Chapter 6: Bones and Bone Structure

Introduction to the Skeletal System

The skeletal system forms the structural framework of the human body, providing support, protection, and facilitating movement. It consists of bones, cartilage, ligaments, and other connective tissues.

• Support: Bones provide a rigid structure for the body and serve as attachment points for muscles.

• Storage of Minerals and Lipids: Bones store essential minerals such as calcium (Ca2+) and phosphate (PO43-), as well as lipids in yellow bone marrow.

• Blood Cell Production: Red bone marrow produces red blood cells (RBCs), white blood cells (WBCs), and platelets.

• Protection: Bones protect vital organs (e.g., skull protects the brain, ribs protect the lungs and heart).

• Leverage: Bones act as levers to change the magnitude and direction of forces generated by muscles.

• Energy Storage: Yellow bone marrow stores fat as an energy reserve.

Classification of Bones by Shape

Bones are classified according to their shapes, which relate to their functions and locations in the body.

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

• Flat Bones: Thin and broad; provide protection (e.g., cranial bones, sternum).

• Sutural Bones: Small, irregular bones found within cranial sutures.

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

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

• Short Bones: Boxlike in appearance; found in wrists and ankles (e.g., carpals, tarsals).

Structure of Bones

Bones have a complex structure that supports their functions. The structure varies between long, short, flat, and irregular bones.

• Long Bones:

  • Diaphysis: Shaft of the bone; composed of compact bone surrounding the medullary cavity.

  • Epiphysis: Ends of the bone; contain spongy bone and red marrow.

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

  • Medullary Cavity: Central cavity containing yellow bone marrow in adults.

  • Periosteum: Outer fibrous layer covering the bone; contains nerves, blood vessels, and bone cells.

  • Endosteum: Thin membrane lining the medullary cavity; contains bone cells involved in growth and repair.

• Short, Flat, and Irregular Bones:

  • Composed of a thin layer of compact bone surrounding spongy bone.

  • Spongy bone contains red marrow and is covered by periosteum.

Microscopic Structure of Bone

Bones are composed of both compact and spongy tissue, each with distinct microscopic features.

• Compact Bone:

  • Contains osteons (Haversian systems) as the structural unit.

  • Osteons consist of concentric lamellae surrounding a central canal (Haversian canal) with blood vessels and nerves.

  • Perforating (Volkmann's) canals connect osteons and provide pathways for blood vessels.

• Spongy Bone:

  • Composed of trabeculae (meshwork of bony struts) that provide structural support and house marrow.

  • Trabeculae are oriented to withstand stress from multiple directions.

Histology of Bone

Bones are made up of both inorganic and organic components, contributing to their strength and flexibility.

• Inorganic Component: Crystals of hydroxyapatite (), along with sodium, magnesium, and fluoride. These crystals are hard and brittle, making up about 55% of bone weight.

• Organic Component: Collagen fibers (about 30% of bone weight) and osteoid (90% protein, 10% sugar compounds). Collagen provides tensile strength and flexibility.

Bone Cell Types

Bone tissue contains several specialized cell types, each with distinct functions.

• Osteoprogenitor Cells: Stem cells found in the periosteum and endosteum; divide to produce osteoblasts and are important for bone growth and repair.

• Osteoblasts: Immature bone cells that secrete osteoid (organic matrix) and promote mineralization. They are responsible for bone formation.

• Osteocytes: Mature bone cells located in lacunae; maintain bone tissue, regulate mineral content, and repair microfractures.

• Osteoclasts: Large, multinucleated cells derived from white blood cell lineage; responsible for bone resorption and regulation of mineral homeostasis. They secrete acids and proteolytic enzymes to break down bone matrix.

Summary Table: Bone Cell Types

Bone Membranes

Bones are covered and lined by specialized membranes that support growth, repair, and provide routes for blood and nerve supply.

• Periosteum: Outer fibrous layer and inner cellular layer; contains fibroblasts, osteoprogenitors, and osteoblasts. Functions include isolation from surrounding tissue, route for circulatory and nervous supply, and participation in bone growth and repair.

• Endosteum: Incomplete cellular layer lining the medullary cavity; contains osteoblasts, osteoprogenitor cells, and osteoclasts. Functions in bone growth, repair, and remodeling.

Bone Development and Growth

Bone formation is a lifelong process that begins in the embryo and continues through childhood and adulthood.

• Embryonic Development:

  • Endochondral Ossification: Bone replaces a hyaline cartilage model; forms most bones of the body.

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

• Childhood: Interstitial (lengthwise) and appositional (widthwise) growth.

• Adulthood: Bone remodeling and repair.

Intramembranous Ossification Steps

1. Mesenchymal cells cluster and differentiate into osteogenic cells, then osteoblasts.

2. Osteoblasts secrete osteoid, which becomes mineralized.

3. Osteoblasts deposit minerals from the blood, forming bony spicules.

4. Spicules grow together, trapping blood vessels and forming trabeculae (spongy bone).

5. Spongy bone is remodeled to produce compact bone at the edges.

6. Mesenchyme at the periphery develops into the periosteum.

Endochondral Ossification Steps

1. Hyaline cartilage model forms in the embryo.

2. Osteoblasts replace chondrocytes, forming bone tissue.

3. Bone growth continues at the epiphyseal plates until adulthood.

Calcification vs. Ossification

• Ossification: The process of laying down new bone material, including osteocytes, calcium salts, and osteoid.

• Calcification: The formation of calcium salts and crystals within tissue; a step within ossification but can occur in other tissues as well.

• Key Difference: All ossification involves calcification, but not all calcification is ossification.

Example: Bone Formation in the Skull

Flat bones of the skull are formed by intramembranous ossification, where mesenchymal cells differentiate directly into osteoblasts, producing bone without a cartilage precursor.

Additional info: Some details about bone cell signaling, repair mechanisms, and the role of hormones in bone growth were inferred from standard anatomy and physiology knowledge to provide a complete study guide.