Chapter 6: Bones and Bone Structure – Study Notes

Bones and Bone Structure

Major Functions of Bones

Bones are specialized connective tissues that serve several essential functions in the human body:

• Support: Bones provide a rigid framework that supports the body and cradles soft organs.

• Mineral and Lipid Storage: Bones store minerals, primarily calcium (about 39% of bone mass), and lipids in yellow bone marrow.

• Blood Cell Production: Hematopoiesis occurs in the red bone marrow found within the medullary cavities of long bones and the diploë of flat bones.

• Protection: Bones protect vital organs, such as the rib cage protecting the heart and lungs, and the skull protecting the brain.

• Leverage: Bones act as levers that muscles pull on to produce movement.

Classification of Bones by Shape

Bones are classified according to their shapes, which relate to their functions:

• Long Bones: Longer than they are wide (e.g., femur, humerus).

• Short Bones: Approximately equal in length and width (e.g., carpals).

• Flat Bones: Thin, flattened, and usually curved (e.g., sternum, skull bones).

• Irregular Bones: Complex shapes (e.g., vertebrae).

• Sesamoid Bones: Small, round bones embedded within tendons (e.g., patella).

Structure of Long Bones and Flat Bones

The structure of bones reflects their function and classification:

• Long Bones:

  • Diaphysis: The long shaft of the bone.

  • Epiphyses: The rounded ends of the bone.

  • Metaphysis: The region where the diaphysis and epiphysis meet.

  • Medullary Cavity: The hollow center containing bone marrow.

• Flat Bones:

  • Composed of two layers of compact bone with a middle layer of spongy bone called diploë.

Bone Cells

Bone tissue contains four main types of cells, each with distinct functions:

• Osteoprogenitor (Osteogenic) Cells: Stem cells that differentiate into osteoblasts; important for bone growth and repair.

• Osteoblasts: Cells that synthesize the bone matrix (osteoid) before it becomes calcified.

• Osteocytes: Mature bone cells derived from osteoblasts; reside in lacunae and maintain the bone matrix.

• Osteoclasts: Large, multinucleated cells from the macrophage lineage; break down bone matrix using acids and enzymes.

Structure of Compact Bone vs. Spongy (Trabecular) Bone

Bones consist of two types of osseous tissue:

• Compact Bone: Dense and solid; forms the outer layer of bones and provides strength for weight bearing.

• Spongy Bone (Trabecular): Consists of a network of trabeculae; found mainly in the epiphyses and inside flat bones, reducing bone weight while maintaining strength.

Periosteum and Endosteum

Bones are covered and lined by specialized membranes:

• Periosteum: A two-layered outer membrane:

  • Outer Layer: Dense irregular connective tissue; tendons anchor here via Sharpey's fibers.

  • Inner Layer: Cellular layer containing osteoblasts and osteoprogenitor cells.

• Endosteum: A thin, incomplete cellular layer lining the medullary cavity; contains osteoprogenitor cells, osteoblasts, and osteoclasts.

Ossification vs. Calcification

These terms describe processes involved in bone formation:

• Ossification: The process of replacing other tissues with bone; involves both the formation of osteoid and its subsequent calcification.

• Calcification: The deposition of calcium salts within a tissue; occurs during ossification but can also occur abnormally in other tissues.

Development of Bones

Bone formation occurs through two main processes:

• Endochondral Ossification: Bone develops by replacing hyaline cartilage; most bones form this way.

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue; forms flat bones like the skull and clavicle.

Bone Growth

Bones grow in length and diameter through distinct mechanisms:

• Interstitial Growth: Occurs at the epiphyseal plates, resulting in lengthening of long bones until the plates close at maturity.

• Appositional Growth: Increases bone diameter by adding new bone tissue at the surface; continues throughout life.

Regulation of Blood Calcium Levels

Blood calcium is tightly regulated by two hormones:

Hormone	Effect on Blood Calcium	Mechanisms
Parathyroid Hormone (PTH)	Increases	Stimulates osteoclast activity Increases intestinal absorption (via increased calcitriol) Decreases renal excretion
Calcitonin	Decreases	Inhibits osteoclast activity Decreases intestinal absorption Increases renal excretion

Repair of Bone Fractures

Bone fractures are classified and repaired through a series of steps:

• Types of Fractures:

  • Simple (Closed): Bone does not penetrate the skin.

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

• Steps of Fracture Repair:

  1. Formation of a fracture hematoma (blood clot)

  2. Formation of a fibrocartilaginous (soft) callus

  3. Formation of spongy bone (bony callus)

  4. Remodeling into compact bone

Factors Affecting Bone Mass and Density

Bone health is influenced by several factors:

• Hormones: Such as growth hormone, sex hormones, and thyroid hormones.

• Weight-Bearing Exercise: Stimulates bone formation and increases density.

• Nutrition: Adequate intake of calcium, vitamin D, and protein is essential.

Bone Pathologies Associated with Age

With aging, bones may lose mass and strength:

• Osteopenia: Reduced bone mass due to inadequate ossification; common with aging.

• Osteoporosis: Severe loss of bone density, making bones fragile and more susceptible to fractures.