Osseous Tissue & the Skeletal System: Structure, Function, and Disorders

Osseous Tissue & the Skeletal System

Functions of Bone

Bones serve several essential functions in the human body, contributing to both structure and physiology.

• Support: Provides a framework for the body and supports soft organs.

• Protection: Shields vital organs such as the brain, spinal cord, and heart.

• Movement: Acts as levers for muscle action, enabling locomotion.

• Mineral Storage: Stores calcium and phosphorus, and releases them as needed.

• Blood Cell Formation: Houses bone marrow, which produces blood cells.

• Triglyceride Storage: Stores fat in bone cavities for energy.

• Hormone Production: Bones secrete hormones (e.g., osteocalcin) that help regulate insulin secretion, glucose levels, and energy metabolism.

Classification of Bones

Bones are classified by location and shape, which reflect their function and structure.

• Axial Skeleton: Includes skull, vertebral column, and rib cage.

• Appendicular Skeleton: Includes bones of the limbs and girdles.

Bones are also classified by shape:

• Long Bones: Longer than wide (e.g., limb bones).

• Short Bones: Cube-shaped (e.g., wrist and ankle bones); sesamoid bones form within tendons (e.g., patella).

• Flat Bones: Thin, flat, and slightly curved (e.g., sternum, scapulae, ribs, skull bones).

• Irregular Bones: Complicated shapes (e.g., vertebrae, hip bones).

Bone Structure

Bones are organs composed of several tissue types, including bone tissue, cartilage, nervous tissue, and connective tissue.

• Compact Bone: Dense outer layer that appears smooth and solid.

• Spongy Bone: Honeycomb of small, needle-like or flat pieces called trabeculae; spaces between trabeculae are filled with red or yellow bone marrow.

Structure of Short, Irregular, and Flat Bones

• Consist of thin plates of spongy bone (diploë) covered by compact bone.

• Compact bone is sandwiched between connective tissue membranes (periosteum and endosteum).

• Bone marrow is scattered throughout spongy bone; no defined marrow cavity.

• Articular cartilage covers bone ends if part of a movable joint.

Structure of a Typical Long Bone

• Diaphysis: Tubular shaft forming the long axis of bone; contains medullary cavity with yellow marrow in adults.

• Epiphyses: Ends of long bones; compact bone externally, spongy bone internally; articular cartilage covers joint surfaces.

• Epiphyseal Line: Remnant of childhood epiphyseal plate where bone growth occurs.

Membranes

• Periosteum: Double-layered membrane covering external bone surfaces except at joints; contains osteogenic cells.

• Endosteum: Delicate connective tissue membrane covering internal bone surfaces; lines trabeculae of spongy bone.

Hematopoietic Tissue in Bones

• Red Marrow: Found within trabecular cavities of spongy bone and diploë of flat bones; produces blood cells.

• Yellow Marrow: Fat storage; can convert to red marrow if person becomes anemic.

Bone Markings

Sites of muscle, ligament, and tendon attachment, as well as conduits for blood vessels and nerves.

• Projections: Outward bulges of bone.

• Depressions/Openings: Allow passage for blood vessels and nerves.

Microscopic Anatomy: Cells of Bone Tissue

• Osteogenic Cells: Mitotically active stem cells in periosteum and endosteum; differentiate into osteoblasts or bone-lining cells.

• Osteoblasts: Bone-forming cells that secrete unmineralized bone matrix (osteoid); actively mitotic.

• Osteocytes: Mature bone cells that maintain bone matrix and act as stress or strain sensors.

• Bone-Lining Cells: Flat cells on bone surfaces; help maintain matrix.

• Osteoclasts: Derived from hematopoietic stem cells; giant, multinucleate cells for bone resorption.

Compact Bone Structure

• Osteon (Haversian System): Structural unit of compact bone; consists of a cylinder of bone matrix with collagen fibers running in different directions.

• Canals and Canaliculi: Central canal contains blood vessels and nerve fibers; canaliculi connect lacunae to each other and to the central canal.

• Interstitial and Circumferential Lamellae: Lamellae not part of osteon; help long bone resist twisting.

Chemical Composition of Bone

• Organic Components: Includes cells and osteoid (ground substance and collagen fibers); contribute to bone's flexibility and tensile strength.

• Inorganic Components: Mainly hydroxyapatite (mineral salts); account for bone's hardness and resistance to compression.

Bone Development (Ossification)

Ossification is the process of bone formation, beginning in the embryo and continuing through adolescence.

• Endochondral Ossification: Bone forms by replacing hyaline cartilage; forms most bones below the base of the skull.

• Intramembranous Ossification: Bone develops from fibrous membrane; forms flat bones (e.g., skull, clavicles).

Growth in Length of Long Bones

• Occurs at the epiphyseal plate by interstitial growth.

• Epiphyseal plate consists of five zones: resting, proliferation, hypertrophic, calcification, ossification.

• Epiphyseal plate closure occurs when epiphysis and diaphysis fuse (around 18 years in females, 21 years in males).

Growth in Width of Bones

• Occurs by appositional growth; bones thicken in response to stress from muscle activity or added weight.

Hormonal Regulation of Bone Growth

• Growth Hormone: Stimulates epiphyseal plate activity in infancy and childhood.

• Thyroid Hormone: Modulates activity of growth hormone.

• Sex Hormones: Promote adolescent growth spurts and epiphyseal plate closure.

Bone Remodeling

• About 5-7% of bone mass is recycled each week.

• Remodeling involves bone deposit and bone resorption.

• Occurs at surfaces of both periosteum and endosteum.

Bone Deposit and Resorption

• Deposit: Occurs by osteoblasts; requires adequate calcium and phosphate ions.

• Resorption: Function of osteoclasts; dig depressions or grooves, secrete lysosomal enzymes and acids to dissolve bone matrix.

Control of Remodeling

• Hormonal Control: Parathyroid hormone (PTH) and calcitonin regulate blood calcium levels.

• Response to Mechanical Stress: Bones reflect stresses they encounter; thicker where muscles attach or where stress is greatest.

Fractures

Fractures are breaks in bones, classified by position, completeness, and whether skin is penetrated.

• Reduction: Realignment of broken bone ends (closed or open reduction).

• Immobilization: Use of cast or traction for healing.

Repair involves four major stages:

1. Hematoma Formation: Blood vessels hemorrhage, forming a clot.

2. Fibrocartilaginous Callus Formation: Collagen fibers span break, forming soft callus.

3. Bony Callus Formation: New trabeculae appear, forming hard callus.

4. Bone Remodeling: Excess material removed, compact bone laid down.

Bone Disorders

• Osteomalacia/Rickets: Bones poorly mineralized; results in soft, weak bones.

• Osteoporosis: Bone resorption outpaces deposit; bones become fragile.

• Paget's Disease: Excessive bone deposit and resorption; high ratio of spongy to compact bone.

Joints

Classification and Structure

Joints (articulations) are sites where two or more bones meet, providing mobility and stability.

• Structural Classification:

  • Fibrous Joints: Bones joined by dense fibrous connective tissue; mostly immovable (e.g., sutures, syndesmoses, gomphoses).

  • Cartilaginous Joints: Bones united by cartilage; not highly movable (e.g., synchondroses, symphyses).

  • Synovial Joints: Bones separated by fluid-filled joint cavity; freely movable.

Synovial Joint Features

• Articular Cartilage: Hyaline cartilage covering bone ends.

• Joint (Synovial) Cavity: Small, fluid-filled space.

• Articular Capsule: Two layers (external fibrous layer and internal synovial membrane).

• Synovial Fluid: Lubricates and nourishes articular cartilage.

• Reinforcing Ligaments: Strengthen joint.

• Nerves and Blood Vessels: Monitor joint position and supply nutrients.

Joint Injuries and Disorders

• Sprains: Ligaments stretched or torn.

• Dislocations: Bones forced out of alignment.

• Bursitis: Inflammation of bursae.

• Tendonitis: Inflammation of tendon sheaths.

• Arthritis: Includes osteoarthritis (degenerative), rheumatoid arthritis (autoimmune), and gouty arthritis (uric acid crystal deposition).

Table: Bone Disorders Comparison

Disorder	Main Features	Causes	Effects
Osteomalacia/Rickets	Poor mineralization, soft bones	Vitamin D deficiency, insufficient calcium	Bone pain, deformities
Osteoporosis	Bone resorption > deposit, fragile bones	Age, menopause, poor diet, lack of exercise	Fractures, bone loss
Paget's Disease	Excessive bone deposit/resorption	Unknown, possibly viral	Deformed bones, pain

Key Equations

• Bone Remodeling Equation:

$\text{Bone Mass} = \text{Bone Deposition} - \text{Bone Resorption}$

• Calcium Homeostasis:

$\text{PTH} \uparrow \Rightarrow \text{Blood Calcium} \uparrow$

$\text{Calcitonin} \uparrow \Rightarrow \text{Blood Calcium} \downarrow$

Additional info:

• Bone tissue is highly dynamic, constantly undergoing remodeling in response to mechanical and hormonal signals.

• Proper nutrition, physical activity, and hormonal balance are essential for maintaining healthy bones and joints.