Bone Tissue and the Skeletal System: Structure, Function, and Growth

Bone Tissue and the Skeletal System

Functions of the Skeletal System

The skeletal system is essential for multiple physiological functions and structural support in the human body.

• Support: Provides attachment points for soft tissues and organs.

• Storage of Minerals: Especially calcium, which is vital for cellular processes.

• Storage of Lipids: Yellow marrow stores fat.

• Blood Cell Production: Red marrow produces blood cells (hematopoiesis).

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

• Leverage: Facilitates movement by acting as levers for muscles.

Classification of Bones

Bones are classified by their shape and function, which determines their role in the body.

• Shape Classification:

  • Short Bones: e.g., carpal (wrist), tarsal (ankle) bones.

  • Flat Bones: e.g., skull, scapulae, sternum, ribs.

  • Long Bones: e.g., bones of upper and lower limbs.

  • Irregular Bones: e.g., vertebrae, ossa coxae (hip bones).

  • Sesamoid Bones: e.g., patella (kneecap).

• Functional Classification:

  • Compact Bone: Dense, provides support and protection; contains osteons; 80% of bone mass.

  • Spongy Bone: Cancellous, internal to compact bone; no osteons; 20% of bone mass.

  • Cartilage: Includes hyaline, fibrocartilage; also ligaments and tendons.

Anatomy of Long Bone

Long bones have distinct anatomical regions that contribute to their function and growth.

• Diaphysis: Shaft of the bone; contains the medullary cavity.

• Medullary Cavity: Central space; contains red marrow in children, yellow marrow in adults.

• Epiphysis: Ends of the bone; covered by articular cartilage.

• Metaphysis: Region between diaphysis and epiphysis; contains the growth plate (epiphyseal plate/line).

• Epiphyseal Plate: Hyaline cartilage; site of bone growth in length.

• Epiphyseal Line: Remnant after bone stops growing.

Bone Coverings: Periosteum and Endosteum

Bones are covered by specialized membranes that support growth, repair, and attachment.

• Periosteum: Covers the outer surface; has a fibrous layer (dense irregular connective tissue) and a cellular layer (bone cells).

• Endosteum: Lines the internal surface (medullary cavity); contains bone cells.

Bone (Osseous) Tissue Structure

Bone tissue is a composite material with both organic and inorganic components.

• Matrix: Dense, contains calcium salts (inorganic) and collagen fibers (organic).

• Osteocytes: Mature bone cells located in lacunae.

• Canaliculi: Small channels for nutrient and waste exchange.

• Periosteum: Outer covering, as described above.

Bone Matrix Composition

The matrix provides both strength and flexibility to bone.

• Ground Substance: Calcium phosphate and calcium carbonate; gives compression strength.

• Fibers: Collagen and elastic fibers; provide flexible strength.

Bone Cells

Several cell types are involved in bone formation, maintenance, and remodeling.

• Osteoprogenitor Cells: Stem cells found in periosteum and endosteum; differentiate into osteoblasts.

• Osteoblasts: Build new bone; secrete osteoid (unmineralized matrix).

• Osteocytes: Mature bone cells; maintain bone tissue.

• Osteoclasts: Break down bone; derived from white blood cells.

Compact Bone Structure

Compact bone is organized into osteons, which are the basic structural units.

• Osteon: Cylindrical structure; contains concentric lamellae and central canal.

Spongy Bone Structure

Spongy bone lacks osteons and consists of a network of trabeculae.

• Trabeculae: Thin, bony plates; filled with red bone marrow.

• Red Bone Marrow: Site of blood cell formation.

Hyaline Cartilage

Hyaline cartilage is a key component in bone development and joint function.

• Avascular: Contains no blood vessels or nerves.

• Chondroblasts: Produce cartilage matrix.

• Chondrocytes: Mature cartilage cells; reside in lacunae.

• Perichondrium: Dense irregular connective tissue; surrounds cartilage except at articular surfaces.

Bone Formation and Growth

Bone formation (osteogenesis) and growth occur through two main processes: intramembranous and endochondral ossification.

• Osteogenesis: Formation of bone tissue.

• Ossification: Replacement of other tissues with bone.

• Calcification: Deposition of calcium salts.

Intramembranous Ossification

Forms flat bones (e.g., skull, mandible, clavicle) from mesenchymal tissue.

1. Mesenchymal cells cluster to form ossification centers.

2. Cells differentiate into osteoprogenitor cells and osteoblasts; osteoid is secreted.

3. Osteoid calcifies; osteoblasts become osteocytes.

4. Trabecular matrix and periosteum form; woven bone is replaced by lamellar bone.

Endochondral Ossification

Forms most bones (e.g., limbs, pelvis, vertebrae) from hyaline cartilage models.

1. Chondroblasts secrete hyaline cartilage (8th-12th week of pregnancy).

2. Cartilage calcifies; chondrocytes die; blood vessels invade.

3. Primary ossification center forms in diaphysis; osteoblasts produce osteoid.

4. Secondary ossification centers form in epiphysis; bone replaces cartilage except at epiphyseal plates and articular cartilage.

Bone Growth

Bones grow in length and diameter through distinct mechanisms.

• Appositional Growth: Increases bone diameter; occurs at periosteum.

• Interstitial Growth: Increases bone length; occurs at cartilage edge (epiphyseal plate).

Calcium Homeostasis

Calcium levels are tightly regulated by hormones to support vital functions.

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclasts.

• Calcitonin: Decreases blood calcium by inhibiting osteoclasts.

Key Equation:

$\text{Bone Matrix} = \text{Osteoid} + \text{Calcium Salts}$

$\text{Ca}^{2+} \text{ Homeostasis:} \quad \text{PTH} \uparrow \Rightarrow \text{Ca}^{2+} \uparrow \quad \text{Calcitonin} \uparrow \Rightarrow \text{Ca}^{2+} \downarrow$

Types of Fractures

Fractures are classified based on their characteristics and severity.

• Simple (Closed) Fracture: Bone breaks but skin remains intact.

• Open (Compound) Fracture: Bone breaks through the skin.

• Other Types: Transverse, spiral, comminuted, impacted, greenstick, oblique.

Fracture Type	Description
Closed	Bone broken, skin intact
Open	Bone broken, skin broken
Transverse	Break occurs straight across bone
Spiral	Twisting force causes spiral break
Comminuted	Bone shattered into several pieces
Impacted	Broken bone ends forced into each other
Greenstick	Partial break, common in children
Oblique	Break occurs at an angle

Stages in the Healing of a Bone Fracture

Bone healing follows a series of steps to restore structure and function.

1. Hematoma Formation: Blood vessels rupture; hematoma forms.

2. Cartilage Callus Formation: Fibrocartilage forms; fibroblasts and osteoblasts migrate.

3. Spongy Bone Callus Formation: New trabeculae form; soft callus becomes hard callus.

4. Remodeling: Compact bone reconstructs shaft walls.

Bone Disorders: Osteopenia, Osteoporosis, Paget's Disease

Bone health can be compromised by various disorders, especially with aging.

• Osteopenia: Thinning and weakening of bones; begins between ages 30-40.

• Osteoporosis: Severe bone loss; common in older adults, especially women.

• Paget's Disease: Bones become porous and deformed.

Example: Osteoporosis affects 29% of women and 18% of men over age 45.