Lecture 5 Notes on Bone Structure and OIOS Tissue

Chapter 5: OIOS Tissue and Bone Structure

Introduction to Bone Structure

  • Welcome to Chapter Five, Lecture Five: the OIOS tissue and bone structure.
  • Overview of the lecture:
    • Components of the bone
    • Types of cells in bone
    • Bone growth
    • Aging issues related to bones
  • Reminder: Engage with material, ask questions if needed, and utilize office hours.

Overview of the Skeletal System

  • The skeletal system is critical for structure and function in the body.
  • Common misconceptions:
    • Bones are often perceived as static like fossils.
    • Bones are dynamic organs that remodel and adapt.
  • Composition of the skeletal system:
    • Composed of various tissue types:
    • Skeletal bones
    • Cartilage
    • Ligaments
    • Connective tissues
  • Two major divisions of the skeleton:
    1. Axial Skeleton:
    • Forms the central axis of the body.
    • Includes:
      • Skull and associated bones (auditory ossicles, hyoid bones)
      • Thoracic cage: sternum and ribs
      • Vertebral column: vertebrae, sacrum, and coccyx
    1. Appendicular Skeleton:
    • Comprises appendages attached to the axial skeleton.
    • Includes:
      • Pectoral girdle (clavicle and scapula)
      • Upper limbs (humerus, radius, ulna, carpals, metacarpals, phalanges)
      • Pelvic girdle (hip bones)
      • Lower limbs (femur, patella, tibia, fibula, tarsals, metatarsals, phalanges)

Functions of the Skeletal System

  • Beyond providing a framework, bones serve many functions:
    • Framework for soft tissue attachment, holds body upright, gives shape.
    • Reservoir for essential minerals, primarily calcium (98% in bones) and phosphate.
    • Blood cell production in bone marrow:
    • Erythrocytes (red blood cells)
    • Leukocytes (white blood cells)
    • Platelets
  • Muscles pull on bones for movement:
    • Important reminder: Muscles pull; they do not push.
  • Protective functions:
    • Ribs protect the heart and lungs.
    • Skull protects the brain.
    • Vertebrae protect the spinal cord.
    • Pelvic girdle protects reproductive organs.

OIOS Tissue: Bone Tissue

  • OIOS tissue is a specialized type of supporting connective tissue.
  • Composition:
    • Mainly composed of a matrix and cells:
    • Matrix:
      • Non-living components giving bones unique properties.
      • Comprises organic and inorganic compounds:
      • Calcium salts (approx. 23% of bone mass): provide rigidity, resist compression.
        • Inflexible; weak against twisting forces.
      • Collagen fibers (approx. 1/3 of bone mass): flexible and tough, provides tensile strength against shattering.
        • Not compressible.
    • Bone Cells: 2% of bone mass, crucial for growth, maintenance, and repair.

Types of Bone Cells

  • Osteocytes:
    • Mature bone cells trapped within the bone matrix.
    • Reside in lacunae (means "little lake").
    • Functions:
    • Maintain bone matrix.
    • Detect mechanical stress in bone.
    • Canaliculi connect osteocytes to blood capillaries for nutrient and waste exchange.
  • Osteoblasts:
    • Immature bone cells actively involved in osteogenesis (bone formation).
    • Secrete osteoid, the organic matter of the bone matrix, which later becomes calcified.
    • Located on inner and outer surfaces of bones; referred to as "bone builders."
  • Osteoprogenitor Cells:
    • Stem cells in the inner layer of the periosteum and endosteum.
    • Divide to produce osteoblasts, ensuring a supply of bone-building cells.
  • Osteoclasts:
    • Large, multinucleated cells performing osteolysis (bone breakdown).
    • Secrete acids and enzymes to reabsorb bone, releasing calcium and phosphate into the bloodstream.
    • Referred to as "bone destroyers" or "remodelers."

Types of Bone Tissue: Compact Bone vs. Spongy Bone

  • Compact Bone:
    • Dense and solid tissue forming the outer layer of bones.
    • Heavier and provides strength against forces parallel to bone axis.
  • Spongy Bone (Cancellous Bone):
    • Open network of plates surrounding the medullary cavity (contains bone marrow).
    • Lighter and better at resisting multidirectional forces due to its network-like structure.
Structural Differences
  • Compact Bone:
    • Functional unit: Osteon (Haversian system).
    • Contains:
    • Central Canal (Haversian Canal): longitudinal channel with blood vessels and nerves.
    • Canaliculi: tiny canals radiating from lacunae, connecting osteocytes to central canal.
    • Laminae: concentric layers of bone matrix around central canal; collagen fibers oriented in different directions (resists twisting).
    • Perforating Canals (Volkmann's canals): connect adjacent central canals and link to periosteum and medullary cavity.
  • Spongy Bone:
    • No osteons; forms an open network of branching plates called Trabeculae.
    • Trabeculae organized along lines of stress; osteocytes are found in lacunae with canaliculi extending to trabecular surfaces, connecting to the vascular system within the marrow.

Hyaline Cartilage: A Supporting Connective Tissue

  • Most common cartilage type in the body.
  • Matrix:
    • Translucent with closely packed, non-prominent collagen fibers.
  • Cartilage cells: Chondrocytes live in lacunae within the matrix.
  • Locations of Hyaline Cartilage:
    • Between tips of ribs and sternum.
    • Covering bone surfaces at synovial joints (articular cartilage).
    • Supporting structures: larynx, trachea, bronchi, part of nasal septum.
  • Function:
    • Provides stiff but flexible support and reduces friction at joints.

Functional Differences between Bone Types

  • Compact Bone: Forms the strong outer layer, resisting longitudinal forces.
  • Spongy Bone: Provides lightweight structural integrity and is arranged to withstand stresses from multiple directions.

Gross Anatomy of Long Bones (e.g., Femur)

  • Composed of three sections:
    1. Diaphysis:
    • Shaft composed mainly of compact bone surrounding the medullary cavity (yellow marrow).
    1. Epiphysis:
    • Ends of the bone containing spongy bone and red marrow (blood cell production).
    1. Metaphysis:
    • Narrow region connecting diaphysis to epiphysis, containing the epiphyseal plate (growth plate).

Bone Coverings

  • Periosteum:
    • Outer fibrous layer covering most bone surfaces.
    • Composed of:
    • Fibrous layer: dense connective tissue for strong tendon and ligament attachment.
    • Osteogenic layer: contains osteoprogenitor cells for growth and repair.
  • Endosteum:
    • Incomplete cellular layer lining the medullary cavity and central canals.
    • One cell layer thick, contains osteoprogenitor cells, osteoblasts, and osteoclasts.

Bone Development and Growth

  • The human skeleton is predominantly hyaline cartilage before 8 weeks of development.
  • Ossification:
    • Process where fibrous tissue and/or cartilage converts to bone cells.
    • Two types of ossification:
    1. Intramembranous Ossification:
      • Develops directly from fibrous connective tissue.
      • Forms flat bones (skull, mandible, and clavicle).
    2. Endochondral Ossification:
      • Develops from a cartilage model.
      • Forms long bones and others in the body.
  • Osteogenesis: General term for bone formation.
  • Calcification: Specific process involving calcium deposition, hardening the matrix.
Steps of Intramembranous Ossification
  • Step 1: Differentiation of osteoblasts from mesenchymal cells, creating an ossification center in vascularized fibrous membrane.
  • Step 2: Formation of bony spicules by osteoblasts secreting osteoid; spicules grow outwards.
  • Step 3: Entrapment of blood vessels within bony spicules.
  • Step 4: Formation of spongy bone as spicules interconnect, creating a complex network.
Steps of Endochondral Ossification
  • Step 1: Enlargement of cartilage and calcification; chondrocytes enlarge, matrix calcifies, leading to cavities.
  • Step 2: Formation of bone collar; perichondrium differentiates into periosteum and creates a bone collar around the diaphysis.
  • Step 3: Primary ossification center forms as blood vessels invade and osteoblasts replace calcified cartilage with spongy bone.
  • Step 4: Formation of the medullary cavity and increased diameter by osteoclasts.
  • Step 5: Secondary ossification centers develop in the epiphyses.
  • Step 6: Epiphyseal closure; the epiphyseal plate is replaced by bone, ceasing growth.
  • Step 7: Formation of the epiphyseal line and articular cartilage remains at joints.

Factors Affecting Bone Growth

  • Minerals:
    • Calcium, phosphate, magnesium, and fluoride are key for growth.
  • Vitamins:
    • Vitamin A stimulates osteoblast activity.
    • Vitamin C crucial for collagen synthesis and osteoblast differentiation.
    • Vitamin D essential for calcium and phosphate absorption.
  • Hormones:
    • Parathyroid hormone (PTH) increases blood calcium.
    • Calcitonin decreases blood calcium by promoting bone deposition.
    • Sex hormones (estrogen and testosterone) promote osteoblast activity and rapid bone growth during puberty.

Bone Remodeling

  • Bones are constantly recycled to remain strong and adapt to stress.
  • Increased muscular development leads to bone remodeling, increasing thickness where stress is applied.
  • Principle: Use it or lose it.
    • Lack of stress decreases osteoblast activity while osteoclast activity may increase, leading to bone mass loss.
    • Example: Astronauts experience significant bone mass loss in space due to inactivity.

Fracture and Repair of Bone

  • Recovery steps:
    1. Hematoma formation: blood clot forms at the fracture site.
    2. Callus formation: external and internal calluses stabilize the fracture.
    3. Replacement of calluses with bone: osteoblasts replace cartilage and calluses with bone.
    4. Remodeling: Excess bone is reabsorbed over time, restoring original bone shape.

Aging and Bone Density

  • In youth, balanced remodeling occurs (osteoblast vs. osteoclast).
  • With aging:
    • Decrease in osteoblast activity leads to more bone breakdown than formation.
    • Resulting condition: Osteoporosis - loss of bone mass and density, increasing fracture risks.
  • Personal note on the impact of osteoporosis, involving family experiences of compression fractures as a result of this condition.

Conclusion

  • Recap of Lecture Five: understanding OIOS tissue, bone structure, and the complex processes involved in growth and repair of bone.
  • Encouragement to ask questions and explore further in next lectures.