The adolescent growth spurt
The adolescent in the photograph on the left is the same juvenile shown in image from «Play defines human childhood», included in the previous Gallery. He was about 18 years old in the photograph above. His height growth continued for a few more years and his final stature is greater than his father’s. The adolescent growth spurt is illustrated on the left as height velocity curves. The velocity curves were calculated using data derived from a sample of healthy, well-nourished girls and boys living in Guatemala. Velocity curves for human growth are explained in image and text «How people grow in size and in shape». The order in which several pubertal and adolescent events occur in girls and boys is imposed upon the height velocity curves, in terms of time before and after «Peak height velocity» (‘PHV’) of the adolescent growth spurt. The average age of PHV is about 12 years for girls and 14 years for boys. The age range for adolescence is 9-18 years for girls and 12-22 years for boys. At PHV, growth is more rapid than any time since early infancy. The Tanner Maturation Staging System for the development of secondary sexual characteristics is used in this illustration. This system is based on five stages, with stage 1 being pre-pubertal. In both girls and boys, puberty begins with changes in the activity of the hypothalamus and other parts of the central nervous system. These changes are labelled as ‘CNS puberty’ in the figure. Note that the CNS events begin at the same relative age in both girls and boys, that is, three years before PHV. This is also the time when growth rates change from decelerating during the juvenile stage to accelerating. In girls, the first outward sign of puberty is the development of the breast bud (‘B2’) and wisps of pubic hair (‘PH2’). This is followed, in order, by a rise in serum levels of estradiol which leads to the laying down of fat on the hips, buttocks, and thighs; the adolescent growth spurt; further growth of the breast and body hair (‘B3 & PH3’); ‘Menarche’ (first menstruation); completion of breast and body hair development (‘B5 & PH5’); and attainment of adult levels of ovulation frequency.
The path of pubertal development in boys starts with a rise in serum levels of luteinizing hormone (‘LH’) and the enlargement of the testes and then penis (‘G2’). This genital maturation begins, on average, only a few months after that of girls. However, the timing and order of other secondary sexual characteristics is unlike that of girls. About a year after CNS puberty, there is a rise in serum testosterone levels (‘T’) which is followed by the appearance of pubic hair (‘PH2’); about a year later motile spermatozoa may be detected in urine; PHV follows after about another year, along with deepening of the voice, and continued growth of facial and body hair; the adult stages of genital and pubic hair development follow the growth spurt (‘G5 & PH5’); and near the end of adolescence boys undergo a spurt in muscular development.
The sex-specific order of pubertal events tends not to vary between early and late maturers, between well-nourished girls and boys and those who suffered from severe malnutrition in early life, between rural and urban dwellers, or between European and African ethnic groups. [Barry Bogin]