Chapter_6_Bone_and_Skeletal_Tissues

Chapter 6: Bones and Skeletal Tissue

Skeletal Cartilages

• Composition of Skeletal Cartilages

  • Human skeleton initially consists solely of cartilage, which is gradually replaced by bone.

  • Areas that require flexibility, such as the tip of the nose or joints, retain cartilage.

• Characteristics of Skeletal Cartilage

  • Made of resilient, molded cartilage tissue primarily composed of water.

  • Contains no blood vessels or nerves, relying on perichondrium for nutrients.

  • Contains chondrocytes encased in lacunae within an extracellular matrix.

Types of Cartilage

• Three Main Types of Cartilage

  1. Hyaline Cartilage

     • Provides support, flexibility, and resilience.

     • Most abundant type that contains only collagen fibers.

     • Located in articular joints, costal regions (ribs), respiratory structures (larynx), and the nose tip.

  2. Elastic Cartilage

     • Similar to hyaline but contains elastic fibers.

     • Found in the external ear and epiglottis.

  3. Fibrocartilage

     • Contains thick collagen fibers, providing great tensile strength.

     • Located in menisci of the knee and vertebral discs.

Growth of Cartilage

• Methods of Cartilage Growth

  • Appositional Growth

    • Cartilage-forming cells in the perichondrium secrete new matrix on the cartilage surface.

  • Interstitial Growth

    • Chondrocytes within lacunae divide and secrete new matrix from within.

• Calcification of Cartilage

  • Occurs during normal bone growth in youth; hardening can occur in old age but differs from bone.

Functions of Bones

• Seven Key Functions of Bones

  1. Support

     • Provides a framework for the body and supports soft organs.

  2. Protection

     • Guards vital structures such as the brain, spinal cord, and organs.

  3. Movement

     • Serves as levers for muscle action.

  4. Mineral and Growth Factor Storage

     • Reservoirs for calcium, phosphorus, and growth factors.

  5. Blood Cell Formation

     • Hematopoiesis occurs in red marrow within certain bones.

  6. Triglyceride Storage

     • Bone cavities store fat as an energy source.

  7. Hormone Production

     • Bone secretes osteocalcin, which regulates insulin and glucose levels.

Classification of Bones

• 206 Named Bones of the Human Skeleton

  • Bones classified into two main groups:

    • Axial Skeleton

      • Includes the skull, vertebral column, and rib cage.

    • Appendicular Skeleton

      • Consists of limb bones and girdles attaching limbs to the axial skeleton.

• Bone Shape Classifications

  1. Long Bones

     • Longer than wide, e.g., limb bones.

  2. Short Bones

     • Cube-shaped bones, such as those in wrists and ankles.

  3. Sesamoid Bones

     • Form within tendons (e.g., patella).

  4. Flat Bones

     • Thin and curved, e.g., sternum, scapulae.

  5. Irregular Bones

     • Complicated shapes, e.g., vertebrae and hip bones.

Bone Structure

• Types of Bone Tissue

  • Compact Bone

    • Dense outer layer appearing smooth and solid.

  • Spongy Bone

    • Contains trabecular structures and has open spaces filled with marrow.

• Membranes

  • Periosteum

    • Double-layered membrane covering external bone surfaces; contains osteogenic and fibrous layers.

  • Endosteum

    • Thin membrane covering internal surfaces and surfaces of trabecular bone.

Microscopic Anatomy of Bone

• Bone Cell Types

  • Osteogenic Cells

    • Stem cells located in periosteum and endosteum; differentiate into osteoblasts.

  • Osteoblasts

    • Bone-forming cells that secrete osteoid (unmineralized matrix).

  • Osteocytes

    • Mature bone cells occupying lacunae; maintain matrix and act as mechanosensors.

  • Bone Lining Cells

    • Flat cells on bone surfaces involved in maintenance of matrix.

  • Osteoclasts

    • Multinucleated cells responsible for bone resorption, breaking down bone matrix.

Chemical Composition of Bone

• Organic Components

  • Include cells and osteoid (collagen and ground substance) contributing to resilience and flexibility.

• Inorganic Components

  • Comprise hydroxyapatite (mineral salts), which provide hardness and resistance to compression.

Bone Development

• Ossification (Osteogenesis)

  • The formation of bony skeleton begins early in development and continues through adulthood.

• Endochondral Ossification

  • Process involves replacing hyaline cartilage with bone in developing skeleton.

• Intramembranous Ossification

  • Involves formation of bone directly from mesenchymal membranes, e.g., frontal and parietal bones.

Growth of Bones

• Lengthwise Growth

  • Achieved via interstitial growth of epiphyseal plates until epiphyses close in late adolescence.

• Thickness Growth

  • Achieved via appositional growth; bones thicken in response to stress.

Hormonal Regulation of Bone Growth

• Growth Hormone

  • Major hormone stimulating epiphyseal plate activity.

• Thyroid Hormone

  • Modulates growth hormone activity.

• Sex Hormones

  • Testosterone and estrogens stimulate growth spurts and induce epiphyseal closure.

Bone Remodeling

• Life Cycle of Bone

  • 5-7% of bone mass is recycled each week.

  • Spongy bone is replaced every 3-4 years; compact bone every 10 years.

• Regulation of Bone Remodeling

  • Controlled by genetic factors, hormonal signals, and mechanical stress.