Moreover, bone biology is different depending on the developmental stage. “What happens in the womb is different than what happens when you’re born, different form puberty, different from adulthood,” he said. Too much calcium prenatally, for instance, would make a fetus grow too large and ultimately harm its mother. At the same time, a fetus that doesn’t grow large enough will be at greater risk of dying once leaving the womb.
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Explore This IssueJuly 2008
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One of the most important cellular factors in keeping bones healthy is the normal development of osteoblasts. Dr. Warman noted that several studies from the past couple of years have shed light on the function of these cells.
One study investigated whether a decrease in bone mineral density (BMD) is reversible. Previous work had shown that many adult women taking depot medroxyprogesterone acetate (Depo-Provera, or DMPA)—an injectable form of contraception—also had decreased BMD. Nobody knew if this also happened in adolescents, whose bones are still accruing minerals, or, if it did, if the process would reverse itself once the DMPA was discontinued. In 2005, Delia Scholes of the Group Health Cooperative in Seattle studied just that, with a sample of 170 women aged 15 to 19. Scholes found that adolescents, too, lose a significant amount of BMD while taking DMPA, especially around the hip and spine. However, once the DMPA was discontinued, the adolescents began to regain the bone mass that they had lost. “Whether that’s at the level of mesenchymal stem cells hasn’t been answered,” said Dr. Warman. And whether being on DMPA allows a woman’s bone mass to get as high as it would have if not on it, “is also an unanswered question,” he noted.
Dr. Warman next described studies from his own lab and others’ that analyzed how stem cells differentiate into osteoblasts. The process depends on a variety of factors, including the expression of tumor necrosis factors (TNF), mononuclear phagocyte colony-stimulating factors (M-CSF), lipoprotein receptors (LRP), and Wnt cell signaling.
Doctors commonly use a T-score to read the results gleaned from [children’s DEXA scans] because that’s what’s used for adults. But they should use a Z-score instead, so that the patient’s bone mass is compared to age-adjusted norms.
In elderly populations, risk factors for osteoporosis are easy to spot, including low calcium and vitamin D intake, physical inactivity, extreme thinness, excessive caffeine or alcohol intake, corticosteroid medications, and cigarette smoking. But, in children, risk factors are far more difficult to determine. Physical activity and calcium intake have been linked to higher BMDs in children, and juvenile rheumatoid arthritis, diabetes, and steroid use have been shown to put kids at increased risk.