Biological Aspects Of Pubertal Development

Pubertal development is a series of interrelated processes resulting in maturation and adult reproductive functioning. The physiological changes of puberty primarily involve the hypothalamic-pituitary-adrenal (HPA) axis and hypothalamic-pituitary-gonadal (HPG) axis. Pubertal development begins in middle childhood and takes five to six years for most adolescents to complete (Brooks-Gunn & Reiter, 1990; Petersen, 1987). A wide range of individual differences exists in the timing of onset and rate of puberty. The following sections describe the physiological, physical, and central nervous system changes of pubertal development, with attention to gender differences in development. An explanation of how the different aspects of pubertal development are measured is also provided.

Physiological Changes of Puberty

Puberty is part of a continuum of events initiated at conception, mostly involving the hypothalamic-pituitary-gonadal (HPG) axis. The hypothalamic gonadotropin releasing hormone (GnRH) pulse generator, or "gonadostat" is active prenatally and during early infancy, suppressed during childhood, then reactivated at the onset of puberty (Fechner, 2003).1 In order for puberty to begin, the brain's sensitivity to the negative feedback of gonadal sex steroids (testosterone in males and estrogen in females) decreases, which then releases the HPA axis from inhibition. Puberty begins with the release of GnRH pulses, which activates pulsatile bursts of gonadotropins, luteinizing hormone (LH), and follicle stimulating hormone (FSH), from the pituitary gland. The LH and FSH pulses secreted in response to the GnRH occur first at night and then during the day. Increases in LH and FSH are some of the earliest measurable hormonal indications of pubertal development, and they have been found to rise progressively during puberty (Reiter & Grumbach, 1982). Episodic nocturnal bursts of low-levels of LH are indicative of early pubertal stages (Grumbach & Styne, 1998). The gonads respond to LH and FSH by enlarging, maturing, and secreting increased amounts of gonadal sex steroids, androgens and estrogens.

Gender Differences in Physiological Changes

Multiple gender differences in the mean levels and functions of hormone secretions are evident during the period of pubertal development. In females, the function of LH and FSH is to initiate follicular development in the ovaries, which stimulates them to produce estrogen. Estrogen sensitive tissues, such as the breasts and uterus, then respond to the increase (Fechner, 2003). In males, increased LH stimulates the testes to secrete testosterone, resulting in an increase in testicular size, and FSH stimulates spermatogenesis. LH levels increase in both girls and boys at puberty, while FSH is higher in girls than boys during the prepubertal and pubertal years. Increased FSH levels simulate the ovaries to produce estrogen. Whereas LH and FSH levels in both sexes are regulated by the negative feedback of the gonadal steroids and by the hormone inhibin, girls have a second control mechanism associated with their menstrual cycles which is under positive feedback and is cyclic. When estradiol level is high enough, it triggers an LH, and to a lesser extent, an FSH surge, each which lasts less than 2 days and stimulates ovulation. A corpus luteum forms from the ruptured follicle and begins to secrete progesterone. In the absence of pregnancy, the corpus luteum regresses and the progesterone and estrogen levels drop, triggering withdrawal bleeding and menstruation (Fechner, 2002).

Estrogen and testosterone levels also differ between the two sexes at puberty. Estradiol levels at puberty increase in females then remain elevated during periods of each menstrual cycle. In males, estradiol levels increase until their growth spurt (at midpuberty) then decrease again. On the other hand, while males experience substantial increases in testosterone and androstenedione (a weaker androgen than testosterone) at puberty, there is only a slight rise in females. The sexes also differ in their levels of dehydroepiandrosterone (DHEA) and DHEAS (sulfated form of DHEA), hormones which mark the beginning of adrenarche, the period of initial increases in adrenal androgen hormones, at 6 to 7 years of age in both sexes. Levels of DHEA and DHEAS are similar between the sexes until late puberty, when males begin to have higher levels than females. This difference persists into adulthood (Fechner, 2002).

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