SEXUALITY IN EARLY ADOLESCENCE: HORMONES
Underlying the conspicuous changes of puberty in growth and sexual maturation are changes in the hypothalamic-pituitary-gonadal and -adrenal systems which lead to an increased secretion of male and female sex hormones. Since reliable methods for the measurement of pituitary, gonadal, and adrenal hormones have been developed only recently, our knowledge of the complex endocrinology of puberty is still limited. Some representative findings follow.
For girls, Winter and Faiman found in a cross-sectional study that systematic changes in hormone levels coincided with the first signs of puberty. The first appearance of labial hair or of a subareolar breast bud (the-larche) was accompanied by rises in the mean plasma concentrations of FSH, estradiol and testosterone, whereas levels of serum LH did not become significantly elevated over prepubertal values before pubertal stage 3. It is not until several months after menarche that adult levels of estradiol appear as well as its cyclical variation which, in association with LH peaks of ovulatory magnitude and a normal luteal rise in serum progesterone levels, is the prerequisite of the final attainment of reproductive capacity. It is noteworthy that hormonal changes predate the onset of conspicuous pubertal changes. Mixed cross-sectional/ longitudinal data of the same authors (Faiman and Winter) showed small increments of gonadotropins in prepubertal girls between ages six and ten, and a steep rise thereafter, whereas serum estradiol did not change noticeably before age ten.
Analogous data are available for boys. Cross-sectional studies show that serum gonadotropins in males rise gradually between ages six and ten—again, in the prepubertal period—whereas significant rises in plasma testosterone tend to occur after this period. These findings have been confirmed basically in a mixed cross-sectional/longitudinal study by Lee and coworkers. LH levels rose throughout puberty, the initial rise of LH occurring before testosterone concentrations were significantly increased. Testosterone levels rose progressively from before the appearance of sexual hair until an adult distribution was achieved. Noticeable genital growth started before testosterone concentrations increased and continued after elevations of testosterone were no longer detectable. The question of which hormone or hormone combination causes which pubertal event cannot yet be answered in satisfactory detail except for the well established fact that testosterone has a major role in the development of male sexual characteristics and estrogens a major role in female puberty. There are numerous gonadal hormones in both sexes and some of the pubertal changes cannot be explained without taking into account adrenal androgens whose secretion shows significant increases already well before the gonadal changes of puberty in both sexes. Pubertal changes in other than sex hormones are reviewed by Hays.
An important methodological question for psychoendocrine research is whether hormone measures contribute valuable information above and beyond the assessment of physical development during puberty. The answer is that they probably do. One argument is the previously mentioned finding that several puberty-related hormones rise before puberty becomes visible. Some more arguments follow.
Compared over the total span of pubertal development in normal children, indices of physical maturation (Tanner stages) and endocrine measures averaged over individuals are well correlated in both boys and girls, but the slope of the regression of interindividual means of hormone measures on Tanner stages varies from hormone to hormone between the sexes. Tanner stages constitute ordinal scales of only five to six points whereas hormones are measured on a ratio scale and thereby much more precisely. Thus, for each Tanner stage, there exists a tremendous variability of plasma hormone levels.
Intraindividually, considerable increases in hormone secretion occur during one or two Tanner stages of pubertal maturation. A particularly dramatic example of this is the longitudinal study of boys by Knorr and others, which shows steep intraindividual rises in plasma testosterone levels from 40 to 240 ng/100 ml (i.e., from the average adult female range to the lower limit of the adult male range) within ten months; the gradients were surprisingly uniform across individuals, irrespective of chronological age at the onset of this rise. Lee and others found that individual plasma testosterone levels rose from prepubertal to adult levels within two years. These findings imply that pubertal subjects with similar Tanner ratings may, in fact, show considerable differences in hormone secretion which are likely to be relevant to behavior effects. This is a particularly important consideration for the lower ranges of testosterone values characteristic of the early stages of male puberty in which dose-response relationships between hormones and sexual behavior appear to be more likely than later.
Apparently Tanner stages are only very crude indicators of pubertal development and are far from sufficient in elaborating the role of hormones in behavioral development at puberty. The hormones in question have to be assessed directly. Tanner staging will be useful, however, for the systematic assessment of pubertal changes in body build and their relationships to body image and perception by others.
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