Chapter 22. Skeletal System
Recommended Article: 【Biology】 Table of Contents for Biology
1. Overview
2. Structure and Formation of Bones
3. Mechanical Connections of Bones
4. Mechanical Properties of Bones
5. Bone Diseases and Treatment
1. Overview
⑴ Functions
① Body support
② Protection of internal organs
③ Providing links for movement and muscle attachment
④ Mineral storage: calcium, phosphorus storage
⑤ Blood cell production in bone marrow
⑵ Classification of Skeleton
① Hydrostatic skeleton: Formed inside the body when muscles compress a cavity filled with an incompressible fluid.
○ Examples: coelenterates (sea anemones), squids and octopuses (jet propulsion), earthworm locomotion
○ Earthworm Locomotion:
○ 1st. Contraction of the longitudinal muscles and relaxation of the circular muscles in the head and tail segments make them short and thick; their setae anchor to the ground. The remaining segments, with relaxed longitudinal muscles and contracted circular muscles, become long and slender.
○ 2nd. Contraction of the circular muscles in the head region pushes the head forward. The segments just behind the head and those toward the rear become thick, contact the ground, and anchor, preventing the worm from slipping backward.
○ 3rd. The anterior segments thicken again and anchor at the new position, while the posterior segments lift off the ground and are pulled forward.
② Exoskeleton: A hard shell that forms the outer part of the body, with muscles attached.
○ Examples: Mollusca, arthropods.
③ Endoskeleton: Internal structures that support the body, with muscles attached and acting upon them.
○ Example: Human skeleton
⑶ Human Skeleton (Endoskeleton)
① Composition
○ 206 bones in total
○ Main Skeleton: Spinal column, skull, rib cage.
○ Appendicular Skeleton: Joints, shoulders, limb bones.
Figure 1. Human Skeleton
② Spine (vertebral column)
○ Classification of the spine
○ Cervical Vertebrae: Neck vertebrae, 7 vertebrae.
○ Thoracic Vertebrae: Chest vertebrae, 12 vertebrae.
○ Lumbar Vertebrae: Lower back vertebrae, 5 vertebrae.
○ Sacrum: Vertebrae near the hips, 5 vertebrae.
○ Coccyx: Vertebrae near the tailbone, 4-5 vertebrae.
○ Functional Segments: Functional units of the spine.
Figure 2. Structure of Functional Segments
○ Front Part of Functional Segment: Consists of vertebral body, intervertebral disc, longitudinal ligament.
○ Back Part of Functional Segment: Consists of vertebral arch, intervertebral joint, various processes, various ligaments.
○ Intervertebral Disc: Physical cushioning action. Disc bears about 1.5 times its external load. Load borne by the disc increases towards the tail end.
○ Ligaments (excluding yellow ligaments): High collagen content that limits spinal elongation.
○ Yellow Ligaments: High elasticity. Always under tension. Induces contraction during spinal stretching.
○ Disorder 1: Herniated Disc: Condition where intervertebral discs between vertebrae rupture. May compress the spinal cord and cause pain.
○ Cervical Disc: Herniation in cervical vertebrae.
○ Lumbar Disc: Herniation in lumbar vertebrae.
○ Disorder 2: Spinal Stenosis: Narrowing of the canal through which nerves pass in the spinal cord. Causes pain and other neurological symptoms.
○ Often more severe than herniated disc. In contrast to herniated discs, spinal stenosis is often not covered by insurance.
③ Sexual Differences in Skeletal Structure
○ Sex hormones influence skeletal and muscle development. Sex differences in physical performance become evident during puberty.
○ Females experience puberty earlier, resulting in taller stature during this period.
○ The later puberty begins, the more likely growth will continue for a longer period.
○ On average, males have longer limbs than females, resulting in a 15 cm height difference.
○ Longer arms and legs in males result in greater leverage and force.
○ Females have a lower center of gravity, leading to better balance (e.g., rhythmic gymnastics).
○ Q Angle
○ Definition: Angle formed between the femur and the tibia (load direction).
○ Male Q Angle: 12°, Female Q Angle: 16°.
○ Females have a larger Q angle, which can contribute to knee injuries upon landing.
Figure 3. Q Angle
2. Structure and Formation of Bones
⑴ Components of Bones
① Abundance of inorganic minerals compared to regular tissues.
② Water constitutes 5-8% of bone weight.
○ 85% of water is distributed in organic components, collagen fibers, substrates, and the hydration layer of bone crystals.
○ 15% of water is distributed in bone canals and cavities.
③ Organic components constitute 33% of dry bone weight and 50% of bone volume.
○ Function: Provides flexibility and elasticity to bones.
○ Mostly composed of collagen.
○ Collagen (collagenous fiber, type I collagen): 90%. Less resistant to tension.
○ Amorphous ground substance: 10%.
○ Osteogenesis Imperfecta (brittle bone syndrome): Collagen mutation and deficiency.
④ Inorganic minerals constitute 67% of dry bone weight and 25% of bone volume.
○ Function: Provides hardness to bones.
○ 85% of inorganic minerals are calcium phosphate. 15% are calcium carbonate.
○ Calcium constitutes 39% of dry bone weight: 99% of body’s calcium (rest is in teeth).
○ Phosphate constitutes 17% of dry bone weight: 90% of body’s phosphorus.
○ Carbonate constitutes 9.7% of dry bone weight: 80% of body’s carbonate.
○ Sodium constitutes 0.7% of dry bone weight: 35% of body’s sodium.
○ Magnesium constitutes 0.4% of dry bone weight: 50% of body’s magnesium.
○ Potassium constitutes 0.2% of dry bone weight: 4% of body’s potassium.
⑤ Calcium Replacement
○ 99% of calcium is distributed in teeth and bones.
○ Bone’s calcium content is replaced 100% in the first year after birth, about 10% during childhood, and 2-4% annually during adulthood.
○ Synthesis and breakdown of calcium increase during adulthood.
○ Bone loss accelerates over the age of 40-50 due to increased breakdown compared to synthesis.
⑵ Bone Structure
① Bones consist of dense bone and spongy bone.
○ Dense Bone (Cortical Bone, Compact Bone): Outer layer of bone.
○ Forms the dense and tough outer surface of bones.
○ Strength: 100-150 MPa.
○ Spongy Bone (Trabecular Bone): Inner part of bones.
○ Porous with a honeycomb-like internal structure.
○ Strength: 8-50 MPa.
○ Periosteum
○ Dense fibrous membrane. Traversed by Volkmann’s canals connected to blood vessels. Serves as a passageway for nerve fibers.
○ Inner Layer: Also known as the cambium layer or osteogenic layer. Osteoblasts are loosely arranged. Rich in blood vessels. Active bone formation occurs.
○ Middle Layer: Area containing undifferentiated osteoprogenitor cells.
○ Outer Layer: Thick layer of dense connective tissue composed of irregularly arranged collagen and elastic fibers. Also known as the fibrous layer.
○ Sharpey’s fibers: Bundles of fibers that extend deeply from the periosteum into the cortical bone.
○ Endosteum
○ Thin layer of cellular connective tissue covering the bone cavity wall and spongy bone surfaces.
○ Mostly composed of osteoprogenitor cells (endosteal cells), the precursor cells of osteoblasts.
○ Present only in long bones.
○ Surrounds the medullary cavity, the central part of bones.
Figure. 4. Bone Structure
② Composition of Compact Bone
○ Compact bone is composed of numerous osteons.
③ Osteon (Haversian System): the shape of a tree ring
Figure. 5. Structure of Osteon
○ Size of Osteon: Diameter of 200 μm.
○ Haversian Canal: Central canal of an osteon.
○ Blood vessels → Volkmann’s canal → Haversian canal → Bone canaliculus → Bone lacuna ⥇ Cement line.
○ Volkmann’s canal: Penetrates the periosteum from its outer layer and enters the cortical bone, passing through horizontally.
○ Nerve fibers are distributed.
○ Lamellae: Inorganic structures of concentric circles.
○ Bone Lacuna: Spaces along the boundaries of lamellae. Each bone lacuna contains one osteocyte.
○ Bone Canaliculus: Channels that extend vertically from bone canaliculus cavity to each lamella. Pathway connecting to Haversian system.
○ Cement Line: Outermost layer of an osteon.
○ Bone canaliculi and collagen fibers cannot pass through. The weakest point of bone’s microstructure.
○ Composed of GAGs (glycosaminoglycans).
○ Interstitial Lamellae: Located between osteons. Although different in appearance, they have similar composition to osteons.
④ Composition of Spongy Bone
○ Presence of bone lamellae and bone lacunae.
○ No Haversian system in spongy bone.
○ Spaces between spongy bones filled with red bone marrow, which supplies nutrients.
○ Bone Marrow: Produces blood cells within the bone.
⑤ Cells within Bones
Figure. 6. Osteoblasts and Others
○ Nutrient and Gas Exchange: Transported through blood vessels and Volkmann’s canals.
○ Component 1: Osteoclast
○ Breaks down bone (osteocyte) and releases calcium into the bloodstream.
○ Diameter: 20-100 μm.
○ A type of monocyte-macrophage within white blood cells.
○ Mononuclear osteoclasts transform into multinuclear osteoclasts (with around 50 giant nuclei) and adhere to bone surfaces, secreting lysosomal enzymes for bone resorption.
○ Component 2: Osteoblast
○ Forms bone (osteocyte) and removes calcium from the blood.
○ Also known as bone-forming cells.
○ Synthesize and secrete bone matrix and deposit minerals like calcium and magnesium onto the matrix. Mineralization of the bone tissue occurs.
○ Component 3: Osteocyte (Bone Cell): Limited division ability ×
○ Located within bone cavities.
○ Osteocytes are connected through canaliculi and exchange small molecules via gap junctions.
○ Component 4: Osteogenic Cell: Division ability ○
○ Component 5: Blood Vessels, Nerve Fibers
○ Component 6: Parathyroid Hormone: Stimulates bone resorption.
○ Component 7: Calcitonin from Thyroid: Promotes bone formation.
○ Component 8: Blood Calcium: Involved in blood clotting, muscle contraction, generating action potentials, enzyme activity.
⑥ Cartilage
○ Cartilage has no blood vessels, lymphatic vessels, or nerve supply, and has a low cell density.
○ Water comprises 30% of cell content.
○ Important for lubrication in joints.
○ Cartilage is stained by safranin-O due to secretion of Collagen Type II.
○ Rate of sulfate absorption in cartilage tissue is used as an indicator of bone growth.
○ Growth Factors:
○ Insulin-like Growth Factor (IGF)
○ Transforming Growth Factor-Beta (TGF-β)
○ Fibroblast Growth Factor Basic (b-FGF)
○ Bone Morphogenic Protein (BMP)
○ Platelet-Derived Growth Factor (PDGF)
⑦ Bone Remodeling
○ Definition: The structure of bones changes constantly as osteoclasts break down bone in response to external conditions, and osteoblasts create bone.
○ Bones undergo continuous remodeling processes.
○ Wolff’s Law (also known as Wolff’s law)
○ Definition: Refers to the mechanical changes during bone remodeling.
○ Proposed by Julius Wolff (1836-1902).
○ Summary: Bones become stronger with use.
○ Strong physical impact on bones results in increased strength and stiffness as a compensatory mechanism.
○ Weak physical impact on bones leads to decreased strength and stiffness as the body reduces metabolism.
○ Strength and stiffness are related to bone density, thickness, and more.
⑶ Bone Growth
① Classification of Bone Development Phases
○ Woven Bone
○ Found in tumors, Paget’s bone disease, fetal bones, growing bones, fractured bones, and areas of bone formation after fractures.
○ Paget’s Bone Disease: Common bone disorder following osteoporosis, abnormal bone growth and pain due to excessive activity of osteoclasts.
○ Lamellar Bone
○ Begins forming after the first year of birth.
○ Replaces woven bone as mature bone tissue.
② 1st. Embryonic development stage: All bones are composed of cartilage.
③ 2nd. Phase of Bone Formation
○ Primary ossification centers develop within the center of cartilage, starting the process of ossification.
○ Bone differentiation markers: alkaline phosphate (in vitro; 7 ~ 10 days) → osteopontin → osteonectin → osteocalcin (in vitro; 3 weeks)
④ 3rd. Secondary ossification center formation
○ When sufficient ossification has progressed from the primary ossification center, a new bone-forming center (secondary ossification center) appears at the ends of the bone.
○ Osteoconductive: the conduction of bone cells into adjacent areas.
○ Osteoinductive: the ability to induce the formation of bone tissue by itself.
⑤ 4th. Epiphyseal (growth) plate closure
○ Under hormonal influence, osteoblast precursors proliferate faster than chondroblasts, causing the epiphyseal growth plate between the two ossification centers to close → cessation of longitudinal growth.
○ Hormones: growth hormone, thyroid hormones, sex hormones.
○ Epiphyseal growth plate: the cartilaginous region between the two ossification centers.
Figure. 7. Bone Growth
⑥ 5th. Bone Aging: Decrease in bone density, length, and thickness.
○ Bone Density
○ Increases during growth, peaks around age 25.
○ At 70 years, it may be as low as 60% of the peak value.
Figure. 8. Bone Aging
○ Calcium Absorption Rate
○ Highest in infancy: 60%.
○ Prepuberty: 28%.
○ Post-puberty: 34%.
○ Adult females: 25%.
○ Last 2-3 months of pregnancy: Temporary increase in calcium absorption rate.
○ Menopause, low calcium intake, vitamin D deficiency, dieting, lack of exercise accelerate bone aging.
3. Mechanical Connections of Bones
⑴ Joints: Connections between bones
① Immovable Joints
○ Fibrous Joints
○ Formed by collagen and elastic fibers; three types: syndesmosis (ligamentous joint), suture, and gomphosis (peg-and-socket).
○ A representative example of a suture is the cranial (skull) sutures.
○ Cartilaginous Joints
○ Type 1: Synchondrosis (primary cartilaginous joint) — the union is formed by hyaline cartilage.
○ Type 2: Symphysis — a fibrocartilaginous union.
○ Because the vertebral column allows limited motion, it is sometimes classified as an amphiarthrosis.
② Movable Joints: Typical joints
○ Subdivided into synovial, plane, hinge, pivot, ellipsoid (condyloid), saddle, and ball-and-socket joints.
○ Synovial joint: opposing articular cartilages + synovial fluid + joint capsule.
○ Ball-and-socket joint: a joint that moves about three axes.
○ Examples: the shoulder (where the humerus meets the scapula), the hip (where the femur meets the pelvis); also called a spheroidal/cotyloid joint.
○ Hinge joint: a joint that moves about one axis.
○ Examples: the humeroulnar part of the elbow; the knee joint.
○ Pivot joint: a joint that can rotate around its long axis.
○ Examples: the proximal radioulnar joint at the elbow; rotation in the neck at the atlanto-axial (axle) joint.
③ Synovial Joint (Articular Cartilage): Representative movable joint
○ Considered a heterogeneous material: interstitial fluid form (incompressible, non-miscible) + solid form (porous, permeable).
○ The space between opposing articular surfaces is about 100 nm.
Figure. 9. Structure of Cartilaginous Tissue
○ Structure 1: Superficial Zone
○ Topmost layer
○ Chondrocytes aligned elliptically with the major axis parallel to the surface.
○ Collagen fibers oriented parallel to the surface.
○ Lowest concentration of proteoglycans.
○ Structure 2: Middle Zone
○ Chondrocytes have a round shape.
○ Larger collagen fiber bundles are aligned compared to the superficial zone.
○ Structure 3: Deep Zone
○ Chondrocytes are spherical and arranged in a radial pattern.
○ Large collagen fiber bundles are aligned perpendicular to the joint surface.
○ Highest concentration of proteoglycans.
○ Lowest water content.
○ Structure 4: Calcified Zone
○ The deepest layer
○ Divided into subchondral bone and hyaline cartilage.
○ Differentiation of small chondrocytes with nuclear condensation
○ Appears as a dark blue line with H&E (haematoxylin and eosin) staining.
○ Component 1. Collagen
○ The most abundant protein in the human body.
○ Tropocollagen: the basic unit of collagen.
○ Procollagen α chains → three assemble into a triple helix → helices aggregate into banded fibrils.
○ High resistance to tension.
○ Low resistance to compression: proteoglycans provide most of the compressive resistance.
○ Component 2. Proteoglycan
○ Large glycoproteins composed of a core protein with several glycosaminoglycan (GAG) chains attached.
○ Compression → proteoglycans deform and expel interstitial fluid → fixed charge density rises, increasing resistance.
○ Donnan osmotic swelling pressure: compression → aggrecans cluster, increasing negative charge density → water is drawn in and the tissue swells (edema).
④ Arthritis
Figure. 9. Arthritis
○ Osteoarthritis (degenerative arthritis): age-related loss of articular cartilage and reduced synovial fluid viscosity; treated with injections such as hyaluronic acid (hyaluronan).
○ Rheumatoid arthritis: an autoimmune disease targeting joint tissues, causing synovial inflammation, cartilage damage, and related changes.
⑵ Skeletal and Muscle Connections
① Ligaments: Connect bones to bones.
○ Collagen fibers are not completely parallel, allowing some tolerance for load in various directions.
○ Elastin is almost absent.
② Tendons: Connect muscles to bones, enabling movement.
○ Originate at the central axial skeleton and attach at the peripheral skeleton.
○ Collagen fibers are orderly and parallel, accommodating unidirectional tensile load. Tensile strength of 50 MPa.
○ Collagen content is 80%. Elastin is almost absent. Blood vessels are relatively sparse.
○ Paratenon: Vascularized. Forms a sheath for protection.
○ Epitenon: Avascular. Contains tenocytes and promotes tendon gliding through synovial fluid.
③ Ligaments and tendons have minimal blood supply, leading to slow wound healing.
Figure. 11. Ligaments and Tendons
4. Mechanical Properties of Bones
⑴ Bone’s Anisotropy
① Strength: Longitudinal > Transverse
② Stiffness: Longitudinal > Transverse
③ Ductility: Longitudinal > Transverse
⑵ Fatigue Loading
Figure. 12. Fatigue Loading
⑶ Bone Fracture Patterns
① Compact bone is more resistant to yield stress in the order of compression (170 MPa), tension (130 MPa), and shear (70 MPa) load.
② Compression fracture
○ Bone yielding occurs within inter-osteonal gaps.
○ Bone is stronger in compression than in tension.
○ Stable fracture: typically a clean, non-displaced crack (hairline).
○ Most commonly occurs in the vertebrae.
③ Tensile fracture
○ Bone yields by debonding at the cement lines and between osteons.
○ Where cancellous (trabecular) bone is abundant, tensile-load fractures are frequent.
○ Unstable fracture.
④ Shear fracture
○ Where cancellous (trabecular) bone is abundant, shear-load fractures are frequent.
○ Unstable fracture.
5. Bone Diseases and Treatments
⑴ Bone Diseases
① Fractures
② Osteoporosis: Decreased calcification, porous bones.
⑵ Bone Treatments
① Bone Regeneration
○ Mesenchymal stem cells
○ Scaffolds
○ Endothelial cells
○ Growth Factors (BMP): Typically BMP2, BMP4.
② Cartilage Regeneration Methods
○ Differentiation after stem cell transplantation
○ Bone-cartilage transplantation method
○ Joint surface shaping surgery
○ Chondrocyte transplantation method
③ Microfracture Surgery via Bone Marrow Stimulation
○ Create holes in bone to cause bleeding.
○ Stem cells in blood regenerate cartilage.
○ Bone marrow-derived stem cells are superior to adipose-derived stem cells.
Input: 2015.07.26 10:43