Chapter 6: The Skeletal System - Bone Tissue

Principles of Anatomy and Physiology: The Skeletal System - Bone Tissue

6.1 Skeletal Cartilages

  • The human skeleton initially consists of only cartilage before being replaced by bone. Exceptions exist in areas needing flexibility.

Basic Structure, Types, and Locations of Skeletal Cartilages (1 of 2)

  • Skeletal Cartilage:

    • Composed of resilient cartilage tissue primarily made of water.

    • Lacks blood vessels and nerves.

  • Perichondrium:

    • Dense connective tissue surrounding cartilage, functioning as a girdle.

    • Assists in resisting outward expansion and contains blood vessels for nutrient delivery.

  • Cartilage Composition:

    • Composed of chondrocytes, which are encased in lacunae within a jelly-like extracellular matrix.

Basic Structure, Types, and Locations of Skeletal Cartilages (2 of 2)

  • Three Types of Cartilage:

    • Hyaline Cartilage:

    • Most abundant, provides support, flexibility, and resilience.

    • Contains only collagen fibers.

    • Found in articular surfaces, costal (ribs), respiratory (larynx), and nasal cartilage (nose tip).

    • Elastic Cartilage:

    • Similar to hyaline but contains elastic fibers.

    • Locations include the external ear and epiglottis.

    • Fibrocartilage:

    • Contains thick collagen fibers providing great tensile strength.

    • Found in menisci of the knee and intervertebral discs.

Growth of Cartilage (1 of 2)

  • Cartilage Growth Mechanisms:

    • Appositional Growth:

    • Involves cartilage-forming cells secreting matrix against the external face of existing cartilage; new matrix is added to the surface.

    • Interstitial Growth:

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

Growth of Cartilage (2 of 2)

  • Calcification of Cartilage:

    • Occurs during normal bone growth in youth and can also occur with old age.

    • Note: Hardened cartilage differs from bone.

6.2 Functions of Bones (1 of 2)

  • Seven important functions include:

    • Support:

    • Provides structure for the body and supports soft organs.

    • Protection:

    • Safeguards vital organs like the brain, spinal cord, and others.

    • Anchorage:

    • Acts as levers for muscle action.

    • Mineral and Growth Factor Storage:

    • Stores calcium and phosphorus, acts as a reservoir for growth factors.

6.2 Functions of Bones (2 of 2)

  • Additional functions include:

    • Blood Cell Formation (Hematopoiesis):

    • Occurs in red marrow cavities of certain bones.

    • Triglyceride Storage:

    • Energy-reserving fat stored in bone cavities.

    • Hormone Production:

    • Osteocalcin secreted by bones helps regulate insulin secretion and glucose levels.

6.3 Classification of Bones (1 of 3)

  • Total Number of Bones: 206 named bones in the human skeleton.

    • Divided into two groups based on location:

    • Axial Skeleton:

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

    • Appendicular Skeleton:

      • Consists of bones of upper and lower limbs; includes girdles that attach limbs to axial skeleton.

6.3 Classification of Bones (2 of 3)

  • Classification by Shape:

    • Long Bones:

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

    • Short Bones:

    • Cube-shaped, located in wrist and ankle; sesamoid bones (e.g., patella) form within tendons.

    • Flat Bones:

    • Thin and slightly curved, e.g., sternum, scapulae, ribs, most skull bones.

    • Irregular Bones:

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

6.3 Classification of Bones (3 of 3)

  • Figure 6.2 Classification of Bones Based on Shape:

    • (a) Long bone (humerus)

    • (b) Irregular bone (vertebra, right lateral view)

    • (c) Flat bone (sternum)

    • (d) Short bone (talus)

Structure of Bone

  • Bone Composition:

    • Bone is an organ comprising various tissues including bone, cartilage, connective tissue, adipose tissue, and nervous tissue.

    • Forms the entire framework of the skeletal system.

Functions of the Skeletal System:

  • Provides support and protection for internal organs and assists body movements.

  • Engages in mineral homeostasis by storing/releasing calcium and phosphorus.

  • Participates in blood cell production (hemopoiesis).

  • Stores triglycerides in yellow marrow cells.

Chemical Constituents of Bone

  • Composition of bone includes:

    • 25% water

    • 25% organic proteins

    • 50% mineral salts (hydroxyapatite crystals):

    • Organic Components:

      • Comprised of collagen fibers contributing to flexibility and tensile strength.

    • Inorganic Hydroxyapatite Crystals:

      • Primarily calcium phosphate, (Ca<em>3(PO</em>4)<em>2)(Ca<em>3(PO</em>4)<em>2) and calcium carbonate (CaCO</em>3)(CaCO</em>3), along with trace elements like magnesium, fluoride, and sulfate.

Structure of Bone Components:

  • Long Bone Structure:

    • Consists of:

    • Diaphysis: Bone shaft

    • 2 Epiphyses: Ends of bone at joints

    • 2 Metaphyses: Regions between diaphysis and epiphysis

    • Articular Cartilage: Covering both epiphyses

    • Periosteum: Connective tissue surrounding diaphysis

    • Medullary Cavity: Hollow space within the diaphysis

    • Endosteum: Membrane lining the medullary cavity.

Histology of Bone

  • Extracellular Matrix Composition:

    • Composed of about 15% water, 30% collagen, and 55% crystallized mineral salts.

  • Types of Bone Cells Include:

    • Osteoprogenitor Cells: Bone stem cells that differentiate into other cell types.

    • Osteoblasts: Secrete bone matrix and facilitate bone formation.

    • Osteocytes: Mature bone cells essential in maintaining bone tissue.

    • Osteoclasts: Responsible for bone resorption, release calcium.

Types of Bone Structure:

  • Compact Bone:

    • Provides strength and support.

  • Spongy Bone:

    • Lightweight, contains a mesh of bony spines called trabeculae, providing structural support.

Gross Anatomy of Bones:

  • Structure of Short, Irregular, and Flat Bones:

    • Comprised of thin plates of spongy bone (diploë) between layers of compact bone.

    • Bone marrow is dispersed throughout spongy bone without a defined marrow cavity.

    • Hyaline cartilage covers areas where bones form movable joints.

Blood and Nerve Supply of Bone:

  • Nutrient Arteries:

    • Enter the diaphysis through nutrient foramina; accompanied by nutrient veins.

    • Periosteal Arteries and Veins: Enters bone through Volkmann's canals.

  • Periosteum:

    • Tough dense irregular connective tissue sheath containing osteoblasts that enable thickness growth but not length, assists with fracture repair, and serves as an attachment point for tendons and ligaments.

Membranes: Periosteum and Endosteum

  • Periosteum:

    • Double-layered membrane consisting of:

    • Fibrous Layer: Outer dense connective tissue.

    • Osteogenic Layer: Inner layer containing osteogenic cells.

  • Endosteum:

    • Delicate connective tissue lining internal bone surfaces, covering trabeculae and lining canals in compact bone.

    • Contains osteogenic cells.

Bone Markings

  • Include sites of muscle, ligament, and tendon attachment as well as areas involved in joint formation or blood vessels and nerves passage.

  • Types of Bone Markings:

    • Projections: Sites for muscle/ligament attachment.

    • Surfaces: Contribute to joint formation.

    • Depressions and Openings: Passageways for vessels/nerves.

Bone Formation

  • Ossification (Osteogenesis): The process of bone formation occurring in four scenarios:

    • Development during embryological stages.

    • Growth before adulthood.

    • Remodeling.

    • Fracture healing.

  • Two Forms of Ossification:

    • Intramembranous Ossification: Forms spongy bone directly from mesenchymal tissue.

    • Endochondral Ossification: Replaces cartilage with bone in developing embryo/fetus.

Microscopic Anatomy of Bone (1 of 12)

  • Osteon (Haversian System):

    • Structural unit consisting of elongated cylinders aligned parallel to the bone.

    • Composed of several rings of bone matrix (lamellae), which are organized to withstand stress.

Blood and Nerve Supply of Bone (Continued)

  • Central canals in osteons contain blood vessels and nerves connected by the perforating canals.

  • Lacunae hold osteocytes, with canaliculi connecting them to each other and the central canal.

Microscopic Anatomy of Bone (2 of 12)

  • Interstitial and Circumferential Lamellae:

    • Interstitial Lamellae: Fill gaps between osteons.

    • Circumferential Lamellae: Extend around the surface of the diaphysis aiding in resisting twisting forces.

Microscopic Anatomy of Spongy Bone

  • Spongy bone appears organized along lines of stress, supporting strength without osteons present.

Chemical Composition of Bone (1 of 3)

  • Organic Components:

    • Include bone cells and osteoid (ground substance + collagen).

    • Resilience due to sacrificial bonds in collagen.

Chemical Composition of Bone (2 of 3)

  • Inorganic Components:

    • Hydroxyapatites (mineral salts) make up 65% of bone by mass, responsible for hardness and resisting compression.

Bone Growth and Remodeling

  • Key Factors:

    • Minerals: Essential for growth and remodeling (Calcium and Phosphorus).

    • Vitamins:

    • Vitamin A: Stimulates osteoblast activity.

    • Vitamin C: Crucial for collagen synthesis.

    • Vitamin D: Promotes calcium absorption.

    • Vitamins K and B12: Necessary for bone protein synthesis.

Hormones Influencing Bone Growth

  • During childhood, human growth hormone (hGH) and IGFs are critical for bone growth.

  • Sex hormones play a significant role in bone growth and remodeling, closing the epiphyseal plates post-puberty.

  • Parathyroid Hormone (PTH) and Calcitonin regulate calcium levels, balancing serum calcium and bone mineralization.

Fracture and Repair of Bone

  • Fracture Types: Various types of fractures can occur, healing involves a three-phase process consisting of:

    • Reactive Phase: Formation of fracture hematoma.

    • Reparative Phase: Includes fibrocartilaginous and bony callus formation.

    • Remodeling Phase: Remodeling of the bony callus.

Bone's Role in Calcium Homeostasis

  • Bones store 99% of the body's calcium. PTH regulates calcium levels by stimulating osteoclast activity and increasing calcium absorption through calcitriol production.

Aging and Bone Tissue

  • Bone production decreases after adolescence; older individuals may experience a reduction in bone mass due to resorption outpacing deposition, especially in post-menopausal women.

Summary of Factors Affecting Bone Growth

  • Minerals: Calcium and phosphorus crucial for hardening.

  • Vitamins: A, C, D essential for normal bone functions.

  • Hormones: Growth hormone, IGFs, sex hormones crucial for growth and development.

  • Exercise: Stimulates bone deposition, contributing to stronger bones.

  • Aging: Reduces effectiveness of remodeling processes, increasing fracture risk.

Disorders of the Skeletal System

  • Common Disorders: Include Osteoporosis, Rickets, Osteomalacia, etc. Understand hormonal and dietary influences on bone health.

End of Chapter 6 Remarks

  • This chapter encompasses the essential principles of the skeletal system, from the microscopic anatomy of bone to the overarching regulation of calcium homeostasis and the impact of aging.