Chapter 6 - Skeletal System

Skeletal Cartilages

• Basic Structure, Types, and Locations
  • Skeletal cartilages are made from cartilage, surrounded by a layer of dense irregular connective tissue called the perichondrium.
  • Hyaline cartilage is the most abundant skeletal cartilage, and includes the articular, costal, respiratory, and nasal cartilages.
  • Elastic cartilages are more flexible than hyaline, and are located only in the external ear and the epiglottis of the larynx.
  • Fibrocartilage is located in areas that must withstand a great deal of pressure or stretch, such as the cartilages of the knee and the intervertebral discs.
• Growth of Cartilage
  • Appositional growth results in outward expansion due to the production of cartilage matrix on the outside of the tissue.
  • Interstitial growth results in expansion from within the cartilage matrix due to division of lacunae-bound chondrocytes and secretion of matrix.

Classification of Bones

• Axial skeleton - bones of the skull, vertebral column, and rib cage
• Appendicular skeleton - bones of the upper and lower limbs, shoulder, and hip
• Classification by Shape
  • Long bones - longer than they are wide (e.g., humerus)
  • Short bones - Cube-shaped bones of the wrist and ankle
  • Seasmoid bone develop within a tendon (e.g., patella)
  • Flat bones - thin, flattened, & bit curved (sternum, most skull bones)
  • Irregular bones - complicated shapes (vertebrae; hip bones)
  • Wormian bones tiny bones in between major skull bones

FUNCTIONS OF SKELETON

• Support - form the framework that supports the body and cradles soft organs
• Protection - provide a protective case for the brain, spinal cord, and vital organs
• Movement - provide levers for muscles
• Mineral storage - reservoir for minerals, especially calcium and phosphorus
• Blood cell formation - hematopoiesis occurs within the marrow cavities of bones

Bone Markings are bulges, depressions, and holes that serve as:

• Joint surfaces
  • Fossa - shallow, basinlike depression
  • Condyle - rounded articular projection
  • Epicondyle - raised area above a condyle
  • Head -bony expansion on a narrow neck
• Muscle & ligament attachment
  • Trochanter - large, blunt, irregular surface
  • Tuberosity - rounded projection
  • Process - any bony prominence
  • Crest - narrow, prominent ridge of bone
  • Line - narrow ridge of bone
  • Fovea - small pit like indent
  • Tubercle - small rounded projection
  • Ramus - armlike bar of bone
  • Spine - sharp, slender projection
• Passageways for vessels, etc.
  • Foramen - round or oval opening through a bone
  • Fissure - narrow, slitlike opening
  • Meatus - canal-like passageway
• Space within a bone - Sinus

Structure OF A LONG BONE (gross anatomy)

• Long bones consist of a diaphysis and an epiphysis
• Diaphysis -Tubular shaft that forms the axis of long bones
  • Composed of compact bone that surrounds the medullary cavity
  • Yellow bone marrow (fat) is contained in the medullary cavity
• Epiphyses - Expanded ends of long bones
  • Exterior is compact bone, and the interior is spongy bone
  • Joint surface is covered with articular (hyaline) cartilage
• Metaphyses are the areas between the epiphysis and diaphysis and include the epiphyseal plate in growing bones.
• Bone Membranes
  • Periosteum - double-layered protective membrane
    • Outer fibrous layer is dense regular connective tissue
    • Inner osteogenic layer is composed of osteoblasts and osteoclasts
    • Richly supplied with nerve fibers, blood, and lymphatic vessels, which enter the bone via nutrient foramina
    • Secured to underlying bone by Sharpey's fibers
  • Endosteum - delicate membrane covering internal surfaces of bone

Structure of Short, Irregular, and Flat Bones

• Thin plates of periosteum-covered compact bone on the outside with endosteum-covered spongy bone (diploë) on the inside
• Have no diaphysis or epiphyses
• Contain bone marrow between the trabeculae

Location of Hematopoietic Tissue (Red Marrow)

• In infants
  • Found in the medullary cavity and all areas of spongy bone
• In adults
  • Found in the diploë of flat bones, and the head of the femur and humerus

Microscopic Structure of Compact Bone

• Haversian system, or osteon - the structural unit of compact bone
  • Lamella - weight-bearing, column-like matrix tubes composed mainly of collagen
  • Haversian, or central canal -containing blood vessels and nerves
  • Volkmann's canals - channels lying at right angles to the central canal, connecting blood and nerve supply of periosteum to the Haversian canal
• Osteocytes - mature bone cells
• Lacunae - small cavities in bone that contain osteocytes
• Canaliculi -hairlike canals that connect lacunae to each other & central canal

Chemical Composition of Bone

• Organic
  • Osteoblasts - bone-forming cells
  • Osteocytes - mature bone cells
  • Osteoclasts - large cells that resorb or break down bone matrix
  • Osteoid - unmineralized bone matrix composed of proteoglycans, glycoproteins, and collagen
• Inorganic - hydroxyapatites, or mineral salts
  • Sixty-five percent of bone by mass
  • Mainly calcium phosphates
  • Responsible for bone hardness and its resistance to compression

Osteogenesis and ossification - the process of bone tissue formation, which leads to:

• The formation of the bony skeleton in embryos
• Bone growth until early adulthood
• Bone thickness, remodeling, and repair
• Formation of the Bony Skeleton
  • Begins at week 8 of embryo development
  • Intramembranous Ossification - bone develops from a fibrous membrane - bone develops from a fibrous membrane
    • Formation of most of the flat bones of the skull and the clavicles
    • Stages of Intramembranous Ossification
      • Fibrous connective tissue membranes are formed by mesenchymal cells
      • An ossification center appears in fibrous connective tissue membrane
      • Bone matrix is secreted within the fibrous membrane
      • Woven bone and periosteum form
      • Bone collar of compact bone forms, and red marrow appears
  • Endochondral Ossification
    • Begins in the second month of development
    • Uses hyaline cartilage "bones" as models for bone construction
    • Requires breakdown of hyaline cartilage prior to ossification
    • Stages of Endochondral Ossification
      • Formation of bone collar
      • Cavitation of the hyaline cartilage
      • Invasion of internal cavities by the periosteal bud, and spongy bone formation
      • Formation of the medullary cavity; appearance of secondary ossification centers in the epiphyses
      • Ossification of the epiphyses, with hyaline cartilage remaining only in the epiphyseal plates

Postnatal Bone Growth

• Bone Growth in length
  • Epiphyseal plate or cartilage growth plate
    • cartilage cells are produced by mitosis on epiphyseal side of plate
    • cartilage cells are destroyed and replaced by bone on diaphyseal side of plate
  • Between ages 18 to 25, epiphyseal plates close.
    • cartilage cells stop dividing and bone replaces the cartilage (epiphyseal line)
  • Growth in length stops at age 25
  • 4 Zones of Growth in Epiphyseal Plate
    • Zone of resting cartilage
      • anchors growth plate to bone
    • Zone of proliferating cartilage
      • rapid cell division (stacked coins)
    • Zone of hypertrophic cartilage
      • cells enlarged & remain in columns
    • Zone of calcified cartilage
      • thin zone, cells mostly dead since matrix calcified
      • osteoclasts removing matrix
      • osteoblasts & capillaries move in to create bone over calcified cartilage

  Long Bone Growth and Remodeling

  • Growth in length - cartilage continually grows and is replaced by bone as shown
  • Remodeling - bone is resorbed and added by appositional growth as shown
• Appositional Growth of Bone
  • Osteoblasts beneath the periosteum secrete bone matrix, forming ridges that follow the course of periosteal blood vessels.
  • As the bony ridges enlarge and meet, the groove containing the blood vessel becomes a tunnel.
  • The periosteum lining the tunnel is transformed into an endosteum and the osteoblasts just deep to the tunnel endosteum secrete bone matrix, narrowing the canal
  • As the osteoblasts beneath the endosteum form new lamellae, a new osteon is created. Meanwhile new circumferential lamellae are elaborated beneath the periosteum and the process is repeated, continuing to enlarge bone diameter.

Hormonal Regulation of Bone Growth During Youth

• During infancy and childhood, epiphyseal plate activity is stimulated by growth hormone
• During puberty, testosterone and estrogens:
  • Initially promote adolescent growth spurts
  • Cause masculinization and feminization of specific parts of the skeleton
  • Later induce epiphyseal plate closure, ending longitudinal bone growth

Bone Remodeling

• In adult skeletons, bone remodeling is balanced bone deposit and removal, bone deposit occurs at a greater rate when bone is injured, and bone resorption allows minerals of degraded bone matrix to move into the blood.
• Control of Remodeling
  • The hormonal mechanism is mostly used to maintain blood calcium homeostasis, and balances activity of parathyroid hormone and calcitonin.
  • Nutrition - Ca, P, Mg, vitamins, A, C and D.
  • In response to mechanical stress and gravity, bone grows or remodels in ways that allow it to withstand the stresses it experiences.
    • Trabeculae form along lines of stress
    • Large, bony projections occur where heavy, active muscles attach
  • Fracture Repair
    • Hematoma formation
      • Torn blood vessels hemorrhage
      • A mass of clotted blood (hematoma) forms at the fracture site
      • Site becomes swollen, painful, and inflamed
    • Fibrocartilaginous callus forms
      • Capillaries grow into the tissue and phagocytic cells begin cleaning debris
      • Osteoblasts & fibroblasts migrate to fracture & begin reconstructing bone
      • Fibroblasts secrete collagen fibers that connect broken bone ends
      • Osteoblasts begin forming spongy bone
      • Osteoblasts furthest from capillaries secrete an externally bulging
    • Bony callus formation
      • New bone trabeculae appear in the fibrocartilaginous callus
      • Fibrocartilaginous callus converts into a bony (hard) callus
      • Bone callus begins 3-4 weeks after injury, and continues until firm union is formed 2-3 months later
    • Bone remodeling
      • Excess material on the bone shaft exterior and in the medullary canal is removed
      • Compact bone is laid down to reconstruct shaft walls

    Fractures

    • Nondisplaced - bone ends retain their normal position
    • Displaced - bone ends are out of normal alignment
    • Complete - bone is broken all the way through
    • Incomplete - bone is not broken all the way through
    • Linear - the fracture is parallel to the long axis of the bone
    • Transverse - the fracture is perpendicular to the long axis of the bone
    • Compound (open) - bone ends penetrate the skin
    • Simple (closed) - bone ends do not penetrate the skin

    Homeostatic Imbalances of Bone

    • Osteomalacia and Rickets
      • Osteomalacia includes a number of disorders in adults in which the bone is inadequately mineralized.
      • Rickets is inadequate mineralization of bones in children caused by insufficient calcium or vitamin D deficiency.
    • Osteoporosis
      • Group of diseases in which bone reabsorption outpaces bone deposit
      • Spongy bone of the spine is most vulnerable
      • Occurs most often in postmenopausal women
      • Bones become so fragile that sneezing or stepping off a curb can cause fractures
      • Factors that contribute to osteoporosis include a petite body form, insufficient exercise or immobility, a diet poor in calcium and vitamin D, abnormal vitamin D receptors, smoking, and certain hormone-related conditions.
      • Treatment
        • Calcium and vitamin D supplements
        • Increased weight-bearing exercise
        • Hormone (estrogen) replacement therapy (HRT) slows bone loss
        • Natural progesterone cream prompts new bone growth
        • Statins increase bone mineral density
    • Paget's disease is characterized by excessive bone deposition and resorption, with the resulting bone abnormally high in spongy bone. It is a localized condition that results in deformation of the affected bone.

    Developmental Aspects of Bones: Timing of Events

    • The skeleton derives from embryonic mesenchymal cells, with ossification occurring at precise times. Most long bones have obvious primary ossification centers by 12 weeks gestation.
    • At birth, most bones are well ossified, except for the epiphyses, which form secondary ossification centers.
    • Throughout childhood, bone growth exceeds bone resorption; in young adults, these processes are in balance; in old age, resorption exceeds formation.
    Check out the following web sites:
    • Slides on Epiphyseal Plate
    • Consequences of Moving a Patient Argosy Medical
    • Bone Pronunciation
    • Daily Skeletal Quizzes
    • The Virtual Body

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