Skeletal System

1
Skeletal System

• Chapter 7
• Bio160

2
Functions of Bone

• Supports Soft Tissue

• Protection of vital organs such as central
  nervous system housed in cranial cavity and
  vertebral column

• Attachment of skeletal muscles via tendons when
  muscles contract, movement results.

3
Functions of Bone

• Hematopoiesis - manufacture of blood cells in red
  bone marrow (in adults proximal epiphysis of
  humerus and femur ribs, sternum, clavicle, hip
  bones, vertebrae, skull)

• Reservoir of minerals such as calcium and
  phosphorus

• Storage of triglycerides

4
Microscopic Structure of Bone

• Compact Bone

By weight, bone has 25 water, 25 collagen
fibers and 50 calcium phosphate

• Collagen fibers produced by osteoblasts,
  bone-forming cells

• Calcium salts deposit along collagen fibers and
  cement osteoblasts into a small chamber, a lacuna

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Microscopic Structure of Bone
Once osteoblasts become trapped in lacuna, they
are known as osteocytes
Cytoplasmic extensions from osteocytes are
located in canaliculi, small tubes through the
material outside the cell, the extracellular
matrix

• Nutrients reach osteocytes via canaliculi and
  cytoplasmic extensions from Haversian canal

6
Microscopic Structure of Bone
Haversian canal longitudinal tube that forms
the center of microscopic bone structure
Matrix arranged in concentric rings around
Haversian canal called lamella
Haversian canal and surrounding lamella is called
Haversian system (osteon)

• Spongy Bone

7
Bone FormationOssification

• Embryonic skeleton composed of fibrous membrane
  and hyaline cartilage in shape of bones

• Ossification begins 6th week of embryonic life

8
Bone FormationOssification

• Ossification replaces preexisting CT with bone

• Intramembranous formation

occurs in flat bones of skull, part of clavicle
begins at 6 weeks of embryonic life
Osteoblasts secrete collagen within a preexisting
fibrous membrane
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Bone FormationOssification
Spongy bone develops inside the membrane
Surface layers are eventually reconstructed into
compact bone because osteoblasts on surface
reconstruct bone (much of newly formed bone will
be destroyed and reformed)
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Bone FormationOssification

• Endochondral formation

gives rise to all other bones
hyaline cartilage model is first formed which is
replaced by bone
begins at 8 weeks of life
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Bone FormationOssification
Bony collar Outside of cartilage model becomes
calcified cutting off nutrient supply to tissue
underneath

• Cartilage cells begin to die and the interior of
  the bone breaks down and disappears

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Bone FormationOssification
Primary ossification center - blood vessels
invade the center of the bone, where the
cartilage is dying, carrying osteoblasts

• Osteoblasts replace dying cartilage with spongy
  bone, which eventually is remodeled into compact
  bone

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Bone FormationOssification

• The hollow interior of a long bone is formed by
  another bone cell called an osteoclast

• Osteoclasts secrete acids and enzymes that
  dissolve the extracellular matrix of bone

14
Bone FormationOssification
Secondary ossification center - more blood
vessels enter the ends of a bone, bringing with
them osteoblasts which produce spongy bone

• generally secondary centers do not begin until
  after birth

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Bone FormationOssification
after the secondary centers have formed, bone
tissue completely replaces cartilage except in 2
regions

• articular cartilage (prevents damage from bone to
  bone contact within joints)

• epiphyseal plate metaphysis growth plate

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Bone Growth

• Growth in length

Epiphyseal plate
Epiphyseal cartilage stops dividing and is
replaced by bone at puberty with surge of
hormones what remains is the epiphyseal line
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Bone Growth

• Growth in diameter

osteoclasts inside the bone destroy bone lining
the hallow cavity
osteoblasts on surface of bone add new tissue
around outer surface of bone
18
Bone Growth

• Reconstruction remodeling is part of bone
  maintenance old bone is constantly reworked.

Remodeling is replacement of old bone tissue by
new bone tissue like skin.
Initially bone is spongy and is replaced on
outside of bones with compact bone.
Distal end of femur is replaced every 4 months.
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Bone Growth
Value and need for Reconstruction

• Old bone weakened by degeneration of organic
  matrix must be replaced

• Constant exchange of Ca2 (bones store 99 of
  Ca2)

20
Bone Growth

• Bone adjusts to mechanical stress thickens under
  stress

bones of athletes are heavier
movement of teeth in orthodontics involves
reshaping of alveoli by stress applied with braces
21
Bone Growth

• Hormones and vitamins that regulate growth and
  remodeling of bone

Growth hormone and thyroxin - normal bone growth
in young people
Sex hormones - osteoblasts have receptors for sex
hormones
22
Bone Growth

• osteoporosis - with decrease of sex hormones with
  advancing age, porous bones may result more
  common in females because of menopause when
  estrogen production essentially halts

adequate diet which may include Ca2, exercise,
and estrogen replacement may be indicated for
prevention of osteoporosis in females
23
Bone Growth
Parathyroid hormone - increases osteoclast
activity, therefore increases Ca2 in blood
Calcitonin- (from thyroid) increases osteoblast
activity, therefore accelerating deposit of Ca2
into bone blood Ca2 levels decrease
Vitamin D - sun converts cholesterol derivative
into Vitamin D in skin
24
Bone Growth

• Vitamin D is needed to absorb Ca2 from intestine
  (Vitamin D aids in synthesis of a carrier protein
  molecule that is needed to transport Ca2)

Vitamin C - promotes synthesis of collagen