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Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction to enable fertilization. It is initiated by hormonal signals from the brain to the gonads: the ovaries in a girl, the testes in a boy. In response to the signals, the gonads produce hormones that stimulate libido and the growth, function, and transformation of the brain, bones, muscle, blood, skin, hair, breasts, and sex organs. Physical growth—height and weight—accelerates in the first half of puberty and is completed when an adult body has been developed. Until the maturation of their reproductive capabilities, the pre-pubertal physical differences between boys and girls are the external sex organs. On average, girls begin puberty at ages 10–11; boys at ages 11–12.〔"For girls, puberty begins around 10 or 11 years of age and ends around age 16. Boys enter puberty later than girls-usually around 12 years of age-and it lasts until around age 16 or 17." 〕 Girls usually complete puberty by ages 15–17,〔〔(【引用サイトリンク】publisher=University of Maryland )〕 while boys usually complete puberty by ages 16–17.〔〔〔Marshall (1986), p. 176–7〕 The major landmark of puberty for females is menarche, the onset of menstruation, which occurs on average between ages 12–13;〔(Tanner, 1990).〕 for males, it is the first ejaculation, which occurs on average at age 13.〔(Jorgensen & Keiding 1991).〕 In the 21st century, the average age at which children, especially girls, reach puberty is lower compared to the 19th century, when it was 15 for girls and 16 for boys. This can be due to any number of factors, including improved nutrition resulting in rapid body growth, increased weight and fat deposition, or exposure to endocrine disruptors such as xenoestrogens, which can at times be due to food consumption or other environmental factors. Puberty which starts earlier than usual is known as precocious puberty. Puberty which starts later than usual is known as delayed puberty. Notable among the morphologic changes in size, shape, composition, and functioning of the pubertal body, is the development of secondary sex characteristics, the "filling in" of the child's body; from girl to woman, from boy to man. Derived from the Latin ''puberatum'' (age of maturity), the word ''puberty'' describes the physical changes to sexual maturation, not the psychosocial and cultural maturation denoted by the term ''adolescent development'' in Western culture, wherein adolescence is the period of mental transition from childhood to adulthood, which overlaps much of the body's period of puberty.〔''The Oxford Dictionary of English Etymology'', C.T. Onions ed. Oxford University Press 1996 p. 720.〕 ==Differences between male and female puberty== Two of the most significant differences between puberty in girls and puberty in boys are the age at which it begins, and the major sex steroids involved, the testosterones and the estrogens. Although there is a wide range of normal ages, girls typically begin the process of puberty at age 10 or 11; boys at ages 11–12.〔〔 Girls usually complete puberty by ages 15–17,〔〔〔 while boys usually complete puberty by ages 16–17.〔〔〔 Girls attain reproductive maturity about four years after the first physical changes of puberty appear.〔 In contrast, boys accelerate more slowly but continue to grow for about six years after the first visible pubertal changes.〔Garn, SM. Physical growth and development. In: Friedman SB, Fisher M, Schonberg SK., editors. Comprehensive Adolescent Health Care. St Louis: Quality Medical Publishing; 1992. Retrieved on 2009-02-20〕 Any increase in height beyond the post-pubertal age is uncommon. For boys, an androgen called testosterone is the principal sex hormone. While testosterone is produced, all boys' changes are characterized as virilization, a substantial product of testosterone metabolism in males is estradiol. The conversion of testosterone to estradiol depends on the amount of body fat and estradiol levels in boys are typically much lower than in girls. The male "growth spurt" also begins later, accelerates more slowly, and lasts longer before the epiphyses fuse. Although boys are on average shorter than girls before puberty begins, adult men are on average about taller than women. Most of this sex difference in adult heights is attributable to a later onset of the growth spurt and a slower progression to completion, a direct result of the later rise and lower adult male levels of estradiol. The hormone that dominates female development is an estrogen called estradiol. While estradiol promotes growth of the breasts and uterus, it is also the principal hormone driving the pubertal growth spurt and epiphyseal maturation and closure. Estradiol levels rise earlier and reach higher levels in women than in men. The hormonal maturation of females is considerably more complicated than in boys. The main steroid hormones, testosterone, estradiol, and progesterone as well as prolactin play important physiological functions in puberty. Gonadal steroidgenesis in girls starts with production of testosterone which is typically quickly converted to estradiol inside the ovaries. However the rate of conversion from testosterone to estradiol (driven by FSH/LH balance) during early puberty is highly individual, resulting in very diverse development patterns of secondary sexual characteristics. Production of progesterone in the ovaries begins with the development of ovulatory cycles in girls (during the lutheal phase of the cycle), before puberty low levels of progesterone are produced in the adrenal glands of both boys and girls. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Puberty」の詳細全文を読む スポンサード リンク
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