Breast

15 Breast




ANATOMY


‘The breasts from their prominence, the colour of their skin, and the red colour of the nipples, by which they are surmounted, add great beauty to the female form’ (Sir Astley Paston Cooper, On the anatomy of the breast, 1840)



DEVELOPMENT AND FORM


Breasts are skin appendages which represent modified sweat glands on the anterior thoracic wall. They arise from mammary ridges of ectodermal origin which develop embryologically into paired mammary glands along the milk line extending from axillary to inguinal regions. Accessory nipples sometimes with underlying breast tissue can be found along this line but otherwise a single breast develops on each side in the pectoral region. Early growth and differentiation of breast tissue occurs in both sexes, but post-natal development is confined to females and the breast is a vestigial structure in the adult male. This secondary embryological status accounts for the absence of a true capsule surrounding the gland and lack of a specialized vasculature and innervation; the blood vessels, nerves and lymphatics of the breast come from existing structures supplying the anterior thoracic wall.


In humans the breast is a domed structure with a conical lens shape which contrasts with a flatter structure in other mammals. Breasts vary in size and shape, becoming more pendulous after lactation and a shallow disc of tissue in old age or states of malnutrition. The nulliparous gland extends from the 2nd or 3rd to the 7th rib and from the lateral sternal border to just beyond the anterior axillary fold. The lateral and inferior borders are well defined, but superiorly the breast merges with the subcutaneous tissue of the anterior chest wall. It is located on the antero-lateral aspect of the chest wall and overlies predominantly the pectoralis major muscle, extending partially over the serratus anterior laterally, the external oblique infero-laterally, the rectus sheath infero-medially and the costal cartilages medially and superiorly. Most of the glandular tissue is located in the central and upper outer quadrants with part of the breast extending along the lateral border of pectoralis major as the axillary tail (of Spence).



Glandular structure


The breast is a glandular structure lying within the superficial fascia of the anterior chest wall. This disc shaped mass of tissue is composed of epithelial parenchyma together with supporting connective tissue. The breast is essentially a conglomerate gland consisting of 15–20 lobes which are pyramids of glandular tissue with an apex pointing towards the nipple and a base peripherally. Secretions drain centripetally towards the nipple and each lobe drains via a system of branching ducts into a lactiferous sinus and in turn into a collecting duct which opens at the tip of the nipple. Each lobe drains separately and there is no communication between the ducts of adjacent lobes (Fig. 15.1). Individual lobes are composed of tubuloalveolar structures in which several acini or alveoli open into a common duct – the terminal duct. This combination of glandular acini together with their draining duct is termed the terminal duct lobular unit (TDLU) and represents the basic functional unit of the breast. The terminal ducts drain into subsegmental and in turn larger segmental ducts which are interwoven within any single lobe. The epithelial elements are supported by connective tissue which surrounds the gland and extends as collagenous and inelastic septa between the lobes and lobules. This was formerly known as the ‘fascia mammae’ which is derived from fibrous tissue in the subcutaneous layer of the chest wall and does not constitute a true capsule. However, it forms the basis for the suspensory mechanism of the breast by its attachment to the periosteum of the ribs and the skin anteriorly. The deeper layer of the superficial fascia covers the posterior surface of the breast and forms the anterior wall of the retromammary bursa. The more superficial layer of ‘fascia mammae’ extends over the anterior of the gland and penetrates the substance of the organ to provide support for both glandular and ductal elements. Fibrous thickenings formed of collagenous bundles pass from the superficial mammary fascia to the dermis of the skin. These suspensory ligaments of Astley Cooper form projections which spread out to form a white, firm irregular surface of folds between the skin and the glandular tissue (Fig. 15.2). They permit considerable mobility of the breasts in addition to a supportive function.




The ‘panaliculus adiposus’ forms a fatty envelop around the breast and is responsible for the smooth contour of the organ. Arteries, veins, nerves and lymphatics lie within this layer and are distributed throughout the structure.




Applied anatomy


The structure and form of breasts vary greatly with age, hormonal status, pregnancy and lactation. The nulliparous breast tends to be hemispheric in shape whilst those of multiparous women are larger and more pendulous. Breasts involute with advancing age with flattening and loss of firmness. The bases of the pyramids for individual lobes of the breast extend outwards to unequal lengths. The medial aspect of the breast has an irregular outline and superolaterally in the region of the axillary tail, the edge of the breast is turned up like a hem. These features should be borne in mind with a mastectomy which is increasingly being performed using a skin sparing technique which preserves much of the skin envelope of the breast. The NAC should always be excised when mastectomy is undertaken for invasive malignancy. Though breast conserving techniques are widely practiced, there is no natural plane of dissection within the gland; the arrangement of parenchymal and stromal compon-ents admixed into a conglomerate organ precludes definition of any surgical plane.


There is much variation in the physical characteristics of individual breasts; some patients have lumpy (or nodular) breasts which can make it more difficult to palpate a discrete or separate lump. Superimposed on this physical spectrum are the changes influenced by age and hormonal status. The breast undergoes reversible proliferative changes during each menstrual cycle which can exacerbate any pre-existing lumpiness. At puberty, the breast texture is dense, compact, smooth and homogeneous. During lactation, the glands sep-arate into smaller bodies with indentations around them i.e. become lobulated. A similar change occurs in the nulliparous breast towards the menopause. This lobulation must be distinguished from a discrete lump, particularly malignancy. The formal assessment of a breast lump or lumpiness now involves a combination of clinical examination with breast imaging (mammography; ultrasound) and possible percutaneous biopsy. Beyond the menopause, lobulation becomes less apparent due to atrophy of glandular parenchyma and depositionof adipose tissue. By contrast, the glandular tissue undergoes marked proliferation and hypertrophy during pregnancy with an increase in blood supply. The breasts become full, heavy, tense and often painful.



Blood supply


Despite its importance as an organ of lactation, the breast does not have a blood supply from a single defined source. Perforating branches of the internal mammary artery pierce the 2nd, 3rd, 4th (and occasionally 5th) intercostals spaces and traverse the pectoralis major muscle to supply the medial and deep parts of the gland. These vessels can produce troublesome bleeding at operation should they retract into the chest wall once divided. These branches anastomose with those of vessels entering from the superolateral aspect of the breast arising from the axillary artery; the lateral thor-acic artery provides branches which sweep around the lateral border of pectoralis major to reach the gland. Branches from the thoracoacromial trunk together with some intercostals vessels supply the deeper aspects of the gland. There is a rich anastomotic network between these different sources of blood supply and during pregnancy the medial perforators increase consid-erably in diameter.


The venous drainage of the breast generally corres-ponds to the arterial supply. Veins beneath the areola form an anastomotic circle (circulus venosus) which together with deeper veins carry blood to the periphery of the gland where venous outflow is via the internal thoracic, lateral thoracic and upper intercostals veins. Breast cancer can spread haematogenously via these venous routes and also by way of the vertebral venous plexus which is linked to veins of the chest wall by valveless conduits.



Lymphatic drainage




Lymphatic vessels


The lymphatic system of the breast is a complex network of arborising vessels which reflect its embryological origin from anterior thoracic wall structures. The lymphatics of the breast skin and parenchyma are inter-connected and flow within the valveless vessels is passive. This results in a degree of plasticity which isrelevant to malignant infiltration whereby uni-directionallymph flow may be diverted due to blockage at prox-imal sites by tumour emboli. The lymphatics of the skin of the breast represent part of the superficialsystem of the neck, thorax and abdomen. In the region of the NAC, a cutaneous plexus is linked to a subareolar plexus which receives lymphatics from the glandular tissue of the breast. From this subareolar and a related circumareolar plexus, lymph flows principally to the axillary nodes via a lateral lymphatic trunk. This together with the minor inferior and medial lymph-atic trunks drain along the surface of the breast to penetrate the cribriform fascia and reach the various groups of axillary nodes (Fig. 15.3). Anatomical studies suggest that the lymphatics of the breast initially drain to a group of 3–5 ‘sentinel’ nodes usually located at level I. Lymphatic channels exist which bypass the lower axillary nodes and probably account for ‘skip’ metastases and the finite false negative rate of sentinel lymph node biopsy. Lymph drains medially from the circumareolar plexus into lymphatics accompanying the internal mammary vessels to enter the internal mammary (parasternal) nodes. Approximately 75% of lymph flow passes to the axillary nodes and 25% to the internal mammary nodes.



Until recently, most patients with invasive breast cancer underwent formal axillary clearance with surgical excision of all nodes at levels I (4–8 nodes), level II (10–15 nodes) and occasionally level III (30 nodes). Patients now present with smaller tumours where there is less likelihood of nodal involvement (25–30%). Sentinel lymph node biopsy is a new staging proced-ure which can accurately determine the pathological status of axillary nodes. Clearance of nodal tissue is confined to clinically node positive patients and those clinically node negative patients found to have nodal disease on sentinel node biopsy.



PHYSIOLOGY





Dec 12, 2016 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Breast
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