G. Dynamic Tissue

G. dynamic tissue Bone is a very dynamic tissue, unlike the bones and plastic bones we have in lab. The osteoclasts are constantly resorbing bone (breaking down the bone matrix to release 2+ 3- 2+ Ca and PO4 ) and osteoblasts are constantly making bone matrix (storing Ca and 3- PO4 and other minerals in the form of mainly hydroxyapatite crystals). When ossification rate = bone resorption rate the bone “appears” to have one fixed shape and thickness. However, various conditions can change the ossification and/or bone resorption rate for part of the bone or for the whole bone.

H. problems 1. fractures – When a bone fractures it would often breaks osteons and tear the blood vessels in the Haversian canals, causing bleeding. That is why the first phase of a fracture is the appearance of a hematoma (blood swelling). While macrophages clear out the blood clot, osteoblasts in the periosteum and endosteum at the fracture are stimulated to undergo mitosis to form more osteoblasts and to lay down collagen fibers, chondroitin sulfate, and hyaluronic acid forming a cartilaginous callus (2nd phase of a fracture). Osteoclasts then comes in to remove the cartilage while osteoblasts lay down hydroxyapatite crystals forming a bone callus (3rd phase of a fracture). This would be even thicker and stronger than the bone was originally. After a bone callus is form the osteoclasts and osteoblasts keep working on the bone callus as they do the rest of the bone, remodeling (4th phase of a fracture) the callus until it gets back to nearly the same size as the original bone before the fracture, but it usually will remain slightly thicker than the original bone at that site.

2. Vitamin D deficiency – a. consequence: Vitamin D is necessary to make transport molecules in the epithelium of the small intestines allowing these epithelial cells to absorb Ca2+. If there is a deficiency in vitamin D then no matter how much vitamin the person takes in his/her diet most will not be absorbed. This means that the Ca2+ will not be brought into the cardiovascular system and would not be available for the body to use. The Ca2+ will simply continue down the digestive tract and out of the body with the feces. This would lead to a deficiency in Ca2+ which means ossification would be decreased.

b. Rickets: As a result of this vitamin D deficiency causing a Ca2+ deficiency a child’s bones (which are mainly cartilage) will not be able to ossify (endochondral ossification) although the rest of the organs of body of the child continues to grow. With the added weight due to the growth of the child’s body his/her “bones” which are mostly cartilage would bend since it is not as supportive as bone tissue. This “soft” bone condition is referred to as rickets.

c. Osteomalacia: In adults, since their bones have already been fully formed, the lack of Ca2+ due to vitamin D deficiency would inhibit ossification process but bone resorption will continue. This would result progress loss of bone material in the skeletal system, making the bone more porous. This condition looks just like osteoporosis but the causes are different. 3. Osteoporosis The integrity is mostly due to the balance between the ossification rate countering the bone resorption rate. There are three basic factors that contribute to normal healthy ossification rate. They are: a) physical stress on the bone: Whether it is a muscle pulling on the periosteum of a bone or the weight of the body bearing down on the whole bone, this physical tension or compression stimulates osteoblasts to lay down more bone. b) Sex hormones: Both testosterone and estrogen stimulates ossification rate by stimulating osteoblasts. Testosterone has a greater stimulatory effect on osteoblasts than does estrogen, but both increases ossification rate. c) Availability of Ca2+ : There must be ample amount of Ca2+ to allow osteoblasts to lay down hydroxyapatite crystals. A normal healthy dietary intake of Ca2+ will provide osteoblasts with the necessary inorganic material to make bone matrix.

With these 3 factors at an optimum, individual will have healthy strong bones. However, as people reach their middle age, three changes will typically occur that will hamper ossification. 1) As we get older many individuals become more sedentary. At work they may be a desk or computer hour upon hour. When they come home they may go on the computer or use their entertainment system. 2) When women and men enter menopause, the level of sex hormones decreases. 3) As we age our intestines also age and become less efficient in absorbing Ca2+. This means a lower availability of Ca2+ for the cells of your body.

These three situations result in a decrease in ossification rate such that it is slower than bone resorption rate. This condition of losing bone because of the above is referred to as osteoporosis.

To counter osteoporosis, individuals are encouraged to 1) remain active, especially doing weight-bearing exercise like walking or jogging, even carrying/moving weights. 2) Possibly consider Hormone replacement therapy. This is taking estrogen supplement (or testosterone, in the case of male) to stimulate bone growth. This, however, does have a down side. Estrogen tends to stimulate the growth of breast cancer cells, if you have any in your body. 3) Increase your dietary intake of Ca2+ and vitamin D. The normal intake that was sufficient when you are a young adult is not enough later in life because of the decrease in the small intestine’s ability to absorb Ca2+ .