Growth and physical development depend on a multitude of genetic, nutritional, and environmental factors, and may be optimal only in the presence of optimal health. The skeletal apparatus is no exception. From birth to the end of the pubertal phase, the skeleton of a healthy child increases almost three times in length. The faster skeletal growth occurs from birth to about three years of age, although at a rapidly decelerating yearly rate. From age three to the onset of puberty, there is a relatively slower growth in both sexes, with a small spurt at around eight years. Then, during puberty, a rapid acceleration occurs and the skeleton has another phase of rapid growth. After that, another deceleration phase follows, until full resorption of the growth cartilages occurs. This means the complete cessation of bone lengthening, while the bone mineral content (BMC) continues to increase for several more years. During puberty, the differences between genders become evident. Both the starting age of the pubertal spurt and the growth process are earlier in girls, but the duration of the growth spurt and the maximal peak of growth are greater in boys (see Chapter 3).
Bone growth is mainly determined by the hypothalamic-pituitary, gonadal, and pituitary-thyroid axes. The balance between cell proliferation and maturation is controlled by several hormones and local factors.
The best known action is that of growth hormone (GH). GH acts on a variety of target cells, including those of growth plates, inducing the release of insulin-like growth factor 1 (1GF-1). The GH/IGF-1 system and the androgens are the main stimulants of cell proliferation in the growth plate cartilages, and are thus the main factors in the linear growth of long bones. Less known is the role of thyroid hormones, thyroxine (T4), and the active metabolite triiodothyronine (T3). Children with hypothyroidism have a reduced linear growth of bones. T3 is known to have direct effects on the growth plate chondrocytes, starting both their proliferation and their maturation process (9).
Gonadal hormones (mainly testosterone and estradiol) are secreted only in minimal quantities in infancy. After puberty, they are secreted in large quantities and are obviously involved both in the appearance of the pubertal growth spurt and in the subsequent disappearance of the growth plate and permanent cessation of growth. They are thought to promote both the proliferation and the maturation of growth plate cells, even though their specific actions at this level are not fully understood.
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