PUBERTY NORMAL SEQUENCE
The biggest differences between puberty in girls and boys are (a) the age at which it begins and (b) the major sex steroids involved. On average, girls begin puberty about 1 to 2 years earlier than boys (average age 10.5 years in girls) and reach completion in a shorter time. Girls attain adult height and reproductive maturity about 4 years after the ﬁrst changes of puberty. In contrast, boys accelerate more slowly but continue to grow for about 6 years after the ﬁrst visible pubertal changes. Although boys are on average 2 cm shorter than girls before puberty begins, adult men are 13 cm (5.2 inches) taller than women. The hormone that dominates female development during puberty is estradiol, an estrogen. In males, testosterone, an androgen, is the principal sex steroid.
Puberty begins with gonadotropin-releasing hormone (GnRH) pulsing, leading to a rise in gonadotropins, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) and subsequently an increase in sex hormones. Indeed, exogenous GnRH pulses cause the onset of puberty, and brain tumors that increase GnRH may cause premature puberty. Puberty begins consistently at around 47 kg for girls and 55 kg for boys, and this correlation of pubertal onset with weight makes leptin a good candidate for causing GnRH rise. Kisspeptin, a protein responsible for developmentally activating GnRH neurons and triggering GnRH release, is also likely be involved in inducing pubertal onset. In addition, other genetic, environmental, and nutritional factors are thought to regulate pubertal timing.
The ﬁrst physical sign of puberty in females is usually a ﬁrm, tender lump under the areola(e) of the breasts, referred to as thelarche. In the Tanner staging of puberty, this is stage 2 of breast development (stage 1 is a ﬂat, prepubertal breast). Within 6 to 12 months, the swelling is bilateral, softer, and extends beyond the areolae (stage 3). In another year (stage 4), the breasts are approaching mature size and shape, with areolae and papillae forming a secondary mound. This mound usually disappears into the contour of the mature breast (stage 5). Pubic hair is often the second change of puberty, usually within a few months of thelarche, and is termed pubarche. The ﬁrst few pubic hairs visible along the labia are Tanner stage 2. Stage 3 takes another 6 to 12 months, when hairs are too numerous to count and appear on the pubic mound. In stage 4, the hairs densely ﬁll the pubic triangle. In stage 5, pubic hairs spread to the thighs and sometimes upward to the navel. In response to estrogen, the vaginal mucosa also changes, becoming thicker and a dull pink in color. Whitish vaginal secretions (physiologic leukorrhea) can also be found. For 2 years following thelarche, the uterus and ovaries increase in size and follicles in the ovaries also enlarge. The ovaries contain small follicular cysts observable by ultrasound.
The ﬁrst menstrual bleeding is referred to as menarche and typically occurs 2 years after thelarche. The average age of menarche in U.S. girls is 11.7 years. Menses are not always regular for the ﬁrst 2 years after menarche. Ovulation may or may not accompany the earliest menses, as about 80% of cycles are anovulatory in the ﬁrst year after menarche. Although occurring more frequently with age after menarche, ovulation is not inevitably linked to the menstrual cycle, and many girls with cycle irregularity several years from menarche will continue to have irregularity, anovulation, and possibly infertility.
Also in response to rising estrogen levels, the lower half of the pelvis and hips widen, creating a larger birth canal. The proportion of fat in body composition also increases, especially in the breasts, hips, buttocks, thighs, upper arms, and pubis. Rising androgen levels change the fatty acid composition of perspiration, resulting in an adult body odor and increased oil (sebum) secretions from the skin. This change increases the chances of acne.
In males, testicular enlargement is the ﬁrst physical sign of puberty and is termed gonadarche. Testes in prepubertal boys change little in size from 1 year of age until puberty, averaging about 2 to 3 mL in volume. Testicular size increases throughout puberty, reaching maximal adult size 6 years later. Although 18 to 20 mL is the average adult testis size, there is also wide ethnic variation.
The testis Leydig cells produce testosterone that induces most of the changes of sexual maturation and maintains libido. Most of the increasing bulk of testicular tissue is due to growth of seminiferous tubules, including Sertoli cells. The sequence of sperm production and the onset of fertility in males is not as well documented, largely because of the variable timing and onset of ejaculation. Sperm can be detected in the morning urine of most boys after the ﬁrst year of puberty and potential fertility can reached as early as 13 years of age, but full fertility is not achieved until 14 to 16 years of age.
Pubic hair appears shortly after the genitalia start to grow. As in females, the ﬁrst appearance of pubic hair is termed pubarche and hairs are usually ﬁrst visible at the base of the penis. The Tanner stages of hair growth are similarly classiﬁed in males and females, as described earlier. At about Tanner stage 3, the penis starts to grow. Following the appearance of pubic hair, other body areas that respond to androgens develop heavier hair (androgenic hair) in the following sequence: axillary hair, perianal hair, upper lip hair, sideburn hair, periareolar hair, and facial beard. Arm, leg, chest, abdominal, and back hair become heavier more gradually. There is signiﬁcant ethnic variation in the timing and quantity of hair growth.
Under the inﬂuence of androgens, the voice box, or larynx, grows in both genders. Far more prominent in males, this growth causes the male voice to deepen about one octave, as the vocal cords lengthen and thicken. Voice change can be accompanied by unsteadiness of vocalization in the early stages. Most of the voice change occurs in stages 3 to 4 of male puberty, around the time of peak growth. Full adult voice pitch is attained at an average age of 15 years, usually preceding the development of facial hair by months to years.
By the end of puberty, adult men have heavier bones and nearly twice as much skeletal muscle as females. Some of the bone growth (e.g., shoulder width and jaw) is disproportionately greater, resulting in noticeably different male and female shapes. The average adult male has about 150% of the lean body mass of an average female and about 50% of the body fat. Muscle develops mainly during the later stages of puberty. The peak of the “strength spurt” is observed about 1 year after the peak growth rate. As with females, rising levels of androgens change the fatty acid composition of perspiration, resulting in adult body odor and acne. Acne typically resolves at the end of puberty.