Objective Six- Describe briefly the process of bone formation in the fetus and summarize the events of bone remodeling throughout life
Thirteen weeks after conception, the fetus begins to develop bones. They begin with 300 bones, and end up with 206 bones. At first, the bones are not actually bones. They are cartilage that hardens to become bone with the assistance of calcium. This process of bone hardening is called ossification. During childhood, bones grow very differently. Long bones (such as the bones in your arms or legs) grow lengthwise. Flat bones (bones in your skull) grow expand outward. Bone also continuoulsy undergoes remodeling, replacing the old bone with new bone. Remodeling also allows bone to reapond to changes in mechanical forces, such as pregnancy or exterme weight gain or loss.
Bone remodeling cycle
Two main types of cells are responsible for bone renewal: the osteoblasts involved in bone formation and the osteoclasts involved in bone resorption.
Bone Remodeling
The renewal of bone is responsible for bone strength throughout our life. Old bone is removed (resorption) and new bone is created (formation). During childhood and the beginning of adulthood, bone becomes larger, heavier and denser, bone formation is then more important than bone resorption.
The Bone Mass actually increases until the age of 20, 25 where it reaches its maximum value: the Peak Bone Mass in denisty and strength. The higher the Peak Bone Mass is, the lower the risk of osteoporosis is. Bone Mass remains stable for a few years (for women, till about 45 years old). After a certain age, bone mass starts to decrease.
Bone remodeling cycle
Two main types of cells are responsible for bone renewal: the osteoblasts involved in bone formation and the osteoclasts involved in bone resorption.
Osteoblasts
Osteoblasts are responsible for bone matrix synthesis. They secrete a collagen rich ground substance essential for later mineralization of hydroxyapatite and other crystals. Osteoblasts cause calcium salts and phosphorus to precipitate from the blood, these minerals bond with the newly formed osteoid to mineralize the bone tissue. The osteoblasts also have estrogen receptors. Estrogens can actually increase the number of osteoblasts, increasing therefore collagen production.
Osteocytes
Osteoblasts Osteocytes maintain bones, they play a role in controlling the extracellular concentration of calcium and phosphate, and are directly stimulated by calcitonin and inhibited by PTH (Parathyroid hormone). Their exact role is actually still to be defined.
Osteoclasts
These cells derive from bone marrow mononuclear cells. Their characteristic feature is a ruffled edge where active resorption takes place. The osteoclasts secrete bone-reabsorbing enzymes, which digest bone matrix. The mode of differentiation, recruitment and inhibition is controlled by numerous hormonal and growth factors. The osteoclasts also have estrogen receptors (Estrogens can inhibit their recruitment).
Regulation of bone remodeling
Bone formation and resorption are influenced by many factors like:
Parathyroïd Hormone (PTH): The PTH can increase the recruitment and the cavity of osteoblasts and osteoclasts. Therefore if the PTH secretion is too high, there is an acceleration of the bone turnover. If the increase occurs along a Vitamin D deficiency, the bone cycle is accelerated and entails bone loss.
Vitamin D: Vitamin D tends to increase the recruitment of osteoclasts, and also plays a part in the mineralization of bone matrix. A lack of Vitamin D results in osteomalacia (impaired mineralization) and too much Vitamin D entails bone loss.
Many other important factors are involved in bone remodeling such as estrogens, calcitonin, glucocorticoïds, progesterone, androgens... Other factors, like the impact of mechanical constraints, the genetic background are described in the "Risk Factors" section.
Bone remodeling cycle
Two main types of cells are responsible for bone renewal: the osteoblasts involved in bone formation and the osteoclasts involved in bone resorption.
- Activation: Preosteoclasts are stimulated and differentiated under the influence of cytokine and growth factors to mature into active osteoclasts.
- Resorption: Osteoclasts digest mineral matrix, old bone.
- Reversal: End of resorption
Bone Remodeling
The renewal of bone is responsible for bone strength throughout our life. Old bone is removed (resorption) and new bone is created (formation). During childhood and the beginning of adulthood, bone becomes larger, heavier and denser, bone formation is then more important than bone resorption.
The Bone Mass actually increases until the age of 20, 25 where it reaches its maximum value: the Peak Bone Mass in denisty and strength. The higher the Peak Bone Mass is, the lower the risk of osteoporosis is. Bone Mass remains stable for a few years (for women, till about 45 years old). After a certain age, bone mass starts to decrease.
Bone remodeling cycle
Two main types of cells are responsible for bone renewal: the osteoblasts involved in bone formation and the osteoclasts involved in bone resorption.
- Activation: Preosteoclasts are stimulated and differentiated under the influence of cytokine and growth factors to mature into active osteoclasts.
- Resorption: Osteoclasts digest mineral matrix, old bone.
- Reversal: End of resorption
- Formation: Osteoblasts are responsible for bone matrix synthesis (collagen). Two other non-collagenous proteins are also formed: ostocalcin and osteonectin.
Osteoblasts
Osteoblasts are responsible for bone matrix synthesis. They secrete a collagen rich ground substance essential for later mineralization of hydroxyapatite and other crystals. Osteoblasts cause calcium salts and phosphorus to precipitate from the blood, these minerals bond with the newly formed osteoid to mineralize the bone tissue. The osteoblasts also have estrogen receptors. Estrogens can actually increase the number of osteoblasts, increasing therefore collagen production.
Osteocytes
Osteoblasts Osteocytes maintain bones, they play a role in controlling the extracellular concentration of calcium and phosphate, and are directly stimulated by calcitonin and inhibited by PTH (Parathyroid hormone). Their exact role is actually still to be defined.
Osteoclasts
These cells derive from bone marrow mononuclear cells. Their characteristic feature is a ruffled edge where active resorption takes place. The osteoclasts secrete bone-reabsorbing enzymes, which digest bone matrix. The mode of differentiation, recruitment and inhibition is controlled by numerous hormonal and growth factors. The osteoclasts also have estrogen receptors (Estrogens can inhibit their recruitment).
Regulation of bone remodeling
Bone formation and resorption are influenced by many factors like:
Parathyroïd Hormone (PTH): The PTH can increase the recruitment and the cavity of osteoblasts and osteoclasts. Therefore if the PTH secretion is too high, there is an acceleration of the bone turnover. If the increase occurs along a Vitamin D deficiency, the bone cycle is accelerated and entails bone loss.
Vitamin D: Vitamin D tends to increase the recruitment of osteoclasts, and also plays a part in the mineralization of bone matrix. A lack of Vitamin D results in osteomalacia (impaired mineralization) and too much Vitamin D entails bone loss.
Many other important factors are involved in bone remodeling such as estrogens, calcitonin, glucocorticoïds, progesterone, androgens... Other factors, like the impact of mechanical constraints, the genetic background are described in the "Risk Factors" section.
Objective Seven- Name and descrive the various types of fractures.
Transverse- break is a straight line across the bone.
Oblique- diagonal break across the bone.
Spiral-Break spirals around bone, as seen in twisted injuries.
Comminuted (segmental)- Divides a long bone into more than two roughly cylindrical segments
Avulsed- a fragment of bone tears away form the main mass of bone
Impacted- one broken edge is wedged into the other end
Greenstick (torus) - incomplete; broken bone is not completely seperated.
Compression- bone completely destroyed
*closed (simple)-the bone is broken, but the skin is intact
*open (compound)- the bone exits and is visible through the skin, or a deep wound that exposes the bone through the skin.
Oblique- diagonal break across the bone.
Spiral-Break spirals around bone, as seen in twisted injuries.
Comminuted (segmental)- Divides a long bone into more than two roughly cylindrical segments
Avulsed- a fragment of bone tears away form the main mass of bone
Impacted- one broken edge is wedged into the other end
Greenstick (torus) - incomplete; broken bone is not completely seperated.
Compression- bone completely destroyed
*closed (simple)-the bone is broken, but the skin is intact
*open (compound)- the bone exits and is visible through the skin, or a deep wound that exposes the bone through the skin.