The skeleton is a metabolically
active organ that undergoes continuous remodeling throughout life. Bone
remodeling involves the removal of mineralized bone by osteoclasts followed by the
formation of bone matrix through the osteoblasts that subsequently become
mineralized. The remodeling cycle consists of three consecutive phases:
resorption, during which osteoclasts digest old bone; reversal, when
mononuclear cells appear on the bone surface; and formation, when osteoblasts
lay down new bone until the resorbed bone is completely replaced. Bone remodeling serves to adjust bone
architecture to meet changing mechanical needs and it helps to repair micro
damages in bone matrix preventing the accumulation of old bone.
Bone metabolism is
regulated by many systemic hormones and locally acting
substances. Systemic hormones can be divided into two groups: hormones
affecting the metabolism of calcium and phosphates and other
systemic hormones.
Hormones affecting the metabolism of calcium and
phosphates
In the human
body parathormone, calcitriol and calcitonin play a
key role in the regulation of calcium and phosphate content.
1)Parathormone
Parathormone stimulates
the resorption of bone by increasing the activity of osteoclasts (stimulates
transformation of monocytes to osteoclasts). The result is an increased
release of Ca2+ and phosphates from the bone.
2)Calcitonin
Calcitonin inhibits
osteoclast activity (inhibits transformation of monocytes to osteoclasts),
thereby reduces bone resorption and it results in an increased
deposition of Ca2+ in bones.
3) Calcitriol (1,
25-dihydroxycholecalciferol, vitamin D derivative)
Calcitriol stimulates synthesis
of proteins allowing the absorption
of Ca2+ and phosphates in the small intestine. Thus calcitriol
ensures the availability of Ca2+ and phosphates for
bone formation. Simultaneously activates osteoblasts to synthesize
collagen.
Other systemic hormones
1)Estrogens
Estrogens act
like calcitonin. Therefore they inhibit bone resorption – inhibit
osteoclast activity through local factors.
2)Growth
hormone
Growth
hormone stimulates bone formation mediated through the formation of
local growth factors – insulin-like growth
factors (somatomedins – IGF-1 and IGF-2).
3)Insulin
Insulin increases
the synthetic activity of osteoblasts. Bone loss can occur in an untreated
diabetes mellitus.
4)Glucocorticoids
Glucocorticoids reduce
the formation of new bone (they inhibit proteosynthesis in osteoblasts and
their differentiation).
5)Thyroid
hormones
Thyroid
hormones stimulate osteoclasts, activate bone remodeling. At their
overproduction an osteoporosis may develop.
Local factors regulating bone metabolism
1) Factors
stimulating osteoblast differentiation
a) Bone
morphogenic factor (BMF)
b) Insulin-like
growth factor 1 (IGF-1)
c) Platelet-derived growth factor (PDGF)
d) Fibroblast
growth factor (FGF)
2) Factors
stimulating osteoclast differentiation
a) M-Colony
stimulating factor (M-CSF)
b) Interferons
(INF-γ) and interleukins (IL-1, IL-6 etc.)
Bone Related Diseases
Osteoporosis
Osteoporosis
is systemic bone disease characterized by low bone
density and altered bone microarchitecture with
consequent increase in fragility. Its substrate is the loss of
inorganic and organic bone matter which leads to increased fracture
risk. Osteoporosis is divided into two types – primary
osteoporosis (cause is unknown, e.g. postmenopausal or senile
osteoporosis) and secondary osteoporosis (cause is known, e.g.
endocrine problems, medicaments, immobilization).
Fasciitis
Fasciitis is an inflammation of
the fascia, which is the connective tissue surrounding muscles,
blood vessels and nerves, an inflammation of the connective tissue that may be caused by streptococcal or other types of infection, an injury, or anautoimmune reaction, an abnormal benign growth (pseudosarcomatous fasciitis) resembling a tumor that develops in the subcutaneous oraltissues, usually in the cheek. Commonly growing rapidly and then regressing, it consists of young fibroblasts and many
capillaries.
Arthritis
Arthritis is a medical condition that damages the body’s
joints, causing discomfort and pain. It can range from mild to severe and can
affect people of all ages. Arthritis is not one disease alone, but an umbrella
term for more than 100 conditions that affect the joints of the body. Joints are points where two or more bones meet,
such as in the wrist, knuckles, hips, knees and ankles.
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