Physical changes of puberty

In North American and European girls, puberty visibly begins with breast development between the ages of 8 and 10. Other secondary sexual characteristics appear over the ensuing 2.5 years. Puberty culminates with onset of menstruation. The average age of menarche in Caucasian girls is 12.8 ± 1.2 years and, on average, 4–8 months earlier in African-American girls.

The physical changes of puberty in girls have been divided into five stages using a system developed by Marshall and Tanner, who examined groups of English girls as they went through sexual maturation (Fig. 12.1). They then classified the relative and absolute changes in the sexual characteristics of the participants. Although they did not regard their findings as universal, their system has been widely used to describe the timing and progression of typical pubertal changes. Their descriptions must be recognized as specific to the demographics of their study population and to the years covered by the study. Patterns persist, but the characteristics and timing of these changes are affected by race, nutrition and other genetic and environmental factors.

Adrenarche

This describes the contribution of the adrenal gland to puberty in both girls and boys. It is a developmentally programmed increase in adrenal synthesis and secretion of the weak androgens: androstenedione, dehydroepiandrosterone (DHEA) and dehydroepiandrosterone sulfate (DHEA-S). Adrenarche begins at about ages 6–8 years in girls. Secretion of weak adrenal androgens precedes the visible onset of puberty by about 2 years. DHEA and DHEA-S are responsible for initiating growth of pubic and axillary hair as well as growth of and secretion by axillary sebaceous glands. Axillary and pubic hair appear in parallel with the beginning of breast development and visibly mark the onset of puberty in girls.

The exact trigger for adrenarche is not known. It is independent of adenocorticotropic hormone (ACTH) release, gonadotropin release and ovarian function, and appears to be an intrinsic, programmed event within the adrenal gland. Adrenarche is distinct from the other events of puberty (pubarche) and either may occur in the absence of its counterpart.

Breast development (thelarche)

The mammary gland, or breast, is an ectodermal derivative. The breast tissues are remarkably sensitive to hormones. Such hormonal effects are most notable during embryonic development and after puberty. The basic structure of the breast is common to all mammals although there exist wide variations in the number of mammary glands, their size, location and shape. Each mammary gland comprises lobulated masses of glandular tissue. Glandular tissues are embedded in adipose tissue and separated by fibrous connective tissues. Each of the lobes contains lobules of alveoli, blood vessels and lactiferous ducts. See Chapter 23 for a more detailed description of the structure and function of the human breast.

At birth, the breasts consist almost entirely of lactiferous ducts with few, if any, alveoli. These rudimentary mammary glands are capable of a small degree of secretory function (“witch’s milk”) within a few days of birth. Breast secretion in the neonatal period occurs in response to the high prolactin levels in the newborn infant following prior exposure of the fetal breast to high concentrations of placental estrogen during gestation. After placental estrogens are cleared from the neonatal circulation, the breast enters a dormant phase until puberty.

With the onset of puberty, ovarian estrogens induce growth of the lactiferous duct system. The ducts branch as they grow and their ends form into small, solid, spheroidal cell masses. These structures will form the lobular alveoli. The breast and alveoli enlarge. With menarche, cyclic estrogen and progesterone secretion begin and an extra phase of ductal and rudimentary lobular growth will occur. Adrenal corticosteroids further enhance duct development. The breasts continue to increase in size for some time after menarche due to deposition of fat and additional connective tissue. Final breast differentiation and growth will not occur until pregnancy.