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Bone
I. Structure Review
A. Compact bone vs. spongy bone
1. Compact- dense matrix, on surface of bone, resists pressure from one
general direction. Lacunae, canaliculi & lamellae (see below, under “bone cells”)
arranged in concentric circles, forming units called osteons. Blood vessels travel
through a central canal in the center of each osteon.
2. Spongy- matrix bundled into struts called trabeculae, inner part of bone,
resists pressure from many directions, not organized into osteons. Forms in
branches, called trabeculae. Blood vessels extend between trabeculae. Red
marrow common in spongy bone.
B. Coverings
1. Periosteum- contains a fibrous, dense irregular connective layer and a
cellular layer composed of bone cells. The dense irregular connective layer is an
extension of the deep fascia. The cellular layer contacts the bone. Collagen fibers,
"Sharpey's fibers," tie the periosteum to the bone.
2. Endosteum- a layer of bone cells that lines the inner part of bones:
marrow cavities & trabeculae.
C. The matrix: collagen (strong & flexible) laid down by calcium salts (strong and
brittle) creates a strong, non-brittle structure. Calcium salts: Calcium phosphate, calcium
hydroxide, calcium carbonate. Calcium phosphate & calcium hydroxide combine to form
the complex salt, hydroxyapatite. Other minerals (ex, magnesium) are also part of the
matrix crystals.
II. Histology Review
A. Bone cells- what they do and where they live
1. Osteoprogenitor cells- give rise to osteoblasts. Important in repair,
growth & restructuring of bone. Located in periosteum & endosteum.
2. Osteoblasts- produce & secrete the organic matrix (collagen). Collect
and deposit calcium salts upon collagen. Located in periosteum & endosteum.
Give rise to osteocytes.
3. Osteocytes- mature bone cells. In a growing bone, as osteoblasts
produce matrix, the matrix grows around them and they become encapsulated.
Now they are considered osteocytes. Osteocytes live in cavities of the matrix
called lacunae. Osteocytes send extensions through tunnels in the matrix called
canaliculi. Neighboring osteocytes communicate with each other through gap
junctions at the ends of their cytoplasmic extensions. Layers of calcified matrix
between canaliculi are called lamallae. Osteocytes maintain the matrix locally,
breaking it down and building it up.
4. Osteoclasts- dissolve the bone matrix. Not related in origin to other
bone cells. Located in periosteum & endosteum.
B. Remember that bones are constantly being remodeled; that dynamic nature is
because of the activity of the above listed bone cells. When the activity of osteoblasts
outpaces that of osteoclasts, bones grow, and vice-versa.
III. Bone remodeling and repair- bones are constantly being remodeled: built up &
broken down.
A. Repair- when bones are fractured, a blood clot forms around the injured area to
reduce blood loss. Fibroblasts and Osteo -cytes, -blasts, and -progenitors undergo mitosis
and daughter cells migrate to injured area. Some fibroblasts differentiate to chondrocytes
and begin building cartilage around the injured area. Dead cells, pathogens and debris are
cleaned up by macrophages. Over time, the cartilage is replaced by bone. Remodeling of
the affected area may continue for months to years.
B. Factors affecting bone remodeling:
1. Hormones
[Hormones that regulate blood levels of calcium]
a. Calcitriol (active V-D)- required for adequate dietary absorption
of Calcium
b. Parathyroid Hormone (PTH)- stimulates osteoclasts to dissolve
bone & release calcium to blood
c. Calcitonin- inhibits osteoclast activity
[Hormones that are age related and may affect bones indirectly, for example, by affecting
general metabolic rates of cells]
d. Growth Hormone and Thyroxine (thyroid hormone)- stimulate
osteoblast activity, importance in this respect declines after
adolescence.
e. Sex hormones (estrogen, testosterone)- support bone
maintenance in adults. Women's estrogen levels decline more
abruptly (after menopause) than men's testosterone, which may
account for the higher incidence of osteoporosis in women (it does
affect men).
2. Nutrients (these are among the most critically relevant to bone, but
certainly many other nutrients can affect bone either directly or
indirectly)a. Minerals- calcium, phosphorus, magnesium
b. Vitamins- C (required for collagen production), K (importance
in bone health only recently recognized), D (active vitamin D is
calcitriol)
c. Protein
3. Physical stress to bones (weight-bearing exercise)- both compression on
bones and pulling from muscles. Stimulates osteoblast activity.