Download Osteoblasts

“Cartilage and Bone
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Cartilage
Embryo
More
prevalent than in
adult
Skeleton initially mostly
cartilage
Bone replaces cartilage
in fetal and childhood
periods
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Now about bones…like other connective tissue:cells separated by
extracellular matrix with collagen but also mineral crystals
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Bone
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1.
2.
3.
Remember the three germ tissues
Ectoderm - epithelial
Endoderm - epithelial
Mesoderm is a mesenchyme tissue
Mesenchyme cells are star shaped and do
not attach to one another, therefore migrate
freely
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Bones
Functions
Support
Movement:
muscles attach by tendons and use bones
as levers to move body
Protection
Skull – brain
Vertebrae – spinal cord
Rib cage – thoracic organs
Mineral
Calcium and phosphorus
Released as ions into blood as needed
Blood
storage
cell formation and energy storage
Bone marrow: red makes blood, yellow stores fat
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Chemical composition of bones
Cells, matrix of collagen fibers and ground
substance (organic: 35%)
Contribute
to the flexibility and tensile strength
Mineral crystals (inorganic: 65%)
Primarily
calcium phosphate calciumhydroxid
Lie in and around the collagen fibrils in
extracellular matrix
Contribute to bone hardness
Small amount of water
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Classification of bones by shape
Long bones
Short bones
Flat bones
Irregular
bones
Pneumatized
bones
Sesamoid
bones
(Short bones include sesmoid bones)
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Gross anatomy of bones
Compact bone
Spongy (trabecular)
bone
Blood vessels
Medullary cavity
Membranes
Periosteum
Endosteum
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Periosteum
Connective tissue membrane
Covers entire outer surface of bone except at epiphyses
Two sublayers
1. Outer fibrous layer of dense irregular connective tissue
2. Inner (deep) cellular osteogenic layer on the compact bone
containing osteoprogenitor cells (stem cells that give rise to
osteoblasts)
Osteoblasts: bone depositing cells
Also osteoclasts: bone destroying cells (from the white blood cell line)
Secured to bone by perforating fibers (Sharpey’s fibers)
Endosteum
Covers the internal bone surfaces
Is also osteogenic
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Structure of a Long Bone
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Figure 6.3a-c
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Flat, Short, Irregular Bones
Flat Bones
(Spongy bone)
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No diaphyses or
epiphyses
Have bone marrow but no
marrow cavity
A sandwich of (spongy)
bone between compact
bone
Composed of bony plates
known as trabeculae
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(Spongy) Bone
Trabeculae: interconnecting rods or plates of bone.
Like scaffolding.
Spaces filled with marrow.
Covered with endosteum.
Oriented along stress lines PÅL 2011
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Compact Bone
Composed of osteons (haversarian system):
Basic unit of mature compact bone
Osteocytes: mature bone cells arranged in
concentric lamellae (layers)
Surround a central canal containing blood vessels
(deliver nutrients remove waste)
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Internal structure
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Compact Bone
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Compact
bone
Osteons:
Lamellae:
concentric
tubes
Haversian
canals
Osteocytes
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Long bones
Tubular diaphysis
or shaft
Epiphyses at the ends: covered with “articular”
(=joint) cartilage
Epiphyseal line in adults
Kids:
epiphyseal growth plate (disc of hyaline
cartilage that grows to lengthen the bone)
Blood vessels
Nutrient
arteries and veins through nutrient foramen
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Bone Cells
Osteoblasts
Osteocytes
Osteoclasts
Stem
cells or osteochondral progenitor
cells
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Bone Cells
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Bone Cells
Osteoblasts
Active in bone formation, a
process known as ossification or
osteogenesis.
Collagen produced by E.R. and
golgi. Released by exocytosis
Precursors of hydroxyapetite
stored in vesicles, then released
by exocytosis.
Osteocytes
Essentially osteoblasts that have been
trapped in the matrix
Mature bone cells commonly found in
Iacunae
Carry out the normal metabolic
processes of bone
Found in compact and spongy bone
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Osteoclasts and Stem Cells
Osteoclasts.
Cells used to breakdown bone (bone resorption)
Ruffled border: where cell membrane borders bone and
resorption is taking place.
H+ ions pumped across membrane, acid forms, breaks down
mineral salts (demineralization)
Release enzymes that digest the organic proteins of matrix
Derived from monocytes (which are formed from stem cells in
red bone marrow)
Multinucleated and probably arise from fusion of a number of
cells
Stem Cells. Mesenchyme (Osteochondral
Progenitor Cells) become chondroblasts or
osteoblasts.
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Osteoclast – A Bone-Degrading
Cell
A giant cell with many nuclei
Crawls along bone surfaces
Breaks down bone tissue
Secretes
concentrated
hydrochloric acid
Lysosomal
enzymes are
released
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Figure 6.13a
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Bone markings reflect the stresses
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Bone development
Osteogenesis: “formation of bone”
From
osteoblasts
Bone tissue first appears in week 8 (embryo)
Ossification: “to turn into bone”
Intramembranous
ossification
forms directly from mesenchyme (not modeled first in
cartilage)
Most skull bones except a few at base
Clavicles (collar bones)
Sesamoid bones (like the patella)
Endochondral
ossification:
modeled in hyaline cartilage then replaced by bone
tissue
All the rest of the bonesPÅL 2011
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Intramembranous ossification:(osteoid is the organic part)
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Endochondral ossification
Modeled in hyaline cartilage, called cartilage
model
Gradually replaced by bone: begins late in second
month of development
Perichondrium is invaded by vessels and
becomes periosteum
Osteoblasts in periosteum lay down collar of bone
around diaphysis
Calcification in center of diaphysis
Primary ossification centers
Secondary ossification in epiphyses
Epiphyseal growth plates close at end of
adolescence
Diaphysis and epiphysis fuse
No more bone lengthening
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See next slide
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Endochondral ossification
Stages 1-3 during fetal week 9 through 9th
month
Stage 4 is just
before birth
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Stage 5 is process
of long bone growth
during childhood &
adolescence 28
Epiphyseal growth plates in child, (a), and lines in
adult, (b) (see arrows)
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Factors regulating bone growth
Vitamin D: increases calcium from gut
Parathyroid hormone (PTH): increases
blood calcium (some of this comes out of
bone)
Calcitonin: decreases blood calcium
(opposes PTH)
Growth hormone & thyroid hormone:
modulate bone growth
Sex hormones: growth spurt at adolescense
and closure of epiphyses
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Bone remodeling
Osteoclasts
Bone
Osteoblasts
Bone
resorption
deposition
Triggers
Hormonal:
parathyroid hormone
Mechanical stress
Osteocytes are transformed osteoblasts
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Terms (examples)
chondro refers to cartilage
chondrocyte
endochondral
perichondrium
osteo refers to bone
osteogenesis
osteocyte
periostium
blast refers to precursor cell or one that
produces something
osteoblast
cyte refers to cell
osteocyte
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Bone Development
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
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Repair of bone fractures (breaks)
Simple and compound fractures
Closed and open reduction
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The Skeleton Throughout Life
Cartilage grows quickly in youth
Skeleton shows fewer chondrocytes in the
elderly
Bones are a timetable
Mesoderm
– gives rise to embryonic
mesenchyme cells
Mesenchyme – produces membranes and
cartilage
Membranes and cartilage ossify
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The Skeleton Throughout Life
Skeleton grows until the age of 18–21 years
In children and adolescents
Bone
formation exceeds rate of bone
reabsorption
In young adults
Bone
formation and bone reabsorption are in
balance
In old age reabsorption predominates
Bone mass declines with age
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