Although developmental biologists still differ somewhat on the issue, most consider the human breast to be a modified sweat gland. Breast development begins in the fifth embryonic week with the formation of the ectodermal ridges referred to as “milk lines.” Each of these extends from the region which will become the axilla (armpit) to that of the ipsilateral groin. In the sixth week each raises to form a “ridge,” and the mid-thoracic portion of this develops into the “milk hill” while other portions involute. (Incomplete involutions may allow the development of accessory breast tissues: seen in 2-6% of women; 1-3% men- see image below).

During the seventh week the milk hill invaginates into the mesenchymal tissue below, sending epithelium downward and encouraging parenchymal cells to differentiate into needed vessels and smooth muscle cells. By some time between the tenth and fourteenth weeks the breast anlagen is a buried cone with a surface nipple groove. At fifteen weeks, buds of epithelium at the invading parenchymal edge develop into 15 to 25 stalks which will eventually become the major ductal elements. Simultaneous with this, sebaceous glands appear in the surrounding subcutaneous tissues and special apocrine glands (Montgomery) develop around the anlagen.

At this point, the first, fleeting hormonal influence upon development occurs. If testosterone is present, it condenses the mesenchyme around the epithelial stalk leading to its rupture and isolation subdermally. Without this influence the epithelial sprouts canalize and form milk ducts (20 to 32 weeks). From 32 weeks to term a rudimentary lobulo-aveolar structure is formed, first independent of hormone, and then stimulated by maternal and placental sex hormones. With the withdrawal of these steroids at birth, regression occurs with return toward ductular organization. The milk ducts merge beneath the center of the areola in the area which will be the nipple (true nipple prominence develops with puberty). Under the influence of several steroids and pituitary polypeptides, the ductal epithelium further proliferates at puberty, and the functioning lobulo-alveolar apparatus is formed, (the milk-producing and milk-conducting apparatus). Established ovarian cycles thereafter continue lobulo-alveolar development and connective tissue growth until the third decade.

Breast development may therefore be conveniently described as occurring in three phases of concentrated activity:

• an ectodermal phase in the first two to four months with generation of the primitive anlagen (what will be the “nipple/areola”)
• a phase of epithelial proliferation and ingrowth between the fifth month and term
• the pubertal development of the milk-producing structures under hormonal influence

Failure of any of these phases will lead to an “aplasia” of the breast, while incomplete disturbances produce the “hypoplasias.” The interdependence of epithelial and mesodermal progression is demonstrated by the occasionally associated connective tissue deficits seen with breast anomalies, ( absence of pectoralis muscle, ribs, and intercostal muscle, etc).

Although variations in breast location can be from asymmetric primordial cell loci, most result from volume differences during development. Some size variations may be the result of differing hormonal response, and others follow directly from the influences of various traumas like radiation, drugs, and infection upon the developmental process. However, most congenital anomalies can be traced to a difference in the numbers of primordial cells existing on the two sides. It is from this mechanism that the majority of hypoplasias come.

Processes after birth obviously contribute to the pool of patients with complaints of asymmetry. Ablational procedures are, of course, the most common of these, but breast disease and trauma to the organ produce their share. Importantly, the response of the two breasts to cyclical hormonal changes and those following the cessation of lactation and the menses may prove unbalanced, so that the plastic surgeon will often be consulted by a patient once unconcerned with symmetry who, after childbirth or menopause faces a new disparity in breast size.

The breasts normally sit upon the anterior chest wall in relatively symmetrical distribution with respect to the ventral midline. Each overlies the pectoralis major, serratus anterior and upper rectus abdominis muscles. Its clinical limits are considered to be the collar bone above, the rectus sheath below, the anterior border of the latissimus dorsi muscle and the sternum in the midline. In cases of synmastia, the breast tissue may cross the midline, creating a confluence of the two glands.

The anterior surface in convex and smooth and contains a dark ring at its center called the "areola," which surrounds the nipple. The areola is home to 12-20 sebaceous glands that make the surface bumpy. These are referred to as the tubercles of Montgomery. Unlike the surrounding breast skin, the areolar skin is not underpinned by subcutaneous fat. However, it does cover a layer of mostly circular muscle fibers that are responsible for erection of the nipple.

The prominence of the nipple is perched at the center of the areola and is usually cylindrical in form. Its roughened skin overlies muscle fibers that are oriented transversely and longitudinally. The former help to compress the galactophorous ducts and the latter cause retraction of the nipple.

The breast gland lies invested by superficial fascia and by fat that splits to lie behind the gland upon the deep layer of the superficial fascia, and anteriorly to fill the depressions between the 12-20 glandular lobes. Each lobe has its own excretory duct that dilates near the nipple into a sac called the "lactiferous sinus." Each sinus narrows again as it reaches the nipple's tip and empties its content through an opening about 0.5 mm in diameter.

Blood Supply

The arterial blood supply of the breast comes mainly from branches of the internal mammary system medially, the lateral thoracic artery laterally and perforating vessels from the intercostal arteries posteriorly. There is also some contribution from the pectoral branch of the acromio-clavicular artery. The 1st through the 4th internal mammary branches provide the most important contribution to the breast, and these arborize mostly in the superficial substance of the gland, sending some smaller branches into the deeper substance of the breast

The lateral thoracic sends a branch around the lateral border of the pectoralis major and then ramifies throughout the gland lateral, anastomosing with vessels and the internal mammary anterior plexus. The pectoral branch of the acromio-clavicular and the penetrating branches off the intercostal arteries primarily supply the deep lobes of the gland. However, there are rich anastomoses between these various systems in most instances.

The veins of the breast correspond in general to the arteries, being superficial and deep. As with the arteries, the superficial vessels appear to be of greater importance.

The lymphatic plexus serving the breasts is extremely complex and interwoven. The central and lateral breast drain primarily along vessels that run to key axillary glands. The deep upper breast sends channels to the intermammary nodes as well as around the clavicle to supra-clavicular glands. The medial breast sends its main channels along the path of the internal mammary vessels to this mediastinal chain of glands.

Innervation

Nerves to the breast come from the intercostals (to the medial and central breast, from the supra-clavicular branch of the cervical plexus (to the upper breast), and from thoracic branches of the brachial plexus, (to the lateral breast ). Nipple innervation is shared by medial and lateral nerves, but many ascribe primary importance to the antero-lateral branch of the 4th intercostal nerve.

Anatomy of Breast Enhancement

Plastic surgery of the breast depends for its success upon a sound understanding of breast anatomy, and particularly its neurovascular anatomy. In breast reduction and asymmetry-related tissue re-arrangements, denervation is possible, and even more serious sloughs of tissue can occur when vascular anatomy is not respected.

Although breast augmentation involves little risk to the blood supply of the breast, injudicious dissection on the lateral chest wall may divide important sensory nerves to the nipple, and use of a too-large implant may stretch nerves to the point of producing internal damage and dysfunction.

Breast augmentation performed in a so-called "subglandular position really involves placement of the implant in a subfascial plane. The breast gland together with the deep layer of the superficial fascia ia raised from the deep fascia over the pectoralis muscle. When the implant is placed within this plane, the lower 30-50% of the device actually sits in the same location as a typically performed "submuscular" operation, (today ,more properly defined as "dual plane").

Whether an augmentation implant is to be placed submuscularly or subglandularly, (see Breast Augmentation for advantages of different techniques), today's surgeons most often seek to do a precise lower pocket dissection, designing the space to sit behind existing breast tissue and avoiding dissection into areas that will not ultimately cover the implant. The one area of the dissection where the surgeon will often purposely go beyond the dimensions of the proposed implant is superiorly, (see image).

By creating a wider dissection in the upward direction, the surgeon is able to facilitate the re-draping of tissues over the device, preventing constriction of the implant and allowing it to completely unfold onto its entire footprint. This is particularly important when textured surface implants are used, since adherence of un-redraped tissues to the surface can create and trap folds in the surface of the implant, potentially detracting form breast shape and even leading to visible folds post-operatively.

Another anatomic issue that is not intuitively obvious to the prospective submuscular augmentation patient centers upon the relationship between the breast and the underlying pectoralis muscle. First, it should be noted that the breast extends lower on the chest wall than the lateral edge of the pectoralis. If an implant is to sit properly behind the breast in the "up and down" dimension, it must sit, in part, below the level where there is any pectoral muscle. This means that about 1/3 to 1/2 of the implant will not be covered by muscle.

Also, a subpectoral implant will tend to be pushed laterally and not allowed to occupy the proper space with respect to the breast tissue if the muscle is left intact. This is properly dealt with by surgically dividing the lower origins of the muscle to allow those fibers to separate from their osteo-cartilaginous attachments, (see at left). When this is properly accomplished, the implant can sit in the proper location medio-laterally. When it is not done, (as is often the case when non-endoscopic, trans-axillary technique is used), the muscle may squeeze the implants outward and upward, leading to a "high-riding" appearance to the breast prostheses.