STEROIDS AND GROWTH 385 



rather constant rate until sexual maturation and puberty occur. In 

 man this second period lasts from the age of two to the tenth or 

 eleventh year. During this time males and females respond similarly, 

 with an average increase in weight of five pounds per year and an an- 

 nual increment in height of about two inches. 



The third phase of growth is characterized by the well-known 

 adolescent acceleration in growth associated with sexual maturity and 

 puberty. In girls there is a slight acceleration in the annual increment 

 in height and a proportionately greater increase in body weight. As 

 sexual development begins earlier in girls, for a span of one or two 

 years they are heavier and taller than boys. As mentioned above, the 

 growth of boys from the ages of five to tsvelve is relatively slow and 

 constant, averaging about two inches per year; but between the ages 

 of 12 and 15, the growth rate increases to an average of three inches 

 per year (Wetzel, 1941; Jackson and Kelley, 1945), although some boys 

 temporarily may greatly exceed this rate. Body weight increases to an 

 even greater degree, and the body build and musculature character- 

 istics of the male develop. 



The fourth phase of development is characterized by a decelera- 

 tion in the rate of growth which continues until adult size is attained. 

 In general the inhibition of the growth process is apparent in girls by 

 the fifteenth year, and by the seventeenth year in boys. Thus the 

 puberal acceleration of growth occurs earlier and ends sooner in girls 

 than in boys. 



It is evident from the above that growth rates are quite inde- 

 pendent of sex until sexual puberty begins, at which time the respec- 

 tive hormones of the testes and ovaries are secreted in large amounts. 

 In addition to such general factors as changes in height and body 

 weight, other significant diflferences related to sex may be noted. For 

 example, centers of ossification appear later in the male than in the 

 female. Further, when specific bones are studied it is noted that the 

 fusion of the epiphyses with the diaphysis, which takes place during 

 puberty, occurs two years later in boys than in girls, on the average. 



Linear growth: androgenic steroids 



Sources of androgens. Androgens are derived from two sources: 

 the interstitial cells of the testes and the adrenal gland. Testosterone, 

 believed to be the chief androgenic steroid of the testes, has been iso- 

 lated from the bull ( David, ef al., 1935 ) and stallion testis ( Tagman, 

 et al., 1946), and it can be recovered from the human testis perfused 

 with radioactive acetate (Savard, ct al, 1952). Another source of an- 

 drogens is the adrenal gland, which produces materials which, though 

 structurally closely related to testosterone, are weaker androgenically. 



