Human Anatomy and Physiology Chapter 06 - Bones and Skeletal Tissues

Human Anatomy and Physiology - Bones and Skeletal Tissues: Chapter 06 Notes

6.1 Skeletal Cartilages

  • Initial Skeleton Composition: The human skeleton starts as cartilage which is eventually replaced by bone, except in areas requiring flexibility.

Basic Structure, Types, and Locations of Skeletal Cartilage

  • Skeletal Cartilage: Composed of highly resilient molded cartilage tissue primarily consisting of water.

    • Characteristics:

    • Lacks blood vessels and nerves.

  • Perichondrium: A dense connective tissue layer surrounding cartilage.

    • Functions:

    • Resists outward expansion.

    • Contains blood vessels for nutrient delivery to cartilage.

  • Cell Composition:

    • Chondrocytes: Specialized cells encased in small cavities (lacunae) within a jelly-like extracellular matrix.

Types of Cartilage

  1. Hyaline Cartilage:

    • Function: Provides support, flexibility, and resilience.

    • Abundance: Most abundant cartilage; matrix contains only collagen fibers.

    • Locations: Found in articular joints, costal areas (ribs), respiratory structure (larynx), and nasal structure (tip of nose).

  2. Elastic Cartilage:

    • Similar to hyaline but contains elastic fibers.

    • Locations: Found only in the external ear and epiglottis (the flap that covers the larynx).

  3. Fibrocartilage:

    • Contains thick collagen fibers, providing great tensile strength.

    • Locations: Found in menisci of the knee and intervertebral discs.

Growth of Cartilage

  • Methods: Cartilage grows in two ways:

    1. Appositional Growth:

    • Cartilage-forming cells in the perichondrium secrete matrix against the external face of existing cartilage.

    • New matrix forms on the surface.

    1. Interstitial Growth:

    • Chondrocytes in lacunae divide and secrete new matrix, expanding cartilage from within.

6.2 Functions of Bones

  • Seven Important Functions:

    1. Support: Provides structural support for body and soft organs.

    2. Protection: Shields vital organs such as the brain and spinal cord.

    3. Movement/Anchorage: Serves as levers for muscle action and movement.

    4. Mineral and Growth Factor Storage: Reservoir for calcium, phosphorus, and other growth factors.

    5. Blood Cell Formation: Hematopoiesis occurs in red marrow cavities of certain bones.

    6. Triglyceride Storage: Fat stored in bone cavities serves as an energy source.

    7. Hormone Production: Bones secrete osteocalcin that helps regulate insulin secretion, glucose levels, and metabolism.

6.3 Classification of Bones

  • Total Bones in the Human Skeleton: 206 named bones.

Division Based on Location

  1. Axial Skeleton:

    • Comprises the long axis of the body: skull, vertebral column, rib cage.

  2. Appendicular Skeleton:

    • Includes bones of upper and lower limbs along with girdles (shoulder and hip) attaching limbs to the axial skeleton.

Classification by Shape

  1. Long Bones:

    • Longer than they are wide (e.g., limb bones).

  2. Short Bones:

    • Cube-shaped bones found in wrists and ankles; includes sesamoid bones (e.g., patella).

    • Variation: Size and number can vary among individuals.

  3. Flat Bones:

    • Thin, flat, slightly curved (e.g., sternum, scapulae, most skull bones).

  4. Irregular Bones:

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

6.4 Bone Structure

  • Bone as an Organ: Contains different tissue types, primarily osseous tissue but also includes nervous tissue, cartilage, fibrous connective tissue, muscle cells, and epithelial cells in its blood vessels.

Three Levels of Structure

  1. Gross Anatomy: This includes examination of bones in their entirety.

  2. Microscopic Anatomy: Involves examining bones through an optical microscope.

  3. Chemical Composition: Analyzes bone at the molecular level.

Types of Bone Tissue

  1. Compact Bone:

    • Dense outer layer that appears smooth and solid.

  2. Spongy Bone:

    • Composed of a network of small, needle-like or flat pieces of bone, referred to as trabeculae; open spaces filled with red or yellow bone marrow.

Structures of Short, Irregular and Flat Bones

  • Comprise thin plates of spongy bone (diploë) covered by compact bone.

  • Connective Tissue Membranes: Periosteum protects outside, and endosteum lines internal portions of compact bone.

  • Bone marrow is scattered throughout and does not have a defined cavity.

  • Hyaline cartilage covers portions of movable joints.

Structure of Long Bone

  • Includes:

    • Diaphysis: Tubular shaft forming long axis, mainly made of compact bone surrounding medullary cavity (often filled with yellow marrow in adults).

    • Epiphyses: Ends of long bones consisting of compact bone externally and spongy bone internally, with articular cartilage covering joint surfaces.

  • Between diaphysis and epiphysis lies the epiphyseal line, a remnant of the growth plate from childhood.

6.5 Bone Development

  • Ossification (Osteogenesis): The process of forming bone tissue, starting in utero at approximately 2 months of development and continuing postnatally into early adulthood.

Types of Ossification

  1. Endochondral Ossification:

    • Bone forms by replacing hyaline cartilage. Involves primary ossification centers arising in the center of the shaft.

    • Essential for forming most of the skeleton below the base of the skull (except clavicles).

  2. Intramembranous Ossification:

    • Bone formation from fibrous membranes, relevant for cranial bones (e.g., frontal, parietal, occipital, temporal) and clavicles.

Endochondral Ossification Steps

  1. Formation of Bone Collar: Around diaphysis of cartilage model.

  2. Central Cartilage Calcification: Cartilage matrix calcifies and develops cavities.

  3. Periosteal Bud Invasion: This bud contains blood vessels, nerves, and specialized cells and it leads to spongy bone formation.

  4. Diaphysis Elongation and Medullary Cavity Formation: Secondary ossification centers form in epiphyses.

  5. Epiphyses Ossification: Hyaline cartilage remains only in epiphyseal plates and articular cartilages.

Intramembranous Ossification Steps

  1. Ossification Center Formation: Mesenchymal cells cluster and differentiate into osteoblasts.

  2. Osteoid Secretion and Calcification: Osteoid is secreted and subsequently calcified, trapping osteoblasts as osteocytes.

  3. Formation of Immature Spongy Bone and Periosteum: Vascularized mesenchyme condenses onto external face.

  4. Mature spongy bone formation: Trabeculae remodel into mature spongy bone filled with red marrow.

6.6 Bone Remodeling

  • Dynamics: About 5–10% of skeleton is replaced annually; spongy bone replaced every 3-4 years, compact bone every 10 years.

    • Consists of both bone deposition (by osteoblasts) and resorption (by osteoclasts).

Bone Resorption Process

  • Osteoclastic Activity: Ossification involves digging depressions, secreting enzymes to digest the matrix, converting calcium salts for resorption, and apoptosis of osteoclasts post-resorption.

Bone Deposition Process

  • Formation of Osteoid Seam: The unmineralized bone matrix marked by a calcification front where osteoid transitions into mineralized bone.

Control of Remodeling

  • Regulatory Mechanisms:

    • Hormonal Controls: Involves a negative feedback loop affecting blood Ca2+ levels (important for many bodily functions).

    • Mechanical Stress Response: Bones adapt and remodel according to mechanical stress (Wolf's Law).

6.7 Bone Repair

  • Fracture Types: Different methods of fracture classification based on positioning, completeness, and skin penetration.

Stages of Repair

  1. Hematoma Formation: Clotted blood forms at the fracture site, leading to inflammation.

  2. Fibrocartilaginous Callus Formation: Connective tissue spans the break; spongy bone starts forming within the cartilage matrix.

  3. Bony Callus Formation: Trabeculae appear in fibrocartilaginous callus and form bony callus from the outer spongy bone.

  4. Bone Remodeling: Excess materials are removed, and compact bone reconstructs the shaft, returning to its original form.

6.8 Bone Disorders

  • Major Bone Diseases:

    • Osteomalacia and Rickets: Poorly mineralized bones leading to deformities in children (vitamin D deficiency).

    • Osteoporosis: Imbalance of deposition and resorption resulting in decreased bone mass, particularly in women post-menopause.

Osteoporosis Risk Factors

  • Increased age, insufficient exercise, poor diet, smoking, certain medications, and hormone-related conditions.

Treatments and Prevention

  • Traditional treatments include calcium and vitamin D supplements, weight-training, and hormone replacement therapy; prevention involves early dietary calcium intake and regular physical activity.