Study Notes on Human Anatomy & Physiology: Bones and Bone Tissue

Human Anatomy & Physiology: Bones and Bone Tissue

Functions of the Skeletal System

  • The skeletal system performs various critical functions including:
      - Support: Provides structural support to the body and serves as a framework for muscle attachment.
      - Protection: Safeguards vital organs (e.g., rib cage protects the heart and lungs).
      - Movement: Works in conjunction with muscles to facilitate body movement through skeletal muscle attachment.
      - Mineral Storage: Stores essential minerals, particularly calcium and phosphorus, which can be released into the bloodstream as needed.
      - Blood Cell Production: Houses red bone marrow, which is responsible for the production of blood cells (hematopoiesis).
      - Fat Storage: Yellow bone marrow, found in long bones, serves as a reservoir for fat storage.

Bone Structure and Classification

  • Classification of Bones by Shape:
      - Long Bone: Longer than wide (e.g., Humerus).
      - Short Bone: About as long as it is wide (e.g., Trapezium, a carpal bone).
      - Flat Bone: Broad, flat, and thin (e.g., Sternum).
      - Irregular Bone: Shape does not fit into other classes (e.g., Vertebrae).
      - Sesamoid Bone: Round and flat, found within tendons (e.g., Patella).

Structure of Long Bones

  • Components:
      - Periosteum: Dense irregular connective tissue membrane rich in blood vessels and nerves; surrounds outer surface.
      - Perforating Fibers (Sharpey’s Fibers): Collagen fibers anchoring the periosteum to the underlying bone.
      - Diaphysis: Shaft of the long bone; ends are called epiphyses, which are covered with hyaline cartilage (articular cartilage) within joints.
      - Marrow Cavity: Hollow cavity in diaphysis containing red or yellow bone marrow, varying by individual age and bone type.
      - Compact Bone: Dense outer layer of bone that resists compression and twisting forces.
      - Spongy Bone (Cancellous Bone): Honeycomb-like structure inside cortical bone; resists forces from different directions and provides marrow cavities.
      - Endosteum: Membrane lining inside surfaces of bones, containing populations of bone cells.
      - Epiphyseal Lines: Separate epiphyses from diaphysis, remnants of epiphyseal plates in growing bones.

Structure of Other Bone Types

  • Short, Flat, Irregular, and Sesamoid Bones:
      - Lack diaphyses, epiphyses, and medullary cavities.
      - Covered by periosteum with associated blood vessels and nerves.
      - Composed of outer layers of compact bone with a middle layer of spongy bone (diploë); may contain sinuses.

Blood and Nerve Supply to Bone

  • Bones are well-vascularized and innervated:
      - Short, flat, irregular, and sesamoid bones primarily receive blood supply from periosteal vessels.
      - Long bones receive blood from periosteum and nutrient arteries, predominantly supplying compact bone. Nutrient foramen provides entry point.
      - Epiphyses also receive blood supply, mostly through smaller vessels.

Bone Marrow Types

  • Red Bone Marrow:
      - Loose connective tissue that supports blood-forming hematopoietic cells.
      - Decreases with age; present in adults in specific locations (e.g., pelvis, proximal femur/humerus).
  • Yellow Bone Marrow:
      - Composed of triglycerides, blood vessels, and adipocytes.

Microscopic Structure of Bone

  • Osseous Tissue Composition:
      - Mainly extracellular matrix with a few cells.
      - Inorganic Matrix: Makes up approximately 65% of bone's weight, primarily composed of hydroxyapatite crystals containing calcium and phosphorus.
      - Organic Matrix (Osteoid): Comprises collagen fibers and ECM components, making up the remaining 35% of bone structure.

Types of Bone Cells

  • Osteoblasts: Bone-building cells that secrete organic matrix.
  • Osteocytes: Mature bone cells housed in lacunae; maintain bone matrix and can recruit osteoblasts for reinforcement.
  • Osteoclasts: Responsible for bone resorption; break down matrix via secretion of hydrogen ions and enzymes.

Histology of Bone

  • Compact Bone vs. Spongy Bone:
      - Compact bone is dense, structured in osteons (Haversian systems), and withstands significant stress through lamellar organization.
      - Spongy bone is less dense, contains trabeculae, and provides a flexible framework, protecting bone marrow.

Ossification Process

  • Ossification Types:
      - Intramembranous Ossification: Primary bone formation within mesenchymal membranes, leading to flat bone development (e.g., skull).
      - Endochondral Ossification: Bone formation occurring within a hyaline cartilage model, applicable to all long bones except clavicles.

Growth in Length and Width

  • Longitudinal Growth: Growth in length occurs at the epiphyseal plate through the proliferation of chondrocytes, while closure of the epiphyseal plates signals the end of this growth phase.
  • Appositional Growth: Increase in thickness is due to osteoblast activity in the periosteum and remodeling of existing bone structures.

Role of Hormones in Bone Growth

  • Growth Hormone: Stimulates mitosis in epiphyseal plate and encourages appositional growth.
  • Testosterone: Enhances appositional growth, causing greater calcium deposition and accelerates epiphyseal plate closure.
  • Estrogen: Increases the rate of longitudinal growth and inhibits osteoclast activity, leading to differences in average heights between genders.

Bone Repair Mechanisms

  • Fracture Types:
      - Simple: Skin remains intact; Compound: Skin and tissues around fracture are damaged; Other types include spiral, compression, comminuted, greenstick, avulsion, and epiphyseal plate fractures.
Fracture Healing Steps:
  1. Hematoma formation to fill the fracture gap.
  2. Soft callus formation as fibroblasts and chondroblasts infiltrate hematoma.
  3. Hard callus formation where osteoblasts build new bone.
  4. Remodeling as primary bone is replaced with secondary bone over months.