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 Fluid Skeleton: Fluid-filled spaces that cannot be compressed by muscles within the body, formed by the entry of fluid.
Examples: Strong animals (jellyfish), squids and octopuses (jet propulsion), movement of earthworms.
Earthworm Locomotion:
1st Stage: Contraction of circular muscles and relaxation of longitudinal muscles result in shortening and thickening of the head and tail segments. The segments with circular muscle relaxation and longitudinal muscle contraction become elongated and thin.
2nd Stage: Contraction of circular muscles in the head section moves the head forward. The posterior segments of the head and the back of the body become thicker and are fixed to the ground, preventing the earthworm from sliding backward.
3rd Stage: The head segment thickens again and is fixed in a new position. The posterior segments now lift off the ground and move forward.
② Exoskeleton: A hard shell that forms the outer part of the body, with muscles attached.
Examples: Invertebrates, 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
Axial 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 (Coccyx): Vertebrae near the hips, 5 vertebrae.
Coccyx (Tailbone): 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 (Disk): 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.
Delayed puberty corresponds to greater potential for growth.
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, organic matrix, 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): 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. Contains nutrient vessels and nerve fibers, allowing passage for nerves.
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. Contains Sharpey’s fibers, which extend from 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)
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: Passes horizontally through the periosteal layer of bone. Carries blood vessels.
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 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 Osteoblasts and Others
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. Secretes enzymes (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.
Connected through canaliculi for exchange of small molecules.
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
Not supplied by blood vessels, lymphatics, or nerves. 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 Phase of Bone Formation : All bones are composed of cartilage.
③ 2nd Phase of Bone Formation
○ Primary ossification centers develop within the cartilage, starting the process of ossification.
○ Osteoblast differentiation markers: alkaline phosphate (in vitro; 7 ~ 10 days) → osteopontin → osteonectin → osteocalcin (in vitro; 3 weeks)
④ 3rd Phase of Bone Formation
○ When sufficient ossification occurs from the primary centers, new secondary ossification centers develop at the ends of bones.
○ (Note) Osteoconductive: Bone cells conducting from adjacent areas.
○ (Note) Osteoinductive: Self-induction of bone tissue by cells.
⑤ 4th Closure of Epiphyseal Plate
○ Under hormonal influence, osteoblasts proliferate faster than chondrocytes, closing the growth plate between bone centers → cessation of length growth.
○ Hormones: Growth hormone, thyroid hormone, sex hormones.
○ Growth Plate: Cartilage region between bone 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 sutures, elastic fibers, ligamentous attachments, sutures, three types of sutures, e.g., sutures in the skull.
○ Calvaria Sutures: Represented by the sutures of the skull.
○ Cartilaginous Joints
○ Type 1: Articulation of Cartilage
○ Type 2: Fibrocartilage Articulation
○ Vertebrae are somewhat mobile, classified as amphiarthroses.
② Movable Joints : Typical joints
○ Synovial Joints: Divided into hinge, plane, pivot, condyloid, saddle, and ball-and-socket joints.
○ Ball-and-Socket Joint: Moves along three axes.
○ Examples: Shoulder joint, hip joint.
○ Hinge Joint: Moves along one axis.
○ Examples: Elbow, knee.
○ Pivot Joint: Allows rotation around a long axis.
○ Examples: Elbow, neck, atlantoaxial joint.
③ Synovial Joint (Articular Cartilage) : Representative movable joint
○ Non-homogeneous material: Gelatinous fluid form (non-compressive, non-homogeneous) + Solid form (porous, permeable).
○ Gap between joints is 100 μm.
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 compared to the superficial zone.
○ Structure 3: Deep Zone
○ Chondrocytes are spherical and arranged in a radial pattern.
○ Large collagen fiber bundles perpendicular to the joint surface.
○ Highest concentration of proteoglycans.
○ Lowest water content.
○ Structure 4: Calcified Zone
○ Deepest layer
○ Differentiates into calcified cartilage and subchondral bone.
○ Small chondrocytes with condensed nuclei.
○ Stains dark blue with H&E staining.
○ Component 1: Collagen
○ Most abundant protein in the human body.
○ Basic unit of collagen: Tropocollagen.
○ α Chain Procollagen → Forms a triple helix with 3 tropocollagens → Forms band-shaped fibrils.
○ Resistant to tensile stress.
○ Less resistant to compressive stress: Proteoglycans provide resistance.
○ Component 2: Proteoglycan
○ Large protein composed of a core protein with several glycosaminoglycan (GAG) chains attached.
○ Compression → Proteoglycan deformation and release of interstitial fluid → Increased charge density leading to increased resistance.
○ (Note) Donnan osmosis edema pressure:
Compression → Aggregans increase negative charge density → Draws in water (edema).
④ Arthritis
Figure. 9. Arthritis
○ Osteoarthritis: Age-related reduction in articular cartilage and synovial fluid viscosity. Treated with hyaluronic acid injections and more.
○ Rheumatoid Arthritis: Autoimmune disorder affecting joint tissues, causing inflammation and cartilage damage.
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 Bone
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 compression (170 MPa), tension (130 MPa), and shear (70 MPa) order.
② Compression Fracture
○ Yielding occurs at bone’s weakest points.
○ Bones are stronger in compression than tension.
○ Stable Fracture: A clean break.
○ Commonly occurs in the spine.
③ Tension Fracture
○ Yielding occurs due to separation of cement lines and osteons.
○ Frequent in bones with a lot of cancellous bone.
○ Unstable Fracture.
④ Shear Fracture
○ Frequent in bones with a lot of cancellous bone.
○ Unstable Fracture.
⑷ Other
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 superior to adipose-derived stem cells.
Input: 2015.07.26 10:43