Puberty

Hormonal Regulation of Puberty

The pubertal process of physical maturation results in the attainment of final adult height, sex-specific changes in body fat and lean body mass, and the development of the secondary sex characteristics. There is wide individual variation in the timing of both the onset and duration of this process.

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Figure 2 Female growth reference chart incorporating early, average, and late puberty-onset growth percentiles.

Figure 2 Female growth reference chart incorporating early, average, and late puberty-onset growth percentiles.

However, knowing the average age-of-onset and duration of each phase, the predictable sequential progression through the stages, and an understanding of the overall process will help identify young people not progressing through the process as anticipated.

The exact factor(s) initiating the pubertal process remains unknown, although current thinking proposes a body clock concept (housed in the hypothalamus) under the control of yet to be found "master genes" (3). The ofuath *im1 unflopwnt rrcord

Boys: Birth-19 years Height Velocity

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Figure 3 Male height-velocity chart.

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Figure 3 Male height-velocity chart.

control of subsequent physiologic events via the hypothalamic-pituitary-gonadal (HPG) axis is better defined.

The gonadotrophin releasing hormone (GnRH)-producing hypotha-lamic cells release GnRH in a pulsatile manner throughout life, leading to

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pulsatile secretions of the pituitary luteinizing and follicle stimulating hormones (LH and FSH). The axis is relatively dormant during childhood as a consequence of central inhibitory mechanisms. With the onset of puberty this inhibition is released and there is a progressive reduction in the sensitivity of the GnRH-producing hypothalamic cells to the negative feedback of circulating gonadal steroids (estrogen and testosterone). As a consequence there is an increase in both the amplitude and frequency of LH and FSH release, during sleep initially, with FSH increasing more than LH as puberty approaches. LH primarily regulates the subsequent increases in estrogen and testosterone levels in females and males, respectively (4).

Estrogen and testosterone initiate and maintain the process of development of secondary sexual characteristics as well as contribute to the growth spurt, other physical changes, and the development of fertility. Although breast development is the first overt sign of puberty onset in females, estrogen levels are known to increase well in advance of this, commencing enlargement of internal genital organs two years before breast budding. Estrogen levels in females rise more steadily and less dramatically than testosterone levels in males, with pubertal testosterone levels reaching more than 20 times adult levels (4).

The development of a positive feedback loop between the ovary (estrogen), hypothalamus (GnRH) and pituitary (LH) occurs late in the pubertal process and results in menstrual cycles. This process of positive feedback may take some time to fully mature, hence the irregularity of cycles in the first few years following menarche.

The onset and progress of puberty is also influenced by genetic factors (family history of puberty onset), under- and overnutrition, chronic illness, psychosocial factors, and possibly undefined growth factors. The relationship between body weight and the onset of puberty is illustrated by the steady decrease in age of menarche during the last century (which has recently plateaued) due to a steady increase in body size resulting from improved general health and nutrition (3). Pubertal onset requires an increase in leptin levels from adipose tissue, perhaps signaling the attainment of an appropriate nutritional and metabolic milieu in which puberty can proceed, although this is not the primary initiating mechanism as once thought (3).

While the hormones of the HPG axis have the primary role in the pubertal process, other hormones are still important. Growth hormone (GH), and hence insulin-like growth factor 1 (IGF-1), are increased by the gonadal hormones. Without GH, full height potential will not be attained. Likewise, full height attainment is dependent on the presence of thyroid hormone, although thyroid hormone levels do not alter during puberty. In contrast, the increase in adrenal hormones [dehydroepiandrosterone (DHEA) and DHEA-sulphate] via the hypothalamic-pituitary-adrenal (HPA) axis is not necessary for pubertal height acceleration. Instead increased adrenal hormones, a consequence of the adrenarche, influence development of axillary and pubic hair, especially in females (4).

In terms of the pubertal growth spurt, estrogen and testosterone are the main promoters in females and males, respectively, with GH and IGF-1 the important secondary effectors. The latter affect the rate of growth while estrogens and/or androgens control bone maturation (closure of the epiphyses) and hence duration of the pubertal growth spurt (4).

Physical Changes During Puberty

Around 15% of final adult height is attained during the pubertal spurt (on average 25 cm in females and 30 cm in males). Following the prepubertal deceleration in height velocity, the female accelerates soon after the onset of breast development to a peak velocity in mid puberty, 6 to 12 months prior to the menarche. The remaining attainable height averages 5 cm following the menarche, more in those with earlier menarche and vice versa. Males, on the other hand, start their height spurt later into puberty, usually when the testes are of 8 ml volume, getting on average two extra years of prepubertal growth than females. They have a higher growth rate, reaching an average peak height velocity of 9 cm per year, and continue to grow for longer, leading to an average difference between adult males and females of 12.5 cm (3, 4).

Despite asynchrony between the HPG axis (breast and genital development) and the HPA axis (pubic hair development) and the different stages varying at different rates (Fig. 5), as a general rule, most pubertal events are approximately normally distributed with a standard deviation (SD) of one year. An approximate estimation of the normal range for a particular pubertal event can be calculated by adding and subtracting two years from the average age of the event (3) (Table 1). However, young people found to be outside these ranges need more careful clinical evaluation to differentiate those whose pubertal progress may still be normally variable, from those whose progress is pathological.

Tanner and Marshall devised a five-stage pubertal progression of the secondary sexual characteristics, from the prepubertal Stage 1 through to the adult stage 5 (5,6). These are illustrated and described in Figures 6 and 7. Estimation of testicular volume is commonly undertaken with the Prader orchidometer (Fig. 8). Progression from 3 to 4mL testes with thinning of scrotum indicates the onset of puberty in the male. Up to two-thirds of males may develop breast enlargement (gynecomastia), and this is most prominent in late puberty (G4), and as in the female may be non-pathologically asymmetrical. In the female, estrogen also promotes development of the internal genitalia (vagina, uterus, and ovaries) concurrently with the above external changes.

The other physiologic changes that occur during puberty include:

■ Change in body composition. Females start to lay down more body fat from 7 years of age, developing twice as much as males by age 16 years. Meanwhile males undergo an acceleration of lean body mass growth from around 9 years, doubling their muscle and skeletal mass by 16 to 17 years. This lean body mass increase starts more peripherally in the hands and feet, and progresses centrally, with truncal growth occurring last (4).

Figure 5 Progress of male and female pubertal development.

■ Bone accretion. Estrogen and testosterone primarily regulate bone accretion which peaks in the second decade of life, and is complete by 16 to 17 years in females, but not until the early twenties in males. General nutrition, calcium intake, vitamin D levels, exercise, and menstrual history are also contributing factors.

■ Development of facial hair and a deepening of the voice occur in late male development, stage G3-4.

■ Increase in bone-derived alkaline phosphatase levels during the growth spurt.

■ Increase in hematocrit, hemoglobin, and cholesterol with increased levels of testosterone.

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