Bone Development Research Paper

Skeletal Development. Bone is a living tissue and from birth until death, it is constantly undergoing remodeling processes to maintain integrity and mineral composition. Remodeling is a process that is tightly regulated through the coordination of osteoclasts, which regulate bone resorption and osteoblasts, which mediate bone formation.

Osteoclasts are the cells responsible for bone resorption. The formation and activation of osteoclasts is mediated by the ratio of RANKL to osteoprotegerin (OPG), interleukin-1 (IL-1), interleukin-6 (IL-6), colony-stimulating factor (CSF), parathyroid hormone (PTH), 1,25-dihydroxyvitamin-D, and calcitonin (15, 18). Activated osteoclasts use integrin receptors beta-1 and αvβ3 found on their membrane to attach to bone by connecting to bone matrix peptides by binding to collagen, fibronectin, and laminin while αvβ3 binds to osteopontin and sialoprotein (19). Once bound to bone, osteoclasts form two polarized structures, which give them the ability to degrade bone tissue. This occurs through the

proliferation of acidified vesicles that help form a ruffled border that will secrete hydrogen ions to produce a more acidic environment, lowering the pH to about 4.5, allowing for the exocytoses of cathepsin K, an enzyme responsible for the break down of bone and cartilage by catabolizing elastin, collagen, and gelatin (20). Furthermore, binding with the bone also stimulates osteoclasts to polarize its fibrillar actin into a circular structure called the “actin ring” or “sealing zone,” which envelops the recently acidified environment. Evidence shows that inhibition of both the ruffled border and/or the sealing zone will impede bone resorption (21).
After bone resorption, there is a reversal phase where the bone cells transition to begin bone formation.

During this phase, preosteoblasts are recruited and transform into osteoblasts and will migrate to the bone surface to begin bone formation. Once production begins, transforming growth factor-beta (TGF-β) is released from the matrix and will decrease production of RANKL by osteoblasts, thereby inhibiting osteoclast activity (22).

In order for bone formation to occur, osteoblasts must create new collagenous and non-collagenous substance, for the matrix while also monitoring mineralization of the matrix by ensuring proper calcium and phosphate deposits (22). They also produce high amounts of type 1 collagen, whose purpose is to fill the hollow parts of the bone made during the resorption phase. Throughout this process, various osteoblasts become embedded within the bone matrix, and become osteoclasts (23).

Factors Related to Bone

Mass
Because bone health is of great concern, determining factors that influence bone mass is of great interest. Studies have identified a number of non-modifiable factors including: genetics, gender, and race with various modifiable factors including physical activity and diet that contribute to one’s bone mass.
Non-modifiable Factors. Evidence indicates that genetics play a pivotal role in bone mass and BMD. Studies have identified single-nucleotide polymorphisms (SNPs) that contribute to BMD, osteoporosis and osteoporotic-related bone fractures (24). Sigurdsson and associates demonstrated the heritability of BMD, demonstrating that genetics contribute to bone mass (25).
Gender also has proven to influence BMD. Studies suggest that after adolescence, women will consistently have less BMD than men (26). This may be attributed to the differences in estrogen and testosterone levels in the sexes. Estrogen has been shown to stimulate bone formation and because there is a significant decease in estrogen levels in post-menopausal women, they are more likely to see a significant decrease in bone loss (27). Furthermore, lean body mass has a positive relationship with total body bone mass, suggesting that because females usually have less lean body mass, they are more likely to have less bone mass than men (28).
Race has also proven to be an influential factor in bone mass. At all ages, African Americans have higher bone mass than Caucasians (26). Liel and associates showed that African American women have significantly higher BMD than Caucasian women, even when controlling for height and weight. Bone width is also greater in African Americans when compared to Caucasians (29).
Although there is scant information comparing Asian and Caucasian bone mass, studies that have been done show that BMD is less in Asians. This, however, may be attributed to a lower dairy intake (30).
Modifiable Factors. Physical activity and diet appear to be the only modifiable factors that may increase bone mass. Exercise, particularly strength-training exercise, has been shown to have beneficial effects on both health and for the prevention of osteoporosis (31). Because bone gain is most prominent during adolescence, physical activity during these years significantly influences status of bone health later in life (32). In postmenopausal women, regular physical activity has been shown to be effective in maintaining and building muscle strength and preventing falls, which decreases fracture risk. Although there has been scant research done on the role of exercise in young adult women, studies suggest it has beneficial effects on bone status (33).
Diet and nutrition have also been shown to play a major role in bone mass acquisition. Researchers speculate that the degree of osteoporosis can be reduced by about 50% through proper nutrition. Bone is used as a storage compartment for minerals and various nutrients are needed for proper bone health through the synthesis and activation of specific enzymes, hormones, and bone cells (34).
Inadequate nutrition during the early stages of life negatively influences bone mass. Bone growth and development increase nutritional requirements, making childhood and adolescence the most crucial period for adequate nutrition because it is during this time that peak bone mass is heightened (35).
Poor nutritional status has also been shown to increase the risk of osteoporosis in the elderly. Salminen and associates demonstrated that elderly women who were either malnourished or at risk for being malnourished had a twofold increased risk of having osteoporosis (36). A similar study was done and also concluded that malnutrition increases one’s risk for osteoporosis (37).
Nutrients and Bone Health. Nutrients are important modifiable factors that contribute to bone mass and play a role in the prevention of osteoporosis. Although 80-90% of bone mineral content is made up of calcium and phosphorus, studies have shown that other nutrients also may play a significant role including zinc, phosphorus, and magnesium (38). Despite the fact that abundant research has been done on calcium, vitamin D, and the synergistic effects of both on bone health, minimal research has been done to discover the effects of additional micronutrients.
Calcium. On average, about 99% of the body’s calcium stores is found in bone. Because 40% of the mineral in the bone matrix is calcium, it gives bone its structure, making dietary calcium a major contributor to BMD. The levels of calcium in the blood is tightly controlled and maintained at 9 to 11 mg/dL (39). The acquisition of calcium into bone occurs in response to high calcium concentrations in the blood by the secretion of calcitonin from the thyroid gland. Calcitonin acts to inhibit osteoclast activity, thereby stopping the resorption of bone and allowing for the return of normal blood calcium concentrations and bone mineralization (40).
The sequestering of calcium out of bone is another mechanism used to maintain proper blood calcium levels. When extracellular calcium falls, the major hormone that acts to restore these levels is PTH. PTH is secreted from the parathyroid gland where it regulates calcium levels by regulating three different mechanisms that allow for an increase in calcium concentration. First, PTH increases bone resorption by inhibiting osteoblast activity and the formation of bone. Once stimulated by PTH, osteoblasts will activate osteoclasts, allowing for the resorption of bone and the sequestration of calcium into the blood (41). Secondly, PTH also increases the transformation of vitamin D to calcitriol, which increases the absorption of calcium in the intestines along with increasing osteoclast activity (42). Lastly, PTH also allows for increased calcium reabsorption in the distal nephron of the kidneys (43).
Interestingly, studies have shown that a second hormone related to PTH called parathyroid hormone-related protein (PTHrP) may also play a role in influencing calcium concentrations. PTHrP has shown to stimulate renal tubular calcium reabsorption as well as excessive bone breakdown (44, 45).
Calcitonin is a hormone produced by the parafollicular cells of the thyroid gland and is involved in the regulation of calcium levels by opposing the actions of PTH, meaning it reduces calcium levels in the blood. Calcitonin decreases calcium levels by inhibiting the activity of osteoclasts (46).
Vitamin D. Vitamin D plays an important role in monitoring serum calcium levels (47). The expression of the intracellular calcium binding-protein, calbindin, and the plasma membrane calcium pump are both dependent on vitamin D. If there is not an adequate amount of calcitriol, intestinal absorption of calcium is reduced, causing a temporary decrease in serum calcium levels (48). Calcitriol also serves to increase intestinal absorption of calcium (49).
Zinc. There are about 1.5 to 2.5 grams of zinc in the adult body, 29% of which is found in bone. Zinc is absorbed in the small intestine and its absorptive capabilities are enhanced in the presence of amino acids, which form zinc chelates. Its absorption decreases in the presence of fiber, phytates, and high amounts of copper or zinc (50).
Zinc is a necessary component for adequate bone health. Evidence suggests that it increases the differentiation of osteoblasts and bone formation while simultaneously decreasing osteoclast activity (51). High dietary intakes of zinc (140mg), however, show a decrease in bone integrity and a decrease in calcium absorption when calcium consumption is low (52).
Phosphorus. About 85% of phosphorus is found in the hydroxyapatite making phosphorus a necessary component of bone and bone integrity. Although it is rare to see phosphorus deficiency in the United States, diets high in phosphorus are common mostly due to soft drinks and phosphate-containing processed foods. Studies suggest that this may be problematic because a ratio of phosphorus to calcium consumption greater than 1:1 may have deleterious effects on bone by increasing osteoclast activity (53, 54) However, positive associations have been seen in diets with a high calcium to phosphorus ratio (55).