The anatomy of newborns is different from that of adults, or even of toddlers. These anatomical differences make breastfeeding possible.

All babies are born with a small degree of retrognathia, or physiologic jaw retraction. This results in a relative posterior positioning of the base of the tongue, and renders the newborn a predominantly obligate nasal breather. The tongue is also larger in neonates than adults relative to the jaw. The base of the tongue sits far back in the throat over the epiglottis. The newborn’s larynx is in a higher position relative to an adult and sits near the soft palate (see Fig. 2.1).

This anatomy allows an infant: (1) to breathe even with a teat filling his mouth and his nose pressed up against the breast; and (2) to swallow milk without worry of having the liquid spill into the trachea. However, this also means that the newborn has to coordinate sucking, swallowing, and breathing in a very specific way with little margin of error (Sanches 2004).

On the exterior of the breast are the areola, nipple, and Montgomery glands. The areola is the circular, pigmented area of the breast. Centered in the areola is the nipple. The nipple is composed of horizontal and longitudinal smooth muscle fibers that contract in response to touch. Nipples carry the openings (galactophores) of the lactiferous ducts, which transport and store milk. There are 6–10 such openings in each nipple, corresponding to the 6–10 lobes of milk-producing glandular tissue in each breast (see Fig. 2.2: External and Internal breast anatomy).

Surrounding the nipple, within the areola, are Montgomery glands, which secrete an oily liquid in the lactating breast. The yellow liquid creates a seal on the delicate nipple to prevent damage, but it may actually do much more. Decades of research have shown that the breasts of lactating women secrete compounds that affect newborn behavior. They can elicit arousal in sleepy babies (Sullivan and Toubas 1998; Russell 1976; Soussignan et al. 1997), and calm fussy ones (Schaal et al. 1980; Sullivan and Toubas 1998). The odor can also induce appetite (Russell 1976; Soussignan et al. 1997), directional crawling (Varendi and Porter 2001) and head turning behavior (Schaal et al. 1980; MacFarlane 1975; Makin and Porter 1989). The source of these compounds has been disputed, but Doucet et al. (2009) determined that these powerful compounds may come from the oily liquid secreted from the Montgomery glands.

In 1840, Sir Astley Paston Cooper published On the Anatomy of the Breast, the most advanced medical text of the time (Cooper 1840). Cooper was a passionate surgeon with a keen interest in dissection, going so far as to steal his neighbors’ corpses in order to continue his self-directed education (Burch 2010). (He would also perform public dissections of executed criminals in a combination of egoism and showmanship.)

Cooper injected varying colors of wax into ducts and blood vessels of the breasts of recently deceased women to better understand the internal structures of the breast—including the lactating breast—and to visualize their relationships (see Fig. 2.3).

Cooper’s study was so detailed that it became the definitive medical model of the breast, and remained so until 2005. Many physicians practicing today learned all they know about the breast from data that was more than 150 years old.

In 2005, researchers published the results of a study examining the breasts of lactating women using high-resolution ultrasound imaging (Ramsay et al. 2005). This study, as well as others that followed (Geddes 2009; Hassiotou and Geddes 2013), have highlighted inconsistencies in the anatomical literature that impact breast physiology and breastfeeding management, including differences from one of the most venerated texts, Gray’s Anatomy. The information in this chapter is drawn from the newest research, based on state-of-the-art technologies.

The breast is composed of fatty and glandular tissues. Throughout pregnancy, estrogen causes the breast to change from predominantly adipose tissue to being made up primarily of glandular tissue. Studies have shown that a lactating breast transforms from having twice as much adipose as glandular tissue to having twice as much glandular tissue as adipose, although the percentage can vary greatly from woman to woman. These changes, called lactogenesis I, are complete by gestational week 22 in most women, but there is enormous variation in timing and degree (Cox et al. 1999). Mothers with preterm babies younger than 28 weeks may not have fully experienced the breast changes required for lactation. Therefore lactation may be delayed in preterm births (Cox et al. 1999).

The glandular tissue is comprised of alveolar sacs (also called alveoli) and the milk duct system. These tissues are made up of two layers of epithelial cells. The inner layer contains cuboidal cells, which can differentiate into milk-secreting cells called lactocytes under the influence of progesterone and estrogen. The outer layer is made of myoepithelial cells, which have the properties of smooth muscle and contract in response to hormones. The alveolar and ductal tissues are supported within the breast by a loose framework of fibrous connective tissues called Cooper’s ligaments, named for the same Sir Astley Cooper who was so fond of dissection.

In the lactating breast, tracing inward from the nipple, the terminus of each duct forms into alveoli, which produce and store most of the milk. Alveoli contain lactocytes in the luminal layer, and myoepithelial cells in the outer layer (Sternlicht 2006; Watson and Khaled 2008). Lactocytes are predominantly present in the alveoli and directional, in that they point toward the lumen where milk is secreted (Fig. 2.2: External and Internal breast anatomy).

During lactation, the milk is transported from the alveolar sacs via the ductal system to the nipple, where it is expelled through the nipple ducts.

Ductal structures are loosely associated into lobules, each of which contain between 10 and 100 alveoli (Hartmann 1991). In turn, these lobules are arranged into lobes. Each lobe is surrounded by dense fibrous connective tissue with embedded fat cells, referred to as intra-lobular stroma. This stroma contains mesenchymal cells that are highly responsive to hormonal cues (i.e., oxytocin), and which are associated with the development of lactating breasts (Bissell et al. 1999; Wiseman and Werb 2002). The lobes of glandular tissue are intertwining and of varying sizes, with up to 30-fold differences in volume (Moffat and Going 1996). It was formerly thought that alveolar sacs swell to form “pools” of milk in large sacs, but the size of the alveoli remain relatively consistent—about 0.12 mm in diameter each (Hartmann 1991). There are just more of them to contain more milk in lactating breasts.

Most of the glandular tissue is in the anterior part of the breasts, directly underneath the areola, and therefore more easily accessible to the infant.

In addition to these changes in glandular tissue, estrogen also causes an increase in blood flow to the breasts. By 24 weeks of pregnancy, blood flow is doubled compared to pre-pregnancy. This change persists throughout lactation. Most of the extra blood flow comes from the two main vessels supplying the breasts: the Internal Mammary Artery (IMA) and the Lateral Thoracic Artery (LTA). The breast is additionally fed by the intercostal arteries and the thoracoacromial artery. Superficial veins also become more prominent in pregnancy and throughout lactation. The ratio of blood flow to milk yield is approximately 500:1 (Linzell 1960; Christensen et al. 1989).

Nerve supply to the breasts comes from the second to sixth intercostal nerves, which divide into superficial and deep branches (Cooper 1840). The deep branches supply the glandular tissue and nipple, and the superficial branches supply the nipple and areola (although sensory innervation is predominantly found in the nipple). Nerves have been identified along major ductal systems, but not near smaller ducts. There is motor innervation of the smooth muscle of the areola and nipple, (Courtiss and Goldwyn 1976) and of the mammary arteries (Cowie 1974). But there is no motor innervation of the lactocytes or myoepithelial cells, which means they are under hormonal, not neurological control.

The main hormones responsible for the production and maintenance of milk supply are progesterone, prolactin, oxytocin, cortisol, and Feedback Inhibitor of Lactation (FIL).

During pregnancy, high levels of progesterone suppress the production of milk by inhibiting the release of prolactin. Within 48–72 hours after birth, progesterone levels decrease, triggering prolactin release (Suzuki et al. 2000; Czank 2007; Pang and Hartmann 2007).

Prolactin is stimulated by infant suckling and occurs 7–20 times a day. It is produced in the anterior pituitary gland, and is positively and negatively regulated (it can be inhibited by progesterone, estrogen, norepinephrine, and dopamine). Prolactin causes stimulation of mammary glandular growth and epithelial cell proliferation, as well as milk production (Neville et al. 2002).

Serum prolactin levels are high during the first few weeks, but wane after that. After about week 3, prolactin returns to near prepregnancy levels. This is one of the reasons why early evaluation of potential breastfeeding problems is so crucial. During the first week, frequent stimulation of touch receptors on the breast stimulates an increase in prolactin receptors. With more available prolactin receptors, even as prolactin levels naturally decrease, milk production can be maintained for the duration of nursing (Jacobs 1977; Cox et al. 1996).

Initially, the circulating levels of prolactin vary throughout the day, with the highest level occurring about 1 h after sleep, and the lowest level in midmorning. Prolactin levels peak 30 min after the beginning of a feeding, preparing the breast for the next feeding.

If milk accumulates in the breast, the binding of prolactin is reduced. It is hypothesized that this is due to change in lactocyte shape when the alveoli are filled with milk, effectively deactivating the prolactin receptor (Berry et al. 2007). Full alveoli have lowered uptake of prolactin, and empty alveoli have higher binding affinity of prolactin (Cox et al. 1996; Cregan et al. 2002; Daly et al. 1993). When prolactin cannot bind, milk synthesis slows, so full breasts result in an inhibitory effect on milk production. As the alveoli empty, prolactin can again bind, which allows the alveoli to again fill with milk.

In addition to prolactin, infant suckling also stimulates oxytocin, resulting in milk ejection, or “letdown.” It also results in uterine contraction to help the uterus shrink back down to pre-pregnancy size. By suckling, the infant stimulates touch receptors around the areola and nipple, which create impulses that activate the dorsal root ganglia, spinal cord, and hypothalamus, resulting in oxytocin release from the posterior pituitary gland. When oxytocin is released, it binds to receptors on the myoepithelial cells that line the alveolar ducts, causing these cells to contract and expel the stored milk from alveoli into larger ducts (Prime et al. 2007; Cowie 1974). This is called the milk ejection reflex.

The milk ejection reflex is transient, lasting between 45 s and 3.5 min. It is also pulsatile, meaning that oxytocin is secreted more than once, resulting in multiple ejections during nursing or pumping (Drewett et al. 1982; McNeilly et al. 1983; Uvnas-Moberg et al. 1990). Prior to the milk ejection reflex, very little milk can be removed, so it is critical for successful nursing (Young et al. 1996; Kent et al. 2008). This is why mothers often get less milk from pumping alone: because the touch receptors on the areola are not stimulated in the same way as by the baby’s mouth so there is less oxytocin release and therefore less milk ejection. Oxytocin also prevents the unsuckled breast from ejecting milk to maintain positive pressure in that breast so the baby can nurse more easily from that side when the time comes.

Mothers can sense the milk ejection reflex happening with feelings of warmth, pain, tingling, pressure, and related sensations. The first letdown results in the strongest sensations and the largest volume of milk (Isbister 1954; Prime et al. 2007; Geddes 2009). Gardner et al. (2015) showed that milk ejection occurs asynchronously, suggesting that the timing of myoepithelial cell response differs, resulting in heterogeneous emptying of the breast.

It’s important to note how amazing, reciprocal, and layered this oxytocin process is. In addition to milk ejection, oxytocin also induces a state of calm in the mother and promotes bonding between the mother and her infant. And this warm emotional state, in turn, affects oxytocin levels. While oxytocin is triggered by a suckling infant, levels are also influenced by the mother’s subjective feelings and sensations. Smelling, touching, seeing, or even thinking lovingly about her baby or hearing her baby’s cry can stimulate oxytocin release. Conversely, oxytocin is inhibited by emotional or physical pain.