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GARY RICHARDSON |
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Endocrine Changes Associated With Puberty And Adolescence
GARY S. RICHARDSON, M.D.,  he biological processes associated with puberty include the prominent maturation of several endocrine systems with consequent physical growth and sexual development. During this developmental phase, skeletal growth is completed, lean body mass increases and fat is redistributed, and secondary sexual characteristics are acquired. Maturation of the hypothalamo-hypophyseal axis results in the acquisition of adult patterns of hormone release which, in turn, allow reproductive competence, manifesting as spermatogenesis in the male and ovulation in the female.
The primary neural mechanism regulating the onset of endocrine changes at puberty is the disinhibition of hypothalamic releasing factors. The hypothalamus regulates multiple endocrine systems via neuropeptides that activate of inhibit specific pituitary functions. In the regulation of puberty, the most important hypothalamic neuropeptide is gonadotropin releasing hormone (GnRH) which stimulates the synthesis and release of the pituitary gonadotropins, luteinizing hormone (LH) and follicle stimulating hormone (FSH). These glycoprotein hormones act on the gonads to stimulate sex hormone production and gametogenesis. Transcription of the LH and FSH genes is modified by the frequency and amplitude of GnRH pulses from the hypothalamus. Similarly, the gonadal response to the gonadotropins is a function of both the amplitude and frequency of the pulses of those hormones.
The programmed sequence of events that define puberty begins when a critical frequency of pulses of GnRH results in gonadotropin release from the pituitary (ÒgonadarcheÓ). Sex steroids levels rise resulting in the anatomic, physiologic and behavioral changes of puberty. Sex steroids also feed back on the hypothalamic-pituitary axis to regulate GnRH and gonadotropin secretion. The pulsatile pattern of gonadotropin release also displays an age-related diurnal pattern. The earliest event in the pubertal sequence is the onset of pulsatile gonadotropin secretion at night. Sleep reversal studies have shown that the early onset of gonadotropin secretion is associated with sleep itself, rather than a specific circadian phase. As this pattern continues, a gonadal response is recruited, such that detectable levels of gonadal steroids are initially seen only during sleep. With subsequent maturation, levels of gonadotropins and sex steroids rise during wakefulness so that, in the adult, significant diurnal variation is absent. The exception to this is the early follicular phase of the menstrual cycle, when a sleep-dependent inhibition of gonadotropin release is seen. While gonadarche is defined by the onset of pulsatile gonadotropin release, the factors regulating this gonadotropin disinhibition are not well understood. One neuroendocrine system implicated in the control of the onset of puberty is the pineal gland. Data from animal studies suggest that melatonin can inhibit the reproductive endocrine axis. While, available data do not consistently support an antigonadal effect of melatonin in humans, some studies suggest that the pineal may serve as a brake on sexual development, escape from which marks the onset of puberty. Melatonin levels are highest in prepubertal children and drop steadily advancing chronological and developmental age. Rather than reflecting a decrease in pineal production of melatonin, this drop appear to reflect the increase in body mass combined with a constant melatonin production. Recently, another important change, an age-dependent decrease in the number of hypothalamic receptors, suggests that sensitivity to inhibitory effects of melatonin may be decreasing coincident with the onset of puberty. Direct evidence for a causal role of melatonin in the onset of puberty is necessarily limited. Abnormalities in melatonin levels have been linked to disorders of puberty, with abnormally low levels reported in children with precocious puberty and abnormally high levels reported in children with delayed puberty.
In summary, a host of endocrine systems show alterations in association with the onset of puberty. Variously, these systems may be involved in the timing of puberty itself, as well as the somatic and developmental changes associated with sexual maturation.
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