Unit XII SKELETAL SYSTEM

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Unit XIISKELETAL SYSTEM
Biology 220 Anatomy Physiology I
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• Chapter 6
• pp. 172 - 197

E. Gorski/E. Lathrop-Davis/S. Kabrhel
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Introduction

• The skeleton consists of
• Bone
• Cartilages
• hyaline cartilage - flexible, resilient support
• e.g., articular cartilage, costal cartilages,
  respiratory cartilages, nasal cartilages
• elastic cartilage - bendable
• e.g., pinna of ear, epiglottis
• fibrocartilage - highly compressible acts as
  pads between vertebrae and at knee
• e.g., symphysis pubis, intervertebral disks,
  menisci of knee

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Functions

• Support - provides body framework
• Protection - for brain, spinal cord, soft organs
  (especially in thoracic and pelvic cavities)
• Movement - attachment for skeletal muscles
• Mineral storage - especially calcium, phosphate
• Hematopoiesis - blood cell formation (in red bone
  marrow cavities)

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Bone Structure
Bone organ (tissues osseous, nervous,
cartilage, fibrous connective muscle and
epithelial in blood vessels)

• Gross anatomy of long bone
• 1. Diaphysis (dia through, physis growth)
  shaft
• medullary cavity
• yellow bone marrow
• 2. Epiphysis (epiupon), at bone ends
• compact bone (outside)
• spongy bone (inside)
• joint surface covered with articular cartilage
• red bone marrow
• 3. Epiphyseal line - disc-like region between
  diaphysis and epiphysis (remnant of epiphyseal
  plate)

Fig. 6.3, p. 177
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Gross Anatomy - Membranes

• Periosteum (peri around, osteo bone)
• double-layered membrane
• fibrous - outer layer (dense irregular
  connective)
• osteogenic - abuts the bone contains osteoblasts
  (bone germinators) and osteoclasts (bone
  breakers)
• nerve fibers, lymphatic and blood vessels
• provides insertion for tendons and ligaments
• Endosteum (endo within)
• delicate connective tissue layer
• covers trabeculae of spongy bone in marrow
  cavities lines medullary cavity lines the
  central (Haversian) canal
• contains osteoblasts and osteoclasts

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Chemical Composition of Bone

• Proper combination of organic and inorganic
  matrix elements gives strength and durability
• 1. Organic components ( 35)
• cells osteblasts, osteoclasts, osteocytes
• osteoid (organic component of matrix) consists
  of proteoglycans, glycoproteins, collagen fibers
  (all secreted by osteoblasts)
• 2. Inorganic components (65)
• hydroxyapatites (mineral salts), largely calcium
  phosphates

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Bone Development (Osteogenesis or Ossification)

• The skeleton initially made up of
• hyaline cartilage
• elastic cartilage
• fibrocartilage
• Ossification begins in the second month of
  gestation
• Intramembranous
• within fibrous CT (mesenchyme --gtbone)
• forms flat bones (most cranial bones and
  clavicles)
• Endochondral
• within hyaline cartilage
• mesenchyme --gt cartilage --gt bone
• forms most bones

http//www.sru.edu/depts/pt/histo/neonatal.htm
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Intramembranous Ossification

• Steps
• 1. Mesenchymal cells at centers of ossification
  differentiate into osteoblasts
• 2. Osteoblasts secrete organic bone matrix within
  membrane
• followed by calcification
• trapped osteoblasts mature into osteocytes

Fig. 6.7, p. 181
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Intramembranous Ossification

• 3. Blood vessels enter ossified area resulting in
  formation of spongy bone (red bone marrow inside)
• 4. Outer layer of bone reorganized into compact
  bone
• remaining fibrous tissue outside ossified tissue
  becomes periosteum

Fig. 6.7, p. 181
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Intramembranous Ossification
http//education.vetmed.vt.edu/Curriculum/VM8054/L
abs/Lab8/Examples/exmembos.htm
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Endochondral Bone Formation

• Ossification begins at primary ossification
  center (in cartilage at middle of shaft for long
  bone) continues at secondary ossification
  centers
• in short bones - only have primary ossification
  centers
• in irregular bones - have several ossification
  centers

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Endochondral Bone Formation
Ossification begins at primary ossification
center (in cartilage at middle of shaft for long
bone)

• Steps (long bone)
• 1. Formation of bone collar
• blood vessels enter perichondrium at middle of
  shaft
• selected mesenchymal cells (called
  osteoprogenetor cells) become osteoblasts
• osteoblasts secrete bone matrix creating a bone
  collar

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Endochondral Bone Formation

• 2. Secretion of organic matrix causes
  chondrocytes in shaft to hypertrophy (signals
  calcification) and these cells eventually burst
  resulting in cavity
• 3. Formation of periosteal bud blood vessels,
  nerve fibers, osteoblasts, osteoclasts enter
  shaft --gt form spongy bone

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Endochondral Bone Formation

• 4. Diaphysis elongates as primary ossification
  center spreads proximally and distally medullary
  cavity forms
• 5. Formation of secondary center(s) of
  ossification in one (or both) epiphysis around
  time of birth (like primary but spongy bone
  remains)
• When secondary ossification completed hyaline
  cartilage remains as
• articular cartilage
• epiphyseal plate

Fig. 6.8, p. 182
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Endochondral Bone Formation
http//www.uoguelph.ca/zoology/devobio/210labs/mes
o2.html
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Postnatal Bone Growth 1. Interstitial Growth
(length)

• cartilage cells in epiphysial plate form tall
  columns
• zone 1 - growth zone (dividing chondrocytes)
  (zone of proliferation)
• zone 2 - transformation zone (zone of hypertrophy
  and calcification), closer to shaft, chondrocytes
  enlarge then die, matrix calcifies

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Postnatal Bone Growth 1. Interstitial Growth
(length)

• zone 3 - osteogenic zone (zone of retrogression
  and ossification), long spicules of calcified
  cartilage form at the epiphysis-diaphysis
  junction and become covered with bone matrix of
  spongy bone (tips later digested by osteoclasts
  to enlarge medullary cavity)
• at adolescence, chondrocytes divide less often ?
  epiphysial plate becomes thinner -gt entirely
  replaced by bone between age 18-21 years.

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2. Appositional Growth (thickness)

• osteoblasts in periosteum secrete bone matrix on
  external surface of bone
• osteoclasts on endosteal surface (diaphysis)
  remove bone
• more built than broken matrix -gt bone gets thicker

Fig. 6.10, p. 184
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Hormonal Regulation of Bone Growth

• 1. Growth hormone (GH)
• active from infancy through childhood
• acts on epiphyseal plate to stimulate division of
  chondrocytes
• disorders
• gigantism - hypersecretion of GH
• dwarfism - hyposecretion of GH or T3/T4
• 2. Calcitonin - inhibits osteoclast activity
• 3. Parathyroid hormone - stimulates osteoclast
  activity

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Hormonal Regulation of Bone Growth

• 4. Thyroid hormones (T3 and T4) modulate activity
  of cells in response to GH to ensure proper
  proportions of the skeleton
• 5. Sex hormones (testosterone and estrogens)
• active at puberty
• initial growth spurt (hormones stimulate
  osteoblasts)
• masculinization or feminization (affects shape of
  bone)
• later, cause epiphyseal plate to close

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Bone Remodeling

• 5-7 of bone mass recycled every week (distal
  femur fully replaced every 5-6 month)
• spongy bone replaced every 3-4 years,
• compact bone replaced every 10 years
• Remodeling occurs at periosteal and endosteal
  surfaces
• Bone deposit - formation of osteoid followed by
  later mineralization
• Bone resorption - osteoclasts secrete lysosomal
  enzymes that digest the organic matrix and
  metabolic acids that digest calcium salts
  (release calcium)

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Control of Bone Remodeling

• Blood calcium requirement
• 400-800 mg/day (child)
• 1200-1500 mg/day (young adult)
• 1. Hormonal control
• PTH released when blood calcium level declines
• increases osteoclast activity
• calcitonin released when blood calcium level
  rises
• inhibits osteoclast activity
• promotes osteoblast activity
• 2. Mechanical stress
• heavy usage leads to heavy bones
• disuse leads to wasting

Fig. 6.11, p.186
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Bone Repair
Steps (simple fracture) 1. Formation of hematoma
- mass of clotted blood forms at the fracture site

• 2. Formation of fibrocartilaginous callus (soft
  callus granulation tissue)
• capillaries and phagocytic cells invade hematoma
• fibroblasts secrete collagen fibers
• osteoblasts begin formation of spongy bone

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Bone Repair

• 3. Formation of bony callus (hard callus)
• new bone trabeculae appear in fibrocartilaginous
  callus and gradually convert into hard bone
  (callus)
• 4. Remodeling - excess material in bone shaft
  exterior and within the medullary cavity is
  removed and hard wall are remade

Healing time of simple fracture 6-8 weeks
(sometimes as few as 3 weeks)
Fig. 6.13, p. 189
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Examples of Common Types of Fractures

• closed (simple) - completely internal
• open (compound) - bone ends penetrate the skin
• linear - break parallels the long axis of the
  bone
• transverse - break across the bone long axis
• complete - bone is broken through
• incomplete (greenstick) - on side of the shaft
  breaks, the other side bends

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Bone - Homeostatic Imbalances

• 1. Dietary Causes Insufficient calcium, vitamin
  D, vitamin C
• Osteomalacia - soft bone, inadequate
  mineralization, pain when weight is applied
• Rickets - soft bones in children, bowed legs,
  deformities of pelvis, skull, and rib cage

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Bone - Homeostatic Imbalances

• 2. Other Known Causes Estrogen deficiency,
  insufficient calcium, protein deficiency,
  abnormal vitamin D receptors, immobility,
  hyperthyroidism, diabetes
• Osteoporosis - bone resorption outpaces bone
  deposit
• 3. Unknown Cause (probably initiated by virus)
• Pagets disease - excessive bone formation and
  break down leading to weakened bones, irregular
  thickening, and/or filling of marrow cavities