Osseous Tissue and Bone Structure

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Chapter 6

• Osseous Tissue and Bone Structure

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Bones and Cartilages of the Human Body

• Hyaline
• Most abundant skeletal cartilage articular,
  costal, respiratory, nasal
• Elastic
• External ear, epiglottis
• Fibrocartilage
• IVD, knee menisci

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Functions of the skeletal system

• 1. Support - framework for the body
• 2. Protection - skull, vertebrae, ribcage
• 3. Leverage - bones are levers, joints are
  fulcrums
• 4. Mineral storage (calcium)
• 5. Lipid Storage (yellow marrow)
• 6. Blood cell formation - hematopoiesis

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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

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Classification of Bones By Shape

• Long bones
• longer than they are wide (e.g., humerus)

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Classification of Bones By Shape

• Short bones
• Cube-shaped bones of the wrist and ankle

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Classification of Bones By Shape

• Flat bones
• thin, flattened, and a bit curved (e.g., sternum,
  and most skull bones)

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Classification of Bones By Shape

• Irregular bones
• bones with complicated shapes (e.g., vertebrae
  and hip bones)
• Sesamoid- form w/in a tendon
• Wormian- form in sutures

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Classification of Bones By Shape
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Structure of a typical long bone

• 1. Diaphysis- shaft of the bone
• Diathrough physisgrowth
• Contains medullary cavity (yellow in adults)
• 2. Epiphysis- epiabove physisgrowth
• Epiphyseal line- where diaphysis joins epiphysis
• Located at the metaphysis
• Articular cartilage- thin layer of hyaline cart
  to reduce friction cushion joint

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Bone Membranes

• Periosteum double-layered 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 Sharpeys fibers
• Endosteum delicate membrane covering internal
  surfaces of bone
• Active in bone repair

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Structure of other bones

• No shaft or epiphysis
• Contain bone marrow but no cavity
• Thin plates of periosteum covered compact bone
  b/t endosteum covered spongy bone w/in
• Internal layer of spongy bone diploe

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Types of bone

• 1. Compact- has osteon
• 2. Spongy- no osteons
• Lattice of plates called trabeculae (contain
  lacuna)
• w/in the trabeculae are marrow
• B.V.s from periosteum penetrate into spongy bone
  osteocytes are nourished directly from blood in
  marrow cavity

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Bone cell types ( 2 of bone mass)

• 1. osteoprogenitor cells (osteogenic)
• Derived from mesenchyme have ability to
  differentiate into osteoblasts assist in
  fracture repair
• Found in the osteogenic layer of
  periosteum/endosteum, through Volkmanns canals
• 2. osteoblasts
• No mitotic potential found in osteogenic layer
  of peri/endosteum
• Secrete the organic components some of the
  mineral salts involved in bone formation (called
  osteoid) mature into osteocytes
• 3. osteocytes-cannot divide but maintain cellular
  activity
• Principle cells of bone tissue maintain bone
  matrix repair damaged bone
• 4. osteoclasts-found on inner peri/endosteum
• Derived from circulating monocytes and function
  to resorb bone

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Bone (Osseous) Tissue

• Figure 63 Types of Bone Cells.

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Intercellular substance

• 1. Matrix Proteins- organic matrix (primarily
  collagen, some glycoproteins)
• Somewhat flexible
• Approx. 35 of content
• Osteoblasts, osteocytes, osteoclasts
• 2. Mineral salts (hydroxyapatites)- primarily
  calcium phosphate, Ca3(PO4)2
• Approx 65 of content
• Extremely strong, responsible for bone hardness
  and its resistance to compression

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Microscopic Structure of Compact Bone

• Osteon, or Haversian system the structural unit
  of compact bone
• Lamella weight-bearing, column-like matrix
  tubes composed mainly of collagen
• Haversian, or central canal central channel
  containing blood vessels and nerves
• Volkmanns (perforating) canals channels lying
  at right angles to the central canal, connecting
  blood and nerve supply of the periosteum to that
  of the Haversian canal
• Lacunae small cavities in bone that contain
  osteocytes
• Canaliculi hairlike canals that connect lacunae
  to each other and the central canal

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Microscopic Structure of Compact Bone
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Compact and Spongy Bone

• Figure 64a The Histology of Compact Bone.

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Spongy Bone

• Does not have osteons
• The matrix forms an open network of trabeculae
• Trabeculae have no blood vessels
• The space between trabeculae is filled with red
  bone marrow
• which has blood vessels
• forms red blood cells and supplies nutrients to
  osteocytes
• In some bones, spongy bone holds yellow bone
  marrow which stores fat

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Bone formation (osteogenesis)

• Osteogenesis occurs throughout life but in
  different ways
• 1. embryo responsible for laying down of bony
  skeleton (ossification well started by 8th week)
• 2. bone growth continues until early adulthood
• 3. remodeling repair continues for life
• Ossification - The process of replacing other
  tissues with bone (endochondral and
  intramembranous)
• Calcification - The process of depositing calcium
  salts
• Occurs during bone ossification and in other
  tissues

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2 types of ossification

• 1. Intramembranous (dermal ossification)
• Formation of most of the flat bones of the skull
  and the clavicles from a fibrous membrane
• Fibrous connective tissue membranes are formed by
  mesenchymal cells
• 2. Endochondral
• Formation of bone in hyaline cartilage
• Both lead to the same type of bone
• Both begin with migration of mesenchymal cells
  from c.t. to areas of bone formation
• No blood supply? chondroblasts
• Blood supply?osteoblasts

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Intramembranous Ossification Step 1

• Mesenchymal cells aggregate
• differentiate into osteoblasts
• begin ossification at the ossification center
• develop projections called spicules

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Intramembranous Ossification Step 2

• Blood vessels grow into the area
• to supply the osteoblasts
• Spicules connect
• trapping blood vessels inside bone

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Intramembranous Ossification Step 3

• Spongy bone develops and is remodeled into
• osteons of compact bone
• periosteum
• or marrow cavities

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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

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Endochondral Ossification Step 1

• Chondrocytes in the center of hyaline cartilage
• enlarge
• form struts and calcify
• die, leaving cavities in cartilage

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Endochondral Ossification Step 2

• Blood vessels grow around the edges of the
  cartilage
• Cells in the perichondrium change to osteoblasts
  
• producing a layer of superficial bone around the
  shaft which will continue to grow and become
  compact bone (appositional growth)

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Endochondral Ossification Step 3

• Blood vessels enter the cartilage
• bringing fibroblasts that become osteoblasts
• spongy bone develops at the primary ossification
  center

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Endochondral Ossification Step 4

• Remodeling creates a marrow cavity
• bone replaces cartilage at the metaphyses

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Endochondral Ossification Step 5

• Capillaries and osteoblasts enter the epiphyses
• creating secondary ossification centers

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Endochondral Ossification Step 6

• Epiphyses fill with spongy bone
• cartilage within the joint cavity is articulation
  cartilage
• cartilage at the metaphysis is epiphyseal
  cartilage

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Epiphyseal Lines

• When long bone stops growing, after puberty
• Epiphyseal cartilage disappears
• At young adulthood cartilage division slows and
  the plate becomes smaller until it is reduced to
  a fine line epiphyseal line
• Females? age 18 Males?age 21

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Blood Supply of Mature Bones

• 3 sets of blood vessels develop
• Nutrient artery and vein
• a single pair of large blood vessels enters the
  diaphysis through the nutrient foramen
• Metaphyseal vessels
• supply the epiphyseal cartilage where bone growth
  occurs
• Periosteal vessels provide
• blood to superficial osteons
• secondary ossification centers

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Mature Bones

• As long bone matures
• osteoclasts enlarge marrow cavity
• osteons form around blood vessels in compact bone
• Effects of Exercise on Bone
• Mineral recycling allows bones to adapt to stress
• Heavily stressed bones become thicker and
  stronger
• Bone Degeneration
• Bone degenerates quickly
• Up to 1/3 of bone mass can be lost in a few weeks
  of inactivity

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Bone Formation and Growth

• Figure 69 Heterotopic Bone Formation.

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Effects of Hormones and Nutrition on Bone

• 1. Growth hormone
• Single most important stimulus to the epiphyseal
  plate (dwarfism/gigantism)
• 2. Thyroid hormone
• Moderates growth hormone to insure proper
  proportions of growth
• 3. Sex hormones (estrogens androgens)
• A great rush at puberty growth spurt
• Lead to a breakdown of cartilage that leads to a
  closure of plates steroids!
• 4. Calcitriol
• Made in kidneys synthesis requires
  cholecalciferol
• Helps absorb calcium phosphorus from GI tract

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Additional dietary regulators

• Need adequate calcium, phophorus, magnesium,
  flouride, iron, manganese
• Calcium is necessary for
• Transmission of nerve impulses
• Muscle contraction
• Blood coagulation
• Secretion by glands and nerve cells
• Cell division
• Vitamin D absorption of calcium from GI
• Vitamin C formation of collagen
• Vitamin A stimulates osteoblast activity
• Vitamins K and B12 - help synthesize bone proteins

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Chemical Composition of Bone
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Control of Remodeling

• Two control loops regulate bone remodeling
• Hormonal mechanism maintains calcium homeostasis
  in the blood
• Mechanical and gravitational forces acting on the
  skeleton

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Hormonal (-) feedback mechanism

• 1. PTH (parathyroid hormone)
• Released in response to dropping blood levels of
  calcium
• Stimulates osteoclastsraise blood Ca levels
• Increases absorption from digestive tract
• Causes reabsorption from kidneys
• 2. Calcitonin (secreted by thyroid)
• Causes Ca salts to be deposited in boneinhibits
  osteoclast activity increases calcium excretion
  at kidneys
• Mechanism is designed to maintain blood Ca at
  9-11 mg/100ml
• Ca is vital for nerve conduction muscle
  contraction
• BIG problems w/ breakdown in homeostasis

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Calcium Homeostasis

• Figure 616a Factors That Alter the Concentration
  of Calcium Ions in Body Fluids.

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Hormones for Bone Growth and Maintenance
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Response to Mechanical Stress

• Wolffs Law bone remodels in response to stress
  placed upon it
• A deforming bone produces a minute electrical
  current. (-) on side of compression () on side
  of tension(-) seems to stimulate osteoblasts
  the calcification of bone

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Bone Fractures (Breaks)

• Bone fractures are classified by
• The position of the bone ends after fracture
• The completeness of the break
• The orientation of the bone to the long axis
• Whether or not the bones ends penetrate the skin

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Types of Bone 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
• Comminuted bone fragments into three or more
  pieces common in the elderly
• Spiral ragged break when bone is excessively
  twisted common sports injury
• Depressed broken bone portion pressed inward
  typical skull fracture
• Compression bone is crushed common in porous
  bones
• Epiphyseal epiphysis separates from diaphysis
  along epiphyseal line occurs where cartilage
  cells are dying
• Greenstick incomplete fracture where one side
  of the bone breaks and the other side bends
  common in children

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The Major Types of Fractures
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The Major Types of Fractures
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Fracture Repair Step 1

• Bleeding
• produces a clot (fracture hematoma)
• establishes a fibrous network
• Bone cells in the area die

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Fracture Repair Step 2

• Cells of the endosteum and periosteum
• Divide and migrate into fracture zone
• Calluses stabilize the break
• external callus of cartilage and bone surrounds
  break
• internal callus develops in marrow cavity

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Fracture Repair Step 3

• Osteoblasts
• replace central cartilage of external callus
• with spongy bone

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Fracture Repair Step 4

• Osteoblasts and osteocytes remodel the fracture
  for up to a year
• reducing bone calluses

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Osteoporosis

• Bone reabsorptiongtbone production
• Osteopenia begins between ages 30 and 40
• Women lose 8 of bone mass per decade, men 3
• Decrease in bone mass?increase fracture risk
• Decreased levels of estrogen primarily
• Most important cause of fracture in womengt50
• 35 of bone mass may be gone by age 70
• Vertebrae femur neck are most affected

• Risk Factors
• Body build short women have less bone mass
• Weight thinner at greater risk
• Smoking decreases estrogen levels
• Lack of dietary calcium
• Exercise decrease rate of absorption
• Drugs alcohol, cortisone, tetracycline
• Premature menopause

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Osteopenia

• Figure 619 The Effects of Osteoporosis on Spongy
  Bone.

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Homeostatic Imbalances

• Osteomalacia
• Bones are inadequately mineralized causing
  softened, weakened bones
• Main symptom is pain when weight is put on the
  affected bone
• Caused by insufficient calcium in the diet, or by
  vitamin D deficiency
• Rickets
• Bones of children are inadequately mineralized
  causing softened, weakened bones
• Bowed legs and deformities of the pelvis, skull,
  and rib cage are common
• Caused by insufficient calcium in the diet, or by
  vitamin D deficiency

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Developmental Aspects of Bones

• The embryonic skeleton ossifies in a predictable
  timetable that allows fetal age to be easily
  determined from sonograms
• At birth, most long bones are well ossified
  (except for their epiphyses)
• By age 25, nearly all bones are completely
  ossified
• In old age, bone resorption predominates
• A single gene that codes for vitamin D docking
  determines both the tendency to accumulate bone
  mass early in life, and the risk for osteoporosis
  later in life

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Fetal Primary Ossification Centers at 12 weeks
of age