Eugene J. Barrett
Growth from the fertilized ovum to the adult is an exceedingly complex process involving both hyperplasia (an increase in the number of cells) and hypertrophy (an increase in the size of cells) as well as turnover of the cellular elements of body tissues. The timing and capacity for cell division vary among tissues. In the human central nervous system (CNS), neuronal division is essentially complete by 1 year of age, whereas bone, muscle, and fat cells continue to divide until later in childhood. Other tissues—such as skin, blood, and liver—retain the capacity for hyperplasia throughout life.
In humans, the genetic contribution to growth is evident from the observation that midparental height (i.e., the average of paternal and maternal heights) is one of the better predictors of a child's ultimate stature. For domestic animals, breeding based on desired growth characteristics has been a mainstay of animal husbandry for millennia. For humans, in whom such planned breeding does not occur, body-height differences among different populations suggest a genetic contribution. Even more striking are some of the differences seen with specific genetic mutations that affect skeletal growth (e.g., achondroplasia). The basis of these population height differences, and those related to genetic syndromes, is beyond the scope of this chapter. Rather we will focus on nutritional and hormonal processes in which physiological regulation appears to play an important role across individuals. The impact of environmental and nutritional factors, such as emotional or nutritional deprivation, on growth is most profound when it occurs during periods of tissue hyperplasia, most critically during the first 2 years of life.
In this chapter, the first two subchapters deal with factors that affect linear growth, whereas the third deals with factors that regulate body mass. This division is somewhat artificial because changes in linear growth and body mass often occur simultaneously. The control of linear growth in humans depends on multiple hormones, including growth hormone (GH), insulin-like growth factors 1 and 2 (IGF-1 and IGF-2), insulin, thyroid hormones, glucocorticoids, androgens, and estrogens. Among these, GH and IGF-1 have been implicated as the major determinants of growth in normal postuterine life. However, deficiencies (or excesses) of each of the other hormones can seriously affect the normal growth of the musculoskeletal system as well as the growth and maturation of other tissues. The control of body mass depends on many newly discovered humoral factors made in adipose tissue, intestine, hypothalamus, and other tissues that regulate appetite and energy expenditure.