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.
PUBERTY FEMALE
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.
PUBERTY MALES
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.
