Physiology of Bone
Composition of Bone
1) Organic matrix 30%
A. Collagen fibers 90 to 95%
B. Ground substance (homogeneous
gelatinous medium)
-
- Extracellular fluid
- Proteoglycans (chondroitin
sulfate and hyaluronic acid)
2) Bone salts 70%
The crystalline salts present in bones are
called hydroxyapatites, which contain
calcium and phosphate. Apart from these,
some other salts like sodium, potassium,
magnesium and carbonate are also present
in the bone.
Collagen fibers-provide tensile strength
Hydroxyapatites-provide compression strength
Hydroxyapatite Does Not Precipitate in Extracellular Fluid
Despite Supersaturation of Calcium and Phosphate Ions.
Why??
• Presence of Pyrophosphate-an inhibitor
• Therefore, hydroxyapatite crystals fail to precipitate in
normal tissues except in bone despite the state of
supersaturation of the ions.
Precipitation of Calcium in Nonosseous Tissues Under Abnormal Conditions
• However under abnormal conditions,
calcium precipitate in arterial walls
in arteriosclerosis and cause the arteries to
become bonelike tubes.
• Likewise, calcium salts frequently deposit in
degenerating tissues or in old blood clots.
• Presumably, in these instances, the inhibitor
factors that normally prevent deposition of
calcium salts disappear from the tissues,
thereby allowing precipitation.
Type Of Cells In The Bone
• Bone has three major types of cells:
1. Osteoblasts -concerned with bone formation (osteoblastic activity).
2. Osteocytes –concerned with the exchange of calcium between the
bone and ECF.
3. Osteoclasts-concerned with bone resorption (osteoclastic activity).
OSTEOBLASTS
Bone Deposition
- Secretion of collagen molecules (called collagen monomers) and ground substance (mainly proteoglycans) by osteoblasts.
- The collagen monomers polymerize rapidly to form collagen fibers; the resultant tissue becomes osteoid
- Some of the osteoblasts become entrapped in the osteoid
- Entrapped osteoblast become quiescent. At this stage they are called osteocytes.
- Calcium salts begin to precipitate on the surfaces of the collagen fibers.
- After period of weeks to month, these salts are converted into the hydroxyapatite crystals.
- New bone formed
Osteoblasts also produce osteoprotegerin (OPG), that
inhibit differentiation of preosteoclasts into mature
osteoclasts that resorb bone.
Repair of a Fracture (stress) Activates Osteoblasts
Fracture repair ——holding of end of broken bone by fixation apparatus——patient use the bone —–stress on opposed end of broken bones—–activates osteoblasts + cause increase formation of new osteoblast—-Thus accelerate the rate of fracture healing.
OSTEOCLASTS
PTH binds to receptors on the osteoblasts
Stimulate Osteoblast to release cytokines-osteoprotegerin ligand (OPGL), which is also called RANK ligand
Inhibit OPG released by osteoblasts.
OPGL cause causing differentiation of proosteoclast cells to mature multinucleated osteoclasts.
The mature osteoclasts send out villus-like projections toward the bone, forming a ruffled border adjacent to the bone
The villi secrete
the citric and lactic acids- cause dissolution of the bone salts
The enzymes digest or dissolve the organic matrix of the bone
(bone resorption).
Bone Deposition and Absorption Are Normally in Equilibrium
• Normally, the rates of bone deposition and
absorption are equal to each other, so the
total mass of bone remains constant except
in growing kids.
• Osteoclasts eats away the bone creating a
tunnel.
• Now, the osteoclasts disappear and the
tunnel is invaded by osteoblasts instead;
then new bone begins to develop.
• The new bone being laid down in successive
layers of concentric circles (lamellae) on the
inner surfaces of the cavity until the tunnel
is filled.
Importance of Bone Deposition and Absorption-continual bone remodeling
• First, bone adjusts its strength according to the degree of bone stress e.g.bones thicken when subjected to heavy loads.
• Second, bone adjust its shape according to stress pattern for proper support of mechanical forces.
• Third, because old bone becomes relatively brittle and weak, new organic matrix is needed as the old organic matrix degenerates.