Breast Development, Physiology, and Histology

Embryology

In utero breast development involves two main processes: formation of the primary mammary bud and development of the rudimentary mammary gland. Breast development does not differ between genders.

In the first trimester of gestation, breast development begins with the formation of mammary crests which are bilateral ventral epidermal ridges arising from the ectoderm of the fetus. These thickened ridges extend in a linear manner from the axilla to the groin, forming the milk line ( Fig. 1.1 ). As fetal development proceeds, all except a pair of these thickenings, one on each side of the pectoral region at the fourth intercostal space, regress. These two residual solid epithelial masses form the primary mammary buds.

In its earliest stages, the thickened ridges are a result of condensed mesenchymal tissue around an epithelial bud. Solid epithelial cord–like columns develop from the bud. Portions of the dermis increasingly envelop the epithelial columns and develop into the connective tissue of the breast. More fibrous elements of the dermis extend into the developing breast, and much later form the suspensory ligaments of Cooper (named after Astley Cooper, the English anatomist and surgeon, who described these structures in the 19th century).

During the second trimester, secondary epithelial buds arise from indentations of the primary bud. These secondary epithelial buds grow vertically into the mesenchyme, forming columns surrounding the primary bud. Gradually, these secondary epithelial columns branch, canalize, and transform into ducts (and eventually into lobules). Thus each column ultimately gives rise to a lobe of the breast.

In the third trimester, repeated branching of the secondary epithelial buds and canalization occurs, which ends in rudimentary lobular structures or end buds. The developing mammary glands are responsive to maternal hormones and exhibit mild secretory changes. A pit in the epidermis forms at the convergence of the major (lactiferous) ducts, and shortly thereafter its eversion forms the protuberant nipple ( Fig. 1.2 ). Rarely, the nipple may not evert, resulting in an inverted (or permanently retracted) nipple. This deformity may cause considerable difficulty in suckling. At term, 15 to 20 breast glandular tissue lobes have formed, each of which contains a lactiferous duct that opens onto the breast surface through the mammary pit.

Following parturition, the withdrawal of maternal hormones stimulates prolactin release, which initiates colostrum (“witch’s milk”) secretion. This occurs during the first few days after birth in approximately 90% of infants of both genders. Colostrum is composed of water, fat, and debris; its secretion dissipates within 1 month or so after birth. During this time, and for the following few weeks, the breast is palpably enlarged. Until puberty, in both genders, the breast glandular tissue consists almost exclusively of major ducts.

Gross Anatomy

The female breasts are rounded protuberances on either side of the anterior chest wall. The organ is present in a rudimentary form in prepubertal girls, boys, and adult males. While the bulk of female breast tissue overlies the pectoralis major muscle from the second to the sixth rib in the vertical axis and from the sternal edge to the mid-axilla in the horizontal axis, breast glandular tissue usually extends beyond these arbitrary boundaries. The extension of breast tissue from the upper-outer quadrant into the axilla is referred to as the tail of Spence (named after James Spence, a Scottish surgeon of the 19th century). This tail can be difficult to visualize on routine mammograms. Historically, to allow better visualization of this tissue patients were placed in the Cleopatra pose (a semi-reclining stance in which the patient turns and leans backward, a favored pose of the famed Egyptian pharaohs). The current standard mammographic screening views include the bilateral craniocaudal (CC) and mediolateral oblique (MLO) views.

The breast is enveloped by fascia. Anteriorly, there is superficial pectoral fascia. Posteriorly, there is deep pectoral fascia. These two layers of fascia blend with the cervical fascia superiorly and with that overlying the abdomen inferiorly. The previously described Cooper ligaments are more numerous in the superior half of the breast and connect these two layers of fascia. The retromammary space, which lies between the deep boundary of the breast and the fascia of underlying skeletal muscle, is filled with loose connective tissue and allows the breast some degree of movement over the underlying pectoral fascia. The deep fascia overlying the chest wall occasionally harbors breast glandular tissue; however, this tissue only rarely extends beyond this fascia into underlying skeletal muscle. In modified radical mastectomies, which aim to remove as much of the breast glandular tissue as possible, this extension of breast glandular tissue has clinical implications. Furthermore, residual breast tissue is detected in ~50% of skin- and nipple-sparing mastectomies.

Factors influencing the size and shape of the breast include age, diet, parity, menopausal status, genetics, and race. While typically oval and hemispherical, the breast may be hemispherical, conical, pendulous, piriform (i.e., pear-shaped), or thinned and flattened. There is a distinct flattening of the superficial contour of the breast superior to the nipple.

The normal mature, nonlactating female breast weighs approximately 200 g (±100 g). The typical lactating breast may weigh more than 500 g. The average adult breast typically spans 12 cm in diameter and 6 cm in thickness. The right breast has been shown to be less voluminous than the left, which is correlated with handedness. There is no correlation between breast mass and risk of carcinoma as large breasts do not necessarily contain more glandular parenchyma.

The nipple is centrally located and elevated from the surrounding areola, making it the most distinctive feature of the breast skin. The level of the nipple on the chest wall varies widely but typically overlies the fourth intercostal space in younger women. Both the nipple and areola are pink, light brown, or darker (depending on the overall pigmentation of the body). The nipple and areola are slightly less pigmented in the nulliparous and become increasingly pigmented starting in the second month of pregnancy, a change that is irreversible after pregnancy.

Twelve to 20 minute, rounded protuberances in the dermis, representing prominent sebaceous gland units usually associated with a lactiferous duct, are present on the surface of the areola. These protuberances, known as Montgomery tubercles (named after Dr. William Montgomery, a 19th-century Irish obstetrician, although possibly described earlier by Morgagni, the 18th-century Italian anatomist), become prominent during pregnancy and lactation, reflecting the need to keep the areola moist during feeding. Following menopause, Montgomery tubercles regress. Apocrine and sweat glands are also present in the immediate area. Hair follicles are present at the edge of the areola. The presence of these glands and hair follicles may be involved in the pathogenesis of persistent subareolar abscesses.

While the breast is clinicopathologically divided into 4 quadrants (upper-outer, upper-inner, lower-inner, and lower-outer), it’s important to recognize that these quadrants do not exist in anatomical terms. Furthermore, the terms multifocal and multicentric merit mention. Multifocal is usually defined as disease within the same quadrant, while multicentric is generally used for disease in at least 2 quadrants (or for disease >5.0 cm apart). Multiple scenarios reveal the inadequacies of these definitions (e.g., boundary tumors that traverse 2 quadrants and centrally placed tumors that span multiple quadrants).

In the current tumor-node-metastasis (TNM) staging system, breast carcinoma of any size is staged as T4 if it directly extends to the chest wall and/or involves the skin as edema (including peau d’orange) or as satellite skin nodules, or causes epidermal ulceration. Direct extension of carcinoma into the dermis or adherence/invasion to the pectoralis muscle does not qualify as T4.

Structure and Histology

From the nipple, there are several collecting (aka lactiferous) ducts, each of which drains into a mammary lobe ( Fig. 1.3 ). The lobes are arranged around the breast in a radial (spoke-like) manner. In 3D depictions of breast lobes, the lobes are conical, with the apex at the nipple and the base in deeper breast tissue where lobules reside. While most textbooks depict mammary lobes as discrete anatomical territories within the breast, at their borders the lobes grow intricately into one another and do not constitute distinct, grossly identifiable entities. As such, the lobes cannot be visually demarcated (and dissected) during surgery. Additionally, each duct system differs in anatomical extent, with the larger ones extending beyond a quadrant and the smaller ones occupying less than a quadrant. While the lobes are independent systems, it is possible that a few lobes may interconnect at some level via ducts.

The nipple and areola are lined by stratified squamous epithelium , which is continuous with the surrounding skin over the breast ( Fig. 1.4 ). The opening of the collecting ducts at the nipple is typically plugged by keratinous debris in the nonlactating breast. The squamous epithelium of the collecting ducts undergoes a gradual transition to pseudostratified columnar epithelium and, finally, to cuboidal or low-columnar epithelium.

Approximately 20 orifices of collecting ducts , each representing a lobe of the breast, are present in the nipple. These orifices, which may be as few as 8 and as many as 24, are generally arranged as a central group and a peripheral group. The deeper portion of the collecting ducts has a characteristically serrated contour for a variable distance before opening into its terminal portion. The latter portion has a relatively less convoluted and smoother profile. The lactiferous ducts in the nipple are surrounded by bundles of smooth muscle. The muscle fiber arrangement is principally circular, but some fibers are also arranged vertically, interlacing among collecting and lactiferous ducts. The circular muscle fibers cause nipple erection, equipping it for suckling. By cyclic contraction, the vertically arrayed muscle bundles empty the lactiferous sinuses. There is virtually no adipose tissue immediately beneath the nipple and areola.

The portion of the ductal system immediately below the collecting duct is the lactiferous sinus, in which milk accumulates during lactation. This sinus communicates directly with the segmental duct, which subdivides into subsegmental ducts, which in turn subdivide into terminal ducts. The latter structures drain the lobules. Each lobe contains 20 to 40 lobules. Each lobule is composed of clusters of small glandular structures, the acini. The acini are the terminal point of the ductal system. The serially and dichotomously branching structure of the mammary gland, from the tubular-like collecting duct to the terminal acini, leads to its classification as a compound tubuloacinar (or tubulolobular) gland ( Fig. 1.5 ).

The lobule is not grossly apparent on cut sections of breast tissue. Both lobular size and the number of acini within each lobule are extremely variable. Each lobule consists of approximately 10 to 100 (range 8–200) acini. The intralobular stroma consists of loose connective tissue and may also be populated by a mixed inflammatory cell infiltrate, particularly in the secretory phase of the menstrual cycle. The lobule undergoes a variety of morphological changes under various physiological influences ( Fig. 1.6 ).

The fundamental glandular unit of the breast, and its most actively proliferating portion, is the terminal duct lobular unit ( TDLU ). This unit consists of the lobule and its paired terminal duct. During pregnancy and lactation, the epithelial cells of the terminal ducts and lobules undergo secretory changes; also, most disease processes of the breast arise from the TDLUs. The only common lesion believed to be strictly of ductal origin may be the solitary intraductal papilloma ( Table 1.1 ).

Except for the squamous, epithelium-coated, most distal portion of the collecting ducts, the entire ductal system is lined by low-columnar to cuboidal epithelium . This epithelium lining is supported on its basal surface by a layer of myoepithelial cells. The basement membrane (basal lamina) lies under the layer of myoepithelial cells and rests on connective tissue.

Myoepithelial cells facilitate milk secretion via their contractile property, which is largely under the influence of oxytocin. Receptors for the latter have been detected on the surface of myoepithelial cells, and this hormone is primarily responsible for the mechanical release of milk.

Myoepithelial cells are generally spindle-shaped, with inconspicuous cytoplasm. In fine-needle aspiration cytology preparations, myoepithelial cells appear to be entirely devoid of cytoplasm (i.e., naked). The thin and compressed ( bipolar ) nuclei of the myoepithelial cells are oriented perpendicular to the layout of the epithelial cells. Myoepithelial cells extend from collecting ducts to the tip of the acini and may occasionally appear prominent either de novo ( Fig. 1.7 ) or in certain physiological states (e.g., atrophy) and pathological situations (e.g., post-radiation, adenomyoepithelioma). Myoepithelial cells appear to be inapparent in certain lesions (e.g., in macrocysts, in which these cells get stretched).

A number of immunohistochemical stains can be used to demonstrate the presence of myoepithelial cells around ducts, which vary in sensitivity and specificity ( Table 1.2 and Fig. 1.8 ). Lack of the myoepithelial cell layer around neoplastic glands is generally considered to be diagnostic of invasive carcinoma, with some exceptions for microglandular adenosis and noninvasive forms of papillary carcinoma (i.e., solid papillary and encapsulated papillary carcinoma). Absence of the myoepithelial cell layer has also been reported in some apocrine cysts. The use of double (or even triple) immunolabeling with combinations of epithelial and myoepithelial immunostains is helpful in confirming the presence of invasive (particularly microinvasive) carcinoma ( Fig. 1.9 ).

The basement membrane , composed of a relatively attenuated basal lamina, lies under the myoepithelial cell layer and divides the glands from the stroma. The basement membrane can be highlighted using appropriate immunostains (e.g., laminin and collagen 4) or histochemical stains (e.g., reticulin and periodic acid-Schiff). Stromal tissue lies beyond the basement membrane. The multilayer structure of the mammary glands can be highlighted by using various histochemical and immunohistochemical stains ( Fig. 1.10 ).

The mammary ducts and lobules are embedded within a variable fibrous and fatty stroma (i.e., interlobular stroma ). The relative proportion of glands, fibrous tissue, and fat varies with age and body habitus; however, stromal tissues make up the bulk of the breast in nonlactating and nonpregnant adult women. Adipose tissue is typically present in the interlobar stroma and not among lobules. The fibrous tissue assists in the mechanical coherence of the gland. The fibroblastic and myofibroblastic elements in the stroma of the breast often display an angiomatous appearance (hence the term pseudoangiomatous stromal hyperplasia) ( Fig. 1.11 ). The volume fraction of collagen-rich fibrous tissue is greater in younger adult women and accounts for the greater mammographic density therein. Within the United States, several states have enacted laws that require healthcare facilities to notify patients who are categorized as having dense breast tissue on mammograms. Such legislation is designed to help improve the detection of breast carcinoma via the use of additional imaging modalities.

Apocrine cells are normal constituents of the glands of the breast in adult women, suggesting that this finding is a physiological phenomenon (i.e., a normal line of metaplastic differentiation) rather than a pathological finding ( Fig. 1.12 ). Apocrine cells are typically cuboidal or columnar with eosinophilic cytoplasm. Furthermore, they may exhibit a stubby apical snout and may contain apocrine granules ( Fig. 1.13 ). Inexplicably, cysts lined by apocrine cells are a common finding in magnetic resonance imaging (MRI)-detected breast lesions, and may contain calcium oxalate crystals. The latter may need polarizing microscopy to be optimally visualized ( Fig. 1.14 ). Apocrine cells are almost always negative for both estrogen and progesterone receptors (ER and PgR) and are strongly positive for epithelial membrane antigen (EMA), gross cystic disease fluid protein-15 (GCDFP-15), and androgen receptor (AR).

In some instances, clear cell change can occur in epithelial cells (of both ducts and lobules) as well as myoepithelial cells ( Fig. 1.15 ). In epithelial cells, clear cell change can be commonly seen in association with apocrine metaplasia and following cytoplasmic accumulation of glycogen. Clear cell change can occur either spontaneously or sporadically in myoepithelial cells ( Fig. 1.16 ) and may be seen in association with adenomyoepitheliosis and adenomyoepithelioma. Such a change in either epithelium or myoepithelium has not been associated with progression to any disease process.

Foam cells are normally found within the lumina of glands (which are frequently dilated), in glandular epithelium, and in stroma ( Fig. 1.17 ). These foam cells may be polygonal, epithelioid, or spindled in appearance. Pigment-laden histiocytes (aka ochrocytes) appear in the periductal connective tissue of approximately 15% of breasts ( Fig. 1.18 ). These relatively large cells with a low nuclear-to-cytoplasmic ratio contain pale yellow to dark brown pigment. The pigment seems to have the staining qualities of lipofuscin (i.e., positive for periodic acid-Schiff [but diastase resistant], weakly positive for acid-fast stain, and negative for iron). Multinucleated stromal giant cells may rarely be present in the interlobular fibrous stroma, especially in the myofibroblast-dominant areas ( Fig. 1.19 ), and are of no known clinical significance.

A framework of elastic tissue is present along the length of the duct system from the nipple to the subsegmental ducts. TDLUs are typically surrounded by a cuff of fibrous or myxoid connective tissues that contain virtually no elastic tissue. The larger ducts have sparse specialized connective tissue and possess relatively more elastic tissue. Bundles of elastic tissue are present in the periductal stroma of approximately 50% of women over the age of 50 ( Fig. 1.20 ). Elastosis implies an excess of elastic fibers over normal, although the baseline level of elastic tissue in the female breast remains undefined.

Two types of benign clear cells are present in the nipple among the stratified squamous epithelium. These are the cellules claires and the Toker cells. The more common cellules claire (French for “clear cells”) type, seen in about one-third of the nipples, has clear cytoplasm and a semilunar nucleus that is compressed to the edge ( Fig. 1.21 ). The clarity of the cytoplasm is likely the result of hydropic change. These clear cells are typically numerous and scattered throughout the full thickness of the epidermis. The clear portion of the cytoplasm of cellules claires is nonreactive for various cytokeratins, EMA, carcinoembryonic antigen, and papillomavirus markers. The second type of clear cells (i.e., Toker cells ) is more clinically significant because it can be mistaken for Paget disease of the nipple. These cells, first detailed by Cyril Toker, a pathologist in New York City, are “smaller in size than typical Paget cells” and “larger than their squamous neighbors.” The origin of Toker cells is not well understood but has been postulated to represent extensions of mammary duct epithelial cells into the epidermal surface of the nipple, to represent remnants of the embryonic nipple bud, or to potentially be related to sebaceous glands. These cells have bland nuclei and pale cytoplasm and appear to be most numerous around the openings of lactiferous ducts. Toker cells occur either singly or in aggregates of a few cells, and are most commonly encountered near the basal layer but may also be found in the more superficial layers ( Fig. 1.22 ). Notably, Toker cells can appear dendritic (i.e., stellate) on cytokeratin 7 (CK7) immunoreaction.

Paget disease of the nipple (named after Sir James Paget, the 19th-century British surgeon and pathologist) is the ascending extension of carcinoma cells along the preexisting scaffold of the ductal system of the breast and into the epidermis of the nipple. Rarely, these Paget cells form glands. Except for HER2 (which is strongly immunoreactive in >90% of Paget cells), immunohistochemistry is generally unhelpful in the differential diagnosis of Toker and Paget cells because both cell types are reactive for various cytokeratins (including cell adhesion molecule [CAM] 5.2 and CK7) and EMA and are nonreactive for CK20 and S100-protein ( Fig. 1.23 ).

Ultrastructure

On electron microscopy, the luminal epithelial cells contain mitochondria, rough endoplasmic reticulum, and secretory granules. Surface specialization is present with microvilli projecting into the extracellular lumen. Desmosomes are present along the lateral interface with neighboring epithelial cells. Depending on the physiologic state of the organ, secretory granules and droplets at the apical pole of the cells may be present. A seemingly continuous layer of myoepithelial cells lies under the epithelial cells. This layer is oriented perpendicular to the epithelial cells. Myoepithelial cells contain contractile actin filaments that are more electron dense and contain intacytoplasmic myofibrils with dense bodies and pinocytotic vesicles. The myoepithelial cells are attached to the underlying basement membrane via hemidesmosomes. Where there are gaps between the myoepithelial cells, the epithelial cells appear to rest directly on the basement membrane.

Arterial Supply

The breast receives it arterial supply through the lateral thoracic artery and the internal mammary artery (aka internal thoracic artery). The former supplies the upper outer portions of the breast while the latter serves the central and medial portions. While portions of the internal mammary artery may be used during coronary artery bypass graft surgery, necrosis of breast tissue is only a rare complication. Other vessels that contribute to the arterial supply of the breast include the intercostals (mainly the second to fourth), lateral thoracic, subscapular, thoracoacromial, and thoracodorsal arteries.

Similar to that seen in other organs, mammary arteries may exhibit sclerotic changes and Monckeberg medial calcific sclerosis (named after Johann Monckeberg, a German cardiovascular pathologist) ( Fig. 1.24 ). On screening mammogram, these arterial calcifications are detected in up to 9% of breasts in postmenopausal women; however, these findings are not predictive of coronary heart disease at coronary angiography.

Given the relatively rich arterial network in the breast, it is not surprising that the vessels get traumatized by invasive procedures such as needle core biopsies. Indeed, stereotactic vacuum assisted biopsies frequently (~90%) result in bleeding or hematoma formation, and several cases of arterial pseudoaneurysm formation after core biopsies have been reported.

Venous Drainage

While the venous drainage of the breast mostly follows the arterial system, the veins of the breast are more variable. The superficial venous system of the breast drains into the internal thoracic vein. The deep venous system drains into the perforating branches of the internal thoracic vein, lateral thoracic vein, axillary vein, and upper intercostal veins. A circular venous plexus lies around the areola.

Lymphatic System and Regional Lymph Nodes

Much (>75%) of mammary lymph drains into the axilla. Of the remainder, a majority drains into the internal mammary nodes with lesser lymphatic channels leading to the interpectoral, internal thoracic, supraclavicular, and infraclavicular (and possibly even intramammary) lymph nodes. Lymphatic channels of the breast lead directly to the axillary or internal mammary nodes without involving the rich subareolar lymphatic plexus.

The axillary lymph nodes lie along the axillary vein and its tributaries and are typically divided into three levels. Level 1 nodes lie in the low-axilla, lateral to the axillary border of the pectoralis minor muscle; level 2 nodes lie in the mid-axilla, between the medial and lateral borders of the pectoralis minor muscle; and level 3 nodes lie in the apex of the axilla, medial to the cranial margin of the pectoralis minor muscle and inferior to the clavicle. Level 2 nodes also include the Rotter lymph nodes (named after Josef Rotter, a German surgeon in the late 19th century), which lie between the pectoralis major and pectoralis minor muscles and may comprise up to four nodes. Rotter lymph nodes are characteristically involved in breast carcinomas that arise from the upper-central and upper-outer regions of the breast. Level 3 lymph nodes are also known as apical or infraclavicular nodes ( Fig. 1.25 ). Metastases to the latter group of lymph nodes portend a worse prognosis. Axillary lymph nodes usually range from 20 to 30 in number, with an average of 24; however, up to 81 lymph nodes have been dissected from this group. Historically, it was thought that breast carcinoma involved the various levels of nodes in a stepwise fashion, progressing from levels 1 to 3. However, this traditional subdivision of axillary lymph nodes has been challenged by more recent studies that have studied the location of sentinel lymph nodes. Sentinel lymph nodes, which are the first lymph nodes to receive lymphatic drainage from the breast, are seen at level 2 in up to 23% of patients, and metastases in level 3 lymph nodes only (skipping nodes at levels 1 and 2) are present in about 2% to 3% of cases. Of note, sentinel lymph nodes are rarely found to be in extra-axillary locations, which is typically encountered in cases following irradiation to the breast or axillary surgery.

Intramammary lymph nodes may be incidentally identified in breast biopsies or mastectomies, but are more often identified as an ovoid <2 cm circumscribed density on imaging studies. In one series, intramammary lymph nodes were identified in 28% of mastectomies performed for operable breast carcinoma, although in routine practice, these nodes are rarely encountered (<1% of cases). While up to 10% of these nodes can be positive for metastatic carcinoma, they may not be part of the usual lymphatic drainage system of the breast. Before diagnosing a positive intramammary lymph node, invasive carcinoma with medullary features (which is typified by a prominent lymphoid response) must be considered in the differential diagnosis. A lymph node (regardless of its location) has a capsule, subcapsular sinus, and at least one well-formed lymphoid follicle. Intramammary lymph nodes are considered as axillary lymph nodes for staging purposes.

In the current staging system, isolated tumor cell (ITC) clusters defined as groups of tumor cells measuring ≤0.2 mm and up to 200 cells total (i.e., pN0[i+]). The N1 category includes micrometastasis, which is defined as nodal tumor deposits measuring from >0.2 to 2 mm; and N1, which is defined as 1 to 3 lymph nodes with at least one macrometastasis (i.e., nodal tumor deposit measuring >2 mm). The N2 category includes nodal tumor deposits in 4 to 9 lymph nodes, and the N3 category is defined as nodal tumor deposits in 10 or more lymph nodes. As such, a lymph node dissection with a harvest of more than 10 lymph nodes is considered adequate for staging purposes.

The internal mammary lymph nodes , present in the intercostal spaces 2 to 3 cm from the edge of the sternum in the endothoracic fascia. Supraclavicular lymph nodes are considered regional nodes and lie in the supraclavicular fossa, a triangle defined by the omohyoid muscle (laterally and superiorly), the internal jugular vein (medially), and the clavicle and subclavian vein (inferiorly). In the absence of axillary lymph node metastases, involvement of the internal mammary node with a deposit >0.2 mm is staged as pN1b; it is staged as pN2b when the metastases are detected clinically (with or without microscopic confirmation). Involvement of the ipsilateral infraclavicular lymph nodes is staged as pN3a and of the supraclavicular lymph nodes as pN3c.