In Depth Notes on Bones and Bone Structure

Bones and Bone Structure: Part 1

Function of the Skeletal System

  • The skeletal system consists of:

    • Bones of the skeleton

    • Cartilages

    • Ligaments

    • Connective tissues

Primary functions of the skeletal system:

  1. Support: Provides structural support for the entire body.

  2. Storage of minerals and lipids: Stores calcium and lipids.

  3. Blood cell production:

    • Produces red blood cells, white blood cells, and platelets.

  4. Protection: Protects vital organs (e.g., brain, heart).

  5. Leverage: Bones act as levers, facilitating movement when muscles contract.

Bone Classification

  • Classification of bones: Based on shape and structure.

Types of Bone Shapes:

  1. Sutural bones (Wormian bones):

    • Small, flat, irregularly shaped bones found between cranial bones.

    • Size varies among individuals.

  2. Irregular bones:

    • Have complex shapes (e.g., spinal vertebrae, pelvic bones).

  3. Short bones:

    • Boxy appearance (e.g., carpal bones in wrists, tarsal bones in ankles).

  4. Flat bones:

    • Thin with parallel surfaces (e.g., roof of skull, sternum, ribs, scapulae).

    • Provides protection and surface area for muscle attachment.

  5. Long bones:

    • Long and slender (e.g., femur, humerus).

    • Found in arms, legs, palms, fingers, toes.

  6. Sesamoid bones:

    • Small, round, flat bones developed within tendons (e.g., patella).

    • Location and number can vary between individuals.

Structure of Long Bone

  • Diaphysis (shaft):

    • Wall of compact bone with a central medullary cavity.

  • Epiphysis:

    • The wide part at each end, mostly spongy bone.

  • Metaphysis:

    • Region where diaphysis and epiphysis meet.

Structure of Flat Bones

  • Flat bones consist of:

    • Spongy bone between two layers of compact bone (cortex).

    • In the cranium, the spongy bone layer is referred to as diploë.

Bones and Bone Structure: Part 2

Bone Tissue

  • Bone tissue is a dense, supportive connective tissue that includes specialized cells and a solid extracellular matrix with collagen fibers.

Anatomy of Bone Matrix

  • Minerals:

    • Two-thirds of bone matrix is calcium phosphate.

    • Reacts with calcium hydroxide to form hydroxyapatite, which incorporates calcium carbonates and ions like magnesium. Hydroxyapatite is responsible for dense, solid structure of bone.

  • Matrix proteins:

    • Approximately one-third of bone mass is collagen fibers.

Bone Cells

  • Comprise only 2% of bone mass, classified into four types:

    1. Osteogenic cells:

    • Mesenchymal stem cells (cells that divide and multiply) that differentiate into osteoblasts; located in periosteum and endosteum; assist in fracture repair. Cannot be osteocytes.

    1. Osteoblasts:

    • Immature cells responsible for producing new bone matrix during osteogenesis (process of creating); matrix produced is called osteoid. Building blocks of bone.

    1. Osteocytes:

    • Mature bone cells that maintain bone matrix and do not divide; reside in lacunae between matrix layers. Bone tissue is majorly made up of calcium.

    1. Osteoclasts:

    • Large multinucleate cells that absorb and remove bone matrix; involved in regulating calcium ions in homeostasis.

Bones and Bone Structure: Part 3

Compact Bone and Spongy Bone Structure

Structure of Compact Bone:

  • Osteon: Functional unit of compact bone consisting of concentric lamellae surrounding a central canal that contains blood vessels.

    • Lacunae: Small cavities housing osteocytes.

    • Canaliculi: Small canals connecting lacunae for nutrient exchange.

    • Central Canal: Contains blood vessel(s) running parallel to the bone surface.

Structure of Spongy Bone:

  • Lacks osteons; forms an open network of trabeculae filled with red bone marrow that forms blood cells and provides nutrient diffusion to osteocytes.

Endosteum:

  • An incomplete cellular layer lining the medullary cavity; active during growth, repair, and remodeling.

Periosteum:

  • Membrane covering bone (except at joint capsules); consists of outer fibrous and inner cellular layers; participates in growth and repair of bone.

Bones and Bone Structure: Part 4

Ossification: Bone Formation

  • Ossification: The process of bone formation.

  • Two forms of ossification:

    1. Endochondral ossification: Bone replaces existing cartilage.

    2. Intramembranous ossification: Bone develops from mesenchyme or fibrous connective tissue.

    3. Difference between 2 processes is that endochondral ossification has cartilage being replaces, while intramembranous ossification does not include that step.

  • Bone growth initiates around six weeks after fertilization and continues into late adolescence, completing around age 25.

Interstitial Growth:

  • Bone grows in length through activity at the epiphyseal cartilage (plate).

    • Appositional Growth: Increases bone width through circumferential lamellae deposition by osteoblasts.

Hormonal Regulation:

  • Growth hormone and thyroxine promote bone growth; estrogen and testosterone stimulate osteoblast activity.

Bones and Bone Structure: Part 5

Bone Remodeling

  • Bone remodeling occurs throughout life; involves a balance of bone resorption by osteoclasts and deposition by osteoblasts.

  • Effects of exercise:

    • Promotes thicker and stronger bones; lack of activity can lead to rapid bone loss.

Nutritional Effects on Bone

  • Key minerals: Calcium, phosphorus, and minor ones like magnesium and fluoride.

  • Vitamins A, C, D, K, and B12 play roles in bone health; Vitamin C aids in collagen synthesis.

Fractures and Fracture Repair

  • Fractures can be classified as open or closed; types include transverse, compression, spiral, and greenstick.

  • Fracture Repair Steps:

    1. Formation of fracture hematoma.

    2. Callus formation stabilizes the break.

    3. Spongy bone formation replaces the callus with spongy bone.

    4. Compact bone formation remodels the area, potentially making it thicker and stronger than before.