Skeletal System: Structure, Function, and Physiology

Chapter 6: Skeletal System

6.1 Functions of the Skeletal System

The skeletal system performs several essential functions that support the body and maintain homeostasis. Understanding these functions is foundational to the study of anatomy and physiology.

• Body support: Provides the structural framework for the body.

• Organ protection: Shields vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Body movement/leverage: Serves as levers for muscles to produce movement.

• Mineral storage: Stores minerals such as calcium and phosphorus, releasing them as needed.?

• Blood cell production: Houses bone marrow, which produces blood cells (hematopoiesis).

6.2 Cartilage

Cartilage is a specialized, supporting connective tissue that plays a crucial role in the skeletal system, especially during development and at joints.

• Matrix: Contains collagen and proteoglycans, providing strength and flexibility.

• Cells:

  • Chondroblasts: Produce new cartilage matrix.

  • Chondrocytes: Mature cartilage cells that maintain the matrix.

• Perichondrium: Dense connective tissue membrane surrounding cartilage (except at joints).

Example: Articular cartilage covers the ends of bones at synovial joints, reducing friction and absorbing shock.

6.3 Bone Histology

Bone is a dynamic connective tissue composed of specialized cells embedded in a mineralized Its structure allows for both strength and flexibility.

• Matrix:

  • Protein fibers: Mainly collagen, providing tensile strength.

  • Hydroxyapatite: Crystalline calcium phosphate, giving bone its hardness.

• Specialized cells:

  • Osteoblasts: Bone-forming cells.

  • Osteocytes: Mature bone cells maintaining bone tissue. An

  • Osteoclasts: Bone-resorbing cells.

  • Bone tissue types (formed during ossification):

    • Woven bone: Immature, rapidly formed bone with irregular collagen arrangement.

    • Lamellar bone: Mature bone with organized collagen fibers.

• Bone tissue in formed bone:

  • Compact bone: Dense outer layer; contains osteons (Haversian systems) with concentric lamellae, lacunae, canaliculi, and central canals.

  • Spongy bone: Network of trabeculae; spaces filled with marrow.

Example: The femur's shaft is primarily compact bone, while its ends contain spongy bone.

6.4 Bone Anatomy

Bones have distinct anatomical features that relate to their function and development. The long bone serves as a model for understanding bone structure.

• Diaphysis: Shaft of a long bone; contains the medullary cavity (yellow or red marrow).

• Epiphysis: Ends of the bone; contains articular cartilage, epiphyseal plate (growth plate), and red marrow.

• Periosteum: Dense connective tissue covering the bone's outer surface.

• Endosteum: Thin membrane lining the medullary cavity.

• Bone shapes: Long, flat, short, and irregular.

Example: The humerus is a long bone, while the vertebrae are irregular bones.

6.5 Bone Development

Bone development (ossification) begins around the 5th week of embryonic development and continues through adolescence. Two main processes form bone tissue.

• Intramembranous ossification: Forms flat bones (e.g., skull, clavicle).

  1. Osteoblast formation

  2. Spongy bone formation

  3. Compact bone formation and red bone marrow formation

• Endochondral ossification: Forms most bones of the body (e.g., long bones).

  1. Cartilage model formation (surrounded by perichondrium)

  2. Bone collar formation

  3. Primary ossification center formation in diaphysis

  4. Secondary ossification center formation in epiphysis

Equation: $\text{Bone Growth} = \text{Ossification Rate} - \text{Resorption Rate}$

6.6 Bone Growth

Bone growth occurs in length and diameter, primarily at the epiphyseal plates in long bones. Growth is regulated by hormones and mechanical factors.

• Ossification moves away from the center of the diaphysis.

• New bone forms at the epiphyseal plate through endochondral ossification.

• Cartilage is produced on the epiphyseal side; ossification occurs on the diaphyseal side.

• As the rate of new cartilage production equals ossification, bone length increases.

• At puberty, sex hormones accelerate ossification, eventually closing the epiphyseal plate.

• When the epiphyseal plate is replaced by bone, growth in length stops.

Factors affecting bone growth: Nutrition (calcium, vitamin D), hormones (growth hormone, thyroid hormone, sex hormones), and physical activity.

6.7 Bone Remodeling

Bone remodeling is a continuous process where old bone is replaced by new bone tissue. This process maintains bone strength and mineral homeostasis.

• Involves coordinated activity of osteoclasts (resorption) and osteoblasts (formation).

• Allows bones to adapt to stress and repair microdamage.

• Regulated by mechanical stress, hormones, and calcium levels.

6.8 Bone Fracture Classification and Repair

Bone fractures are classified by their cause, pattern, and the arrangement of bone fragments. Understanding these classifications aids in diagnosis and treatment.

• Mechanism of fracture: Soft-tissue damage, displaced vs. nondisplaced.

• Fracture pattern: Linear, spiral, oblique, stress (hairline), compression.

• Number and arrangement of fragments: Incomplete, complete, comminuted.

• Age-specific: Greenstick (children), epiphyseal (growth plate).

R Fracture Type	Description
Linear	Parallel to bone's long axis
Spiral	Twisting force causes spiral pattern
Comminuted	Bone breaks into several pieces
Greenstick	Partial fracture, common in children
Epiphyseal	Involves growth plate
Compression	Bone is crushed

Bone repair steps:

1. Hematoma formation

2. Callus formation

3. Callus ossification

4. Bone remodeling

6.9 Calcium Homeostasis

Calcium homeostasis is vital for many physiological processes. The skeletal system acts as a reservoir for calcium, regulated by hormones through negative feedback mechanisms.

• Parathyroid Hormone (PTH):

  • Direct effects: Stimulates osteoclasts to increase bone resorption, raising blood calcium. L

  • Indirect effects: Increases calcium reabsorption in kidneys and activates vitamin D for increased intestinal absorption.

• Calcitonin: Lowers blood calcium by inhibiting osteoclast activity and promoting calcium deposition in bone.

Equation (Negative Feedback): $\text{If } [Ca^{2+}]_{blood} \downarrow \rightarrow \text{PTH} \uparrow \rightarrow \text{Bone resorption} \uparrow \rightarrow [Ca^{2+}]_{blood} \uparrow$ $\text{If } [Ca^{2+}]_{blood} \uparrow \rightarrow \text{Calcitonin} \uparrow \rightarrow \text{Bone deposition} \uparrow \rightarrow [Ca^{2+}]_{blood} \downarrow$