Other Factors Affecting Bone

At least 20 more hormones, growth factors, and vitamins affect osseous tissue in complex ways that are still not well understood (table 7.2). Bone growth is especially rapid in puberty and adolescence, when surges of growth hormone, estrogen, and testosterone promote ossification. These hormones stimulate rapid multiplication of osteogenic cells, matrix deposition by osteoblasts, and multiplication and hypertrophy of the chondrocytes in the metaphyses. Adolescent girls grow faster than boys and attain their full height earlier, not only because they begin puberty earlier but also because estrogen has a stronger effect than testosterone. Since males grow for a longer time, however, they usually grow taller. Growth ceases when the cartilage of the epiphyseal plates is depleted. At this time, the epiphyseal plates are said to "close," the epiph-ysis and diaphysis of the bone fuse, and the bones can no longer increase in length. Excessive or deficient secretion of these steroids can therefore cause abnormalities ranging from stunted growth to very tall stature (see chapter 17). The use of anabolic steroids by adolescent athletes can cause premature closure of the epiphyseal plates and result in abnormally short adult stature (see p. 87).

Before You Go On

Answer the following questions to test your understanding of the preceding section:

13. Describe the role of collagen and seed crystals in bone mineralization.

14. Why is it important to regulate blood calcium concentration within such a narrow range?

15. What effect does calcitonin have on blood calcium concentration, and how does it produce this effect? Answer the same questions for parathyroid hormone.

16. How is vitamin D synthesized, and what effect does it have on blood calcium concentration?

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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