THE BREAST: DEVELOPMENTAL STAGES
In a human new born at birth, in the female as well as in the male, the mammary glands have developed sufficiently so that they appear as distinct hemispheroidal elevations, palpable as movable soft masses. This is especially prominent in postterm infants. Histologically, a number of branching channels with layers of lining cells and plugs of basal cells at their ends, the future milk ducts and glandular lobules, respectively, can easily be recognized. In a great number of infants an everted nipple is observed, and in about 10% a greatly enlarged gland can be palpated, a condition that received the unfortunate name of mastitis neonatorum, though no signs of inflammation exist. These early glandular structures may produce a milk like secretion, the “witch’s milk,” starting 2 or 3 days after birth. All these neonatal phenomena in the breast are the result of the very intensive, maternal estrogen driven developmental processes in the last stages of intrauterine life. The changes subside within the first 2 to 3 weeks of life. It is during this period that the breast undergoes marked involutional changes leading to the quiescent stage, which is characteristic of infancy and childhood. During these periods, the male and the female glands consist of a few branching rudimentary ducts lined by flattened epithelium, surrounded by collagenous connective tissue.
For most girls, the first sign of
puberty is usually the appearance of breast budding. In the United States, this
early breast change begins at an average age of 10.8 (±1.1) years of age. With
the onset of puberty and during adolescence, follicular ripening in the
ovaries, in response to follicular-stimulating hormone (FSH) of the anterior
pituitary gland, is accompanied by an increased output of estrogen. In response
to the latter, the mammary ducts elongate and their lining epithelium
reduplicates and proliferates at the ends of the mammary tubules, forming the
sprouts of the future lobules. This growth of ductal epithelium is accompanied
by growth of periductal fibrous tissue, which is largely responsible for the
increasing size and firmness of the adolescent female gland. During this period,
the areola and nipple also grow and become pigmented.
With the onset of maturity, that is,
when ovulation occurs and the progesterone-secreting corpora lutea are formed,
the second stage of mammary development occurs. It is essentially concerned
with the formation of the lobules and acinar structures. Although in the adult
woman, progesterone always asserts its influence when estrogen is simultaneously
present, overwhelming experimental evidence indicates that this beginning
unfolding of the lobules is a specific effect of progesterone. This gives the
mammary gland the characteristic lobular
structure found during the childbearing years. This differentiation into a
lobular gland is finished approximately 1 to 1½ years after the first
menstruation, but further acinar development continues in proportion to the
intensity of the hormonal stimuli during each menstrual cycle and especially
during pregnancies. Fat deposition and formation of fibrous stroma contribute to
the increasing size of the gland in the adolescent period.
The predictable sequence of breast
development brought on during adolescence forms one part of the sexual
maturation scale (Tanner staging) that is used to assess the degree and
sequence of pubertal development. In 1969, Marshall and Tanner defined five
stages of breast development and pubic hair development that are combined and
called Tanner, or pubertal, stages 1 through 5. For most girls, breast budding
is the earliest sign of puberty and menarche
the latest.
