen II, appositional growth, and when surrounded by matrix, they become chondrocytes.
• Chondrocytes originate from chondroblasts, displaying basophilic cells with central nuceli
• Chondrocytes are present in lacunae and are surrounded by a capsule.
• Superficial cells are small, oval, and positioned parallel to the surface.
• Deeper cells are rounded or triangular, displaying rounded dark nuclei.
• They have pale cytoplasm, glycogen dissolves when prepared with lipids.
• They divide giving groups of 2,4,8,... isogenous group cell nest.
• ECM features of protein forming cells include mature chondrocytes showing lipid droplets and glycogen granules.
• Its function is to maintain the cartilage matrix, performing interstitial growth of cartilage from inside.
• Ground substance, produced by chondroblasts and chondrocytes, is rubbery, homogenous, and transparent.
• Ground substance is deeply basophilic and formed of proteoglycans, glycoproteins & water.
• With Aggrecan as the most abundant proteoglycan with sulfated GAGs side chains of chondroitin sulfate and keratan sulfate.
• Territorial matrix is the ECM directly around each lacuna, containing mostly proteoglycans and sparse collagen.
• Interterritorial matrix is ECM further from lacuna containing less basophilic components, rich in collagen.
• Chondronectin is a multiadhesive glycoprotein that binds specifically to GAGs, collagen, and integrins.
• Chondronectin mediates the adherence of chondrocytes to the ECM.
• Elastic cartilage has the same structure as hyaline cartilage + elastic fibers and few collagen (type II) fibers.

White Fibrocartilage

• White fibrocartilage is not surrounded by perichondrium.
• It is formed of dense, parallel, thick bundles of collagen (type I) fibers.
• Separated by rows of chondrocytes inside the lacunae surrounded by matrix.

Bone Cells

• Cells are divided into osteogenic dividable cells, osteoblasts, osteocytes, and osteoclasts.
• Osteogenic cells originate in UMCs and pericytes.
• Osteoblasts and osteocytes originate from osteogenic cells, osteoclasts from bone marrow precursors.
• Osteogenic cells are located in the inner osteogenic layer of the periosteum and the endosteum.
• The inner osteogenic layer of the periosteum and endosteum contain osteoblasts.
• Osteocytes are located in lacunae within the calcified matrix.
• Osteogenic cells are flat or oval-shaped with round central nuclei, and basophilic cytoplasm.
• Osteoblasts have large, oval shape with eccentric, deep basophilic cytoplasm/golgi and high alkaline phosphatase enzyme.
• Osteocytes are oval, and have branched cells with cytoplasmic processes that pass through canaliculi, has a deeply stained central cytoplasm that is pale basophilic.
• Osteoclasts are large irregular motile cells with 50 nuclei, and foamy acidophilic cytoplasm with a striated brush border.
• Electron microscopy reveals abundant ribosomes, Golgi complex, rough ER, and mitochondria in osteogenic cells.
• Osteoblasts have ribosomes, rough ER, Golgi complex, and secretory vesicles containing matrix vesicles.
• Osteocytes have rough ER, ribosomes, Golgi complex, and mitochondria.
• Osteoclasts have four zones, of the ruffled border, clear zone, vesicular zone, and basal zone.
• Osteoblasts form/deposit organic and matrix, while osteocytes help with Maintance calcification.
• Osteoclasts function is to dissolves/resorbing bone.
• Osteoblasts require blood and good oxygen to differentiate into chondroblasts if lacking necessary blood.

Osteoclast Cell Zones

• Ruffled border zone is the finger-like processes with actin filaments, that secrets acidic to dissolve inorganic salts.
• The Clear zone is the ring shaped area with the actin filaments surrounding the ruffled border.
• The Clear zone responsible for movement of fixated cells along the bone.
• The Vesicular zone contains lysosomes, exocytotic vesicles, and endocytotic vesicles that help resorb the debris.
• The basal zone is filled with multiple nuclei and is responsible for secretion of enzymes that can dissolve organic tissues.

Bone Coverings

• There is the periosteum and the endosteum.
• The periosteum has two parts of outer fibrous and inner osteogenic layer.
• The outer fibrous layer has dense irregular C.T that is collagen I with fibroblasts, fibrocytes, and blood vessels.
• The inner osteogenic layer has loose C.T that is rich in blood vessels with osteogenic and osteoblasts.
• The endosteum covers both inside and outside of the bone which makes it thinner than periosteum.

Bone

• Bone consists of compact and cancellous types that is also referred to as spongy bones.
• Under the microscope the compact bone is solid, while the cancellous bone is has loose and spongy bone tissue.
• The shaft of the long bones is compact while the rest are cancellous.
• Bone Intercellular Substance (Matrix) can be made of two things either 75% hard or 25% soft tissue.
• The hard substances contain 35% organic components with 65% inorganic components.
• The soft tissues contain 25% of water.
• There are two types of Matrix:
• The Territorial matrix that surrounds each lacuna from all direction.
• The innerterritorial matrix which is further contains fibers and is rich in collagen.

Bone Composition

• Bone contains 75% hard substances and 25% water.
• Hard substances are 35% organic (osteoid tissue) and 65% inorganic components,
• Under the microscope you can see osteocytes that contains bone tissues with red marrow and trabecula.
• Organic components are 90% type I collagen fibers (staining acidophilic ) alongside macromolecules such as Sulfated and non-sulphated glycosaminoglycans with high amounts of proteoglycans/glycoproteins.
• Inorganic components consists of calcium & phosphate minerals in form/crystals with a Rod-shaped composition of low level molecules.
• Bone development happens at two stages: intramembranous life with formation of bone.
• Bone formation from the long bones develops skull vaulting maxilla.
• The endochondral that inter cartinogolous bone which develop at a longer stage.

Ossification Types

• Intramembranous ossification forms bone from a mesenchymal membrane.
• Endochondral ossification forms bone from a cartilage model that is later destroyed and replaced by bone.
• Bone grows to A- Width (Appositional growth) and also increases in B- Length.
• Growth in width is caused by osteogenic cells and osteoblasts found under the periosteum and around the endosteum.
• Osteogenic cells divide while osteoblasts resorb the bones shaft/structure.
• Growth is caused by the epiphseal cartliage and the bone increases in lenght.
• The epiphseal cartliage maintains a constant rate, allowing a better cartliage thickness.
• There are 5 zone which regulate cartilage and bones structure for growth.

Epiphyseal Plates

1. Resting cartilage includes small clusters of chondrocytes.
2. Proliferation happens in columns of cells parallel to bone axis.
3. Maturation and hypertrophy: chondrocytes enlarge, accumulate glycogen, release alkaline phosphatase enzyme, and thin the matrix.
4. Calcification: the matrix between cells becomes calcified, killing any chondrocytes.
5. New bone marrow and cells develop osteoblast for new bone matrix with cancellous structures for bones and empty structures.