Skeletal System: Bone Structure and Function

Definition: The skeletal system is primarily composed of bones, cartilage, ligaments, and other connective tissue structures.
Components:
- Bones: The primary organs of the skeletal system, providing a rigid framework for the body.
- Cartilage
- Ligaments
- Other connective tissue structures

Support and Protection: Bones provide structural support and protect vital organs.
Levers for Movement: Bones act as levers to facilitate movement by muscles.
Hematopoiesis: Blood cell production occurs in the red bone marrow.
Storage of Mineral and Energy Reserves:
- Minerals: Calcium and phosphate are stored within the bones.
- Energy Reserves: Bones store lipids in yellow bone marrow.

Types of Bones:
- Long Bones: Example: femur
- Short Bones: Example: tarsal bone
- Flat Bones: Example: frontal bone
- Irregular Bones: Example: vertebra

Epiphysis: End part of a long bone.
Articular Cartilage:
- Covers the joint surface.
- Made of a thin layer of hyaline cartilage that reduces friction and absorbs shock.
Diaphysis: The shaft of the bone, primarily made of compact bone.

Yellow Bone Marrow:
- Develops from red bone marrow as children mature.
- Fatty substance that can convert back to red bone marrow during severe anemia to facilitate additional erythrocyte production.
Red Bone Marrow:
- Site of blood cell production in adults, found in certain bones.

Composition of Bone: Composed of cells and extracellular matrix (osseous connective tissue).
Types of Cells in Bone:
1. Osteoprogenitor Cells: Stem cells that can differentiate into osteoblasts.
2. Osteoblasts: Cells that secrete bone matrix (osteoid).
3. Osteocytes: Mature bone cells that maintain the bone matrix.
4. Osteoclasts: Multinucleated cells involved in bone resorption.
Extracellular Matrix of Bone:
- Organic Components: Osteoid, collagen, and ground substance containing proteoglycans and glycoproteins that provide tensile strength.
- Inorganic Components: Hydroxyapatite crystals formed from calcium and phosphate.

Bone Formation:
- Begins with secretion of osteoid and occurs through calcification (mineralization).
- Requires Vitamin D (for calcium absorption) and Vitamin C (for collagen formation).
Bone Resorption:
- Bone matrix is degraded by substances released from osteoclasts, including proteolytic enzymes and hydrochloric acid.

Osteitis Deformans: Recognized by excessive bone resorption followed by excessive bone deposition, characterized by larger, more active osteoclasts leading to deformity and pain.

Chondrogenesis: Cartilage growth starts during embryologic development.
- Interstitial Growth: Growth in length occurring within the internal regions of cartilage.
- Appositional Growth: Growth in width occurring on the outer surface of cartilage.

Ossification (Osteogenesis):
- Formation of bone connective tissue.
- Begins in the embryo and continues through childhood and adolescence.

Bone growth occurs within a thin layer of mesenchyme (embryonic connective tissue) to form flat bones of the skull and mandible.
- Steps include the formation of ossification centers, secretion of osteoid, and its calcification leading to the formation of osteocytes.

Begins with a hyaline cartilage model and typically produces most bones of the skeleton (e.g., upper/lower limbs, pelvis, vertebrae).
- Comprises a series of steps including the formation of a cartilage model, ossification centers, and eventual replacement of cartilage with bone.

Rate of Growth: Lengthwise growth occurs within the epiphyseal plates featuring zones of cartilage:
1. Zone of Resting Cartilage
2. Zone of Proliferating Cartilage
3. Zone of Hypertrophic Cartilage
4. Zone of Calcified Cartilage
5. Zone of Ossification
Appositional Growth: Occurs within the periosteum where new bone matrix is deposited parallel to existing bone, resulting in increase in diameter.

Sex Hormones: Released at puberty (estrogen, testosterone) dramatically accelerate bone growth, increasing cartilage growth.
Glucocorticoids: Can impair growth at the epiphyseal plate leading to increased bone resorption and reduced growth in children.
Serotonin: May inhibit osteoblast differentiation when levels are too high, contributing to low bone density conditions.

Importance of Calcium: Essential for muscle contraction, neuron function, heart stimulation, and blood clotting.
Hormones Regulating Calcium:
- Calcitriol: Active form of Vitamin D that facilitates calcium absorption.
- Parathyroid Hormone (PTH): Released when blood calcium levels are low, works to raise calcium levels by:
  - Stimulating osteoclasts to release calcium from the bone.
  - Increasing calcium reabsorption in kidneys.
  - Enhancing intestinal absorption of calcium (via calcitriol).

Decreased Tensile Strength: Lower rates of protein synthesis lead to brittleness and increased susceptibility to fractures.
Osteoporosis: Characterized by reduced bone mass and density due to hormone level decline with age (e.g., Vitamin D, estrogen, testosterone).

Classification of Fractures: Types include:
- Avulsion
- Colles
- Comminuted
- Complete
- Compound (open)
- Compression
- Depressed
- Displaced
- Greenstick
- Hairline
- Impacted
- Linear
- Oblique
- Pathologic
- Pott
- Simple (closed)
- Spiral
- Stress

Process of Fracture Repair:
1. Formation of a hematoma.
2. Development of a fibrocartilaginous (soft) callus.
3. Formation of a bony (hard) callus.
4. Bone remodeling occurs, restoring original structure.