Chapter 7: Skeletal System - Bone Structure and Function

Chapter 7: Skeletal System - Bone Structure and Function

Dr. Wilson

Key Topics

  • Functions of the Skeletal System
  • Cartilage – Location, Function, and Growth
  • Classification of Bones – Location and Shape
  • Gross Anatomy of Bones, Microscopic Anatomy of Bones, Chemical Composition of Bones
  • Ossification – Endochondral and Intramembranous
  • Bone Growth – Appositional and Interstitial
  • Bone Remodeling
  • Fractures – Classification and Repair
  • Bone Disorders

Functions of the Skeletal System and Bones

  • There are seven important functions:
    1. Support: Provides a framework for the body.
    2. Protection: Safeguards vital organs (e.g., skull protects the brain).
    3. Movement: Serves as attachment points for muscles, which facilitate movement.
    4. Mineral storage: Stores minerals such as calcium and phosphorus.
    5. Triglyceride storage: Yellow bone marrow stores fat.
    6. Hematopoiesis: Formation of blood cells occurs in bone marrow.
    7. Hormone production: Produces hormones such as osteocalcin, which regulates bone formation.

Skeletal System Composition

  • Primarily composed of:
    • Cartilage: Flexible connective tissue.
    • Osseous tissue (bone): Hard, dense tissue that forms the skeletal structure.

Skeletal Cartilages

  • The human skeleton initially consists of just cartilage which is later replaced by bone, except for regions that require flexibility.

Basic Structure, Types, and Locations of Cartilage

  • Chondrocytes: Located in lacunae, surrounded by a jelly-like extracellular matrix.
  • Matrix: Contains no blood vessels or nerves.
  • Perichondrium: A layer of dense connective tissue that surrounds cartilage and contains blood vessels for nutrient delivery.
Types of Cartilage

  1. Hyaline cartilage

    • Provides support, flexibility, and resilience.
    • Most abundant type; consists of only collagen fibers.
    • Locations: Articular surfaces (joints), costal cartilage (ribs), respiratory structures (larynx), nasal cartilage (nose).

  2. Elastic cartilage

    • Similar to hyaline but contains elastic fibers.
    • Locations: External ear, epiglottis.

  3. Fibrocartilage

    • Contains thick collagen fibers; provides great tensile strength.
    • Locations: Menisci of the knee, intervertebral discs.

Growth of Cartilage

  • Cartilage grows through two processes:
    1. Appositional Growth:
    • Chondroblasts in the perichondrium secrete matrix to the surface of existing cartilage.
    1. Interstitial Growth:
    • Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within.

Classification of Bones by Location

  • The human skeleton has 206 named bones, classified into two groups:
    1. Axial Skeleton:
    • Comprises the long axis of the body: skull, vertebral column, rib cage.
    1. Appendicular Skeleton:
    • Bones of upper and lower limbs; girdles that attach limbs to the axial skeleton.

Classification of Bones by Shape

  • Bones are categorized into four shapes:
    1. Long Bones:
    • Longer than they are wide (e.g., humerus, femur).
    1. Short Bones:
    • Cube-shaped (e.g., carpals, tarsals); includes sesamoid bones like the patella.
    1. Flat Bones:
    • Thin, flat, slightly curved (e.g., sternum, scapula, ribs).
    1. Irregular Bones:
    • Complex shapes (e.g., vertebrae, pelvic bones).

Bone Anatomy

  • Bones are organs comprised of various tissue types:
    • Osseous tissue (predominantly)
    • Nervous tissue
    • Cartilage
    • Fibrous connective tissue
    • Muscle cells
    • Epithelial cells in blood vessels.
  • Structure can be analyzed at three levels:
    • Gross Anatomy
    • Microscopic Anatomy
    • Chemical Composition
Gross Anatomy of Bone
  • Compact Bone: Dense outer layer appearing smooth and solid.
  • Spongy Bone: Composed of trabeculae forming a honeycomb structure with open spaces filled with red or yellow bone marrow.
Bone Marrow
  • Red Bone Marrow:
    • Site for blood cell formation.
    • Present mainly in flat bones and epiphyses of long bones in adults.
  • Yellow Bone Marrow:
    • Composed mainly of adipocytes; serves as an energy reserve.
Long Bone Structure
  • Diaphysis: Tubular shaft forming the long axis; consists of compact bone surrounding a medullary cavity containing yellow marrow.
  • Epiphyses: Ends of long bones; consist of compact bone externally and spongy bone internally.
  • Articular Cartilage: Covers joint surfaces.
  • Epiphyseal Line: A remnant of childhood epiphyseal plate indicating where bone growth occurred.
Two Membranes
  1. Periosteum:
    • Covers external surfaces, contains osteogenic stem cells for bone growth and repair, and supports attachment points for tendons and ligaments.
  2. Endosteum:
    • Covers internal bone surfaces, houses osteogenic cells, and lines canals.
Structure of Other Bones
  • Bones of thin plates of spongy bone (diploë) are covered by compact bone; bone marrow is scattered within spongy bone, and hyaline cartilage covers movable joint areas.

Microscopic Anatomy of Bone

  • Five Major Cell Types:
    1. Osteogenic Cells: Stem cells in periosteum and endosteum that differentiate into osteoblasts.
    2. Osteoblasts: Bone-forming cells secreting unmineralized bone matrix known as osteoid.
    3. Osteocytes: Mature bone cells maintaining bone matrix, acting as stress sensors.
    4. Bone-lining Cells: Flat cells on bone surfaces believed to maintain matrix (periosteal and endosteal cells).
    5. Osteoclasts: Multinucleate cells derived from macrophages, responsible for bone resorption, with ruffled borders to increase surface area for breakdown.
Compact Bone Structure
  • Also referred to as lamellar bone, structured into:
    • Osteon (Haversian system): The structural unit running parallel to the bone's long axis, acting as weight-bearing pillars.
    • Central Canal: Contains blood vessels and nerve fibers.
    • Canaliculi: Hairlike canals connecting lacunae.
    • Lamellae: Rings of bone matrix providing tensile strength.
Spongy Bone Structure
  • Organized to resist stress, forms trabeculae containing irregularly arranged lamellae connected via canaliculi.

Chemical Composition of Bone

  • Composed of both organic and inorganic constituents.
    • Organic Components:
    • Mostly collagen (33% of the total amount in the body).
    • Inorganic Components:
    • Hydroxyapatites (mineral salts, 67% by mass).
    • Contribute to the hardness and resistance to compression.

Bone Development

  • Ossification: The process of bone tissue formation occurring from the second month of fetal development through early adulthood.
  • Types of ossification:
    • Endochondral Ossification: Bone replaces hyaline cartilage.
    • Intramembranous Ossification: Bone develops from fibrous membranes.
Endochondral Ossification - Process
  1. Bone Collar Formation:
    • Osteoblasts secrete osteoid against diaphysis, forming a hard structure around the cartilage.
  2. Calcification:
    • Central cartilage calcifies, leading to cavities as chondrocytes die.
  3. Periosteal Bud Invasion:
    • Blood vessels and osteogenic cells invade, initiating the formation of spongy bone.
  4. Diaphysis Elongation:
    • Continued osteoclast and osteoblast activity leads to elongation and the formation of the medullary cavity.
  5. Epiphyseal Ossification:
    • Secondary ossification centers form in epiphyses, continuing to develop until bones reach maturity.
Intramembranous Ossification - Process
  1. Formation of Ossification Centers:
    • Mesenchymal cells cluster to form osteoblasts.
  2. Osteoid Secretion and Calcification:
    • Osteoblasts secrete osteoid, which calcifies within days.
  3. Woven Bone Formation:
    • Osteoid surrounds blood vessels, forming trabeculae and subsequently periosteum on the surface.
  4. Compact Bone Replacement:
    • Trabecular bone remodels into compact bone with the appearance of red marrow.

Postnatal Bone Growth

  • Long bones grow in length via interstitial growth in the epiphyseal plate and increase in thickness through appositional growth.
  • Growth ceases after adolescence, although some facial bones continue to grow.
Epiphyseal Plate Zones
  1. Resting Zone: Attaches to epiphysis; relatively inactive.
  2. Proliferation Zone: Rapid cell division pushes the epiphysis away from the diaphysis, promoting lengthening.
  3. Hypertrophic Zone: Contains large chondrocytes that enlarge and erode, forming spaces.
  4. Calcification Zone: Matrix calcifies and chondrocytes die.
  5. Ossification Zone: Chondral spicules are replaced by spongy bone; medullary cavity expands.

Hormonal Regulation of Bone Growth

  • Growth Hormone: Stimulates epiphyseal plate activity during childhood.
  • Thyroid Hormone: Modulates activity of growth hormone for proper proportions.
  • Testosterone and Estrogen: Promote growth spurts during puberty and induce epiphyseal plate closure.
Clinical - Homeostatic Imbalance
  • Abnormal skeletal growth can occur due to hormonal imbalances:
    • Gigantism: Resulting from growth hormone over-secretion.
    • Dwarfism: Resulting from under-secretion of growth hormone.

Bone Remodeling

  • As bones grow, remodeling maintains their shape through processes of bone resorption and deposition.
  • Occurs at periosteum and endosteum surfaces.
  • Involves packets of osteoblasts and osteoclasts coordinating the remodeling process.
Bone Resorption
  • Function of osteoclasts; they break down bone by secreting enzymes and protons that digest the matrix.
  • Osteoclasts also phagocytize demineralized matrix and dead osteocytes.
  • Activation of osteoclasts is regulated by parathyroid hormone (PTH) and immune T cell proteins.
Bone Deposit
  • New bone matrix deposited by osteoblasts, which may be activated by mechanical signals and increased concentrations of calcium and phosphate.
Control of Remodeling
  • Remodeling is continuous and regulated by genetic factors and two loops:
    • Hormonal Controls
    • Response to Mechanical Stress
Hormonal Controls
  • Parathyroid Hormone (PTH):
    • Produced in response to low blood calcium; it stimulates osteoclasts to resorb bone and release calcium into the bloodstream.
  • Calcitonin:
    • Produced in response to elevated blood calcium levels; its effects are considered negligible in humans.
Response to Stress
  • Bone stress correlates with bone growth; they adapt according to stresses placed upon them (Wolf’s Law).
  • Compression and tension create electrical signals that may stimulate remodeling by influencing fluid flow in canaliculi.

Bone Repair

  • Fractures, resulting from trauma or bone thinning, are repaired through several stages:
    1. Hematoma Formation: Formation of a blood clot at the fracture site.
    2. Fibrocartilaginous Callus Formation: Capillary growth in hematoma; remodeling via collagen fiber secretion and cartilage matrix creation.
    3. Bony Callus Formation: Conversion of fibrocartilaginous callus to a bony (hard) callus of spongy bone.
    4. Bone Remodeling: Removal of excess material and reconstruction of the original structure in response to mechanical stressors.

Osteoporosis

  • Osteoporosis is characterized by a decrease in bone density that occurs when resorption exceeds deposition.
  • Spongy bone in the spine and neck of the femur is most susceptible to fracture in osteoporosis.
  • Risk Factors:
    • Age, hormonal changes (especially post-menopausal women), insufficient exercise, poor diet (low in calcium and protein), smoking, genetics, and certain medications.

Review Questions

  1. Growth of cartilage from within is termed:
    a) Interstitial growth
    b) Remodeling
    c) Appositional growth
    d) Endochondral growth

  2. An adult male donating red bone marrow would most likely come from his:
    a) femoral diaphysis
    b) humeral diaphysis
    c) sternum
    d) skull

  3. Concentric rings of bone matrix are known as a(n):
    a) lamella
    b) osteon
    c) pillar system
    d) Sharpey’s system

  4. Communication among osteocytes occurs through:
    a) lacunae
    b) Volkmann’s canals
    c) Haversian canals
    d) canaliculi

  5. The component of bone contributing to hardness is:
    a) hydroxyapatite
    b) collagen
    c) osteoid
    d) organic

  6. Long bone growth would cease if epiphyseal cartilage stopped dividing.

  7. The bone cell type responsible for initiating ossification is:
    a) Osteoblasts
    b) Osteoclasts
    c) Osteocytes
    d) Chondroblasts

  8. Adding new bony matrix to injury sites is known as:
    a) bone sizing
    b) bone deposition
    c) bone resorption
    d) bone addition

  9. In a patient without parathyroid glands, blood calcium levels would likely:
    a) decrease
    b) increase
    c) stay the same
    d) increase twofold

Conclusion

  • Understanding the skeletal system's structure, function, and its dynamic processes such as growth, remodeling, and repair is essential for the study of human biology as these principles apply to health and disease states.