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Female Breast Anatomy


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Breast Development:
  • Mammary glands begin to develop at 6wks LMP
  • Multiple paired areas of ectodermal thickening occur along the mammary ridges but only one pair remains to form the breasts
  • Mammary ridges are also known as milk lines
  • At birth only the main lactiferous ducts have developed
  • Ripening of the breast normally occurs between 9-16yrs
  • Estrogen causes stromal (connective) tissue to elongate and grow, vascularity increases and fat deposition increases
  • Progesterone stimulates TDLU growth
  • Thelarche refers to rapid breast tissue development at puberty
  • Premature Thelarche refers to isolated early ripening of the breasts which usually occurs at 6-8yrs of age
  • Precocious Puberty refers to breast enlargement, onset of menses, underarm/pubic hair, and acne that occurs before age 8; Causes include adrenal gland tumor, primary hypothyroidism or ovarian cyst
  • Breast tissue is not considered completely mature or ripened until pregnancy and lactation occurs
  • If there is no "immediate" pregnancy, the breast tissue fully matures 2 years post menarche
  • Breasts are normally asymmetric in size; usually L>R
Congenital Anomalies:
  1. Nipple Inversion: usually bilateral if congenital; associated pathology can be present if the nipple change is a new finding and/or occurs unilaterally
  2. Athelia: absence of the nipple
  3. Polythelia: accessory nipples, can develop anywhere along the milk line; most commonly seen just inferior to the normal nipple; #1 congenital anomaly of the breast in females and males
  4. Amastia: failure of the breast and nipple to develop
  5. Polymastia: accessory breast tissue; most commonly forms without a nipple; usually found in the axilla
  6. Amazia: absence of development of the functional breast tissue beneath a normal nipple/areola
Supernumerary Breast Tissue:
  • Classification developed in 1915 by Dr. Kajava
  • Polymastia and/or polythelia
  • Polythelia most common
Breast Anatomy:
  • AKA Mammary glands
  • Milk Line: AKA mammary ridges refers to the area between the axilla and inguinal region where breast tissue may be formed, EX: Accessory nipples
  • Sonographically there are 6 layers of tissue identified:
    • skin
    • premammary layer
    • mammary layer
    • retromammary layer
    • muscle layer
    • chest wall
  • Skin layer:
    • Usually between 0.5 – 2mm thick
    • Composed of epidermal cells
    • Contains sebaceous glands and hair follicles
    • Covers the subcutaneous layer of breast tissue
    • Thickest at the base of the breast (inferior crease line)
    • Thicker in younger patients and thins as the patient ages
  • Nipple: composed of erectile tissue and contains small excretory ducts to drain milk
  • Areola: small area of pigmented skin that encircles the nipple; composed of smooth muscle tissue
  • Montgomery glands: small sebaceous glands on the surface of the areola that secrete "protective" oily substance during lactation
  • Axillary Tail of Spence refers to the part of the mammary tissue that extends into the upper outer quadrant and axilla area
  • Pectoralis muscles lie posterior to retromammary layer and line the chest wall from the 2nd - 6th rib from sternum to axilla
  • The pectoralis minor muscle lies posterior to the pectoralis major muscle
  • Ribs lie posterior to the pectoralis muscles; a rib is the most common palpable finding that is mistaken for a mass
  • Right and left intercostal nerves innervate each breast
Three Layers of Breast Tissue:

1.  Premammary Layer:
  • AKA subcutaneous fat layer
  • Contains varied levels of fat related to age, obesity and pregnancy
  • Present across the breast except immediately posterior to the nipple
2. Mammary Layer 
  • AKA glandular or parenchymal layer
  • Only layer that contains glandular tissues (functional tissues)
  • Contains epithelial and stromal tissues
  • Epithelial tissue is considered the functional tissue of the breast includes TDLU, lobules, lobes, lactiferous ducts
  • Stromal Tissue is the structural tissue of the breast includes fat and connective tissue
  • Each breast is composed of 15-20 lobes of glandular tissue arranged in a radial fashion around the nipple and separated by fatty tissues
  • Lobes consist of multiple lobules each with an associated intralobular duct
  • Lobules are composed of glandular tissues which contain the functional epithelial cells (acini cells) that produce milk
  • Intralobular terminal ducts from the smaller lobules drain milk into the ductal system, once the milk exits the lobule it enters the  extralobular duct
  • Extralobular terminal ducts converge into the main interlobular duct which drains all the milk from that cluster of lobules; end at the main duct
  • Main ducts drain the milk from each breast lobe (15-20 ducts); ducts form a pyramid shape, focused to an apex at the areola/nipple
  • Widening of the distal end of each of the main ducts is referred to as the ampulla; serves as a reservoir for milk just prior to it leaving the breast
  • Cooper’s ligaments course between lobes from the chest wall/axilla toward the nipple to support the breast parenchyma; AKA interlobular connective tissue
  • Intralobular connective tissue is a less dense form of connective tissue that surrounds each individual lobule of breast tissue
  • Upper outer quadrant of each breast contains the most glandular tissue
  • The amount of glandular tissue in the mammary layer varies with age and menopause
3.  Retromammary Layer:
  • Located posterior to the mammary layer
  • Contains varied levels of fat related to age, obesity and pregnancy
  • Deep fascia also within the retromammary space
  • This layer of tissue allows breast motion over the chest wall
Superficial Fascia:
  • Surrounds all mammary tissues and separates the breast into superficial and deep tissues
  • Composed of two layers; superficial and deep
  • Superficial layer (superficial fascia) within the premammary layer
  • Deep layer (deep fascia) lies posterior to the mammary layer within the retromammary space
  • Irregularities of the deep fascia posterior to a malignancy can indicate metastasis to the chest wall
Terminal Duct Lobular Unit (TDLU):
  • Considered the "functional unit" of the breast
  • Several hundred acini cells are in the breast
  • TDLU refers to 30-50 acinar cells grouped together in a lobule and their associated ducts
  • Acini cells are the smallest functional unit of the breast
  • A terminal duct arises from each acinar cell
  • TDLU usually less than 2mm in size
  • # and size of TDLUs vary with age and hormone levels
  • Proliferation of the TDLUs normally occurs during reproductive years and pregnancy/lactation
  • Atrophy of the breast lobules normally occurs with cessation of breast feeding and in postmenopausal women
  • Nearly all breast pathology originates in the TDLU
Ductal System:
  • Lined with epithelial cells to reduce friction for easier milk flow
  • Middle layer composed of myoepithelial cells which aids in the movement of milk
  • Basement membrane forms the outer layer of the duct which is in contact with the intralobular stroma
  • Normal ducts in a non-lactating female should measure <2mm diameter and increase in size closer to the nipple
  • Normal ducts in a lactating female should measure <8mm diameter
  1. Intralobular terminal ducts - carry milk from the acini
  2. Extralobular terminal ducts
  3. Interlobular terminal ducts - travel between the breast lobes
  4. Main Terminal Duct/Lactiferous Sinus or Ampulla
  5. Collection ducts or Excretory ducts - 15-20 openings that excrete the milk from the nipple
  6. Nipple
Vasculature:

Arterial Supply:
  • Lateral thoracic artery:
    • Originates from the axillary artery
    • Supplies lateral breast tissues
  • Internal thoracic artery (AKA internal mammary artery)
    • Originates at the subclavian artery
    • Courses lateral to the sternum
    • Supplies medial breast tissues
    • Used for coronary bypass surgery when other vessels unavailable (great saphenous vein, radial artery)
  • Thoracoacromial artery supplies superior breast tissue
  • Intercostal artery supplies the inferior breast tissue
Venous Drainage:
  • Superficial venous system lies within the subcutaneous fat tissue
    • blood moves from the superficial system into the deep system
    • most distal veins line the superficial fascia and drain blood centrally
    • provides connection between right and left breast with potential for metastasis to occur between breasts
  • Deep venous system consists of veins that follow the arterial system
    • Primary route of venous drainage of the breast
    • Communicates with the axillary vein, subclavian vein and SVC
    • Includes internal mammary vein, lateral thoracic, axillary, subclavian and intercostal veins
Lymphatic System:
  • Intramammary nodes found within the breast parenchyma; most concentrated in the upper outer quadrants
  • Lymph vessels closely follow the venous drainage system of the breast
  • Flow in the deep system moves centrally toward the nipple
  • Lymph flow is from the deep nodes into the superficial system
  • Flow in the superficial lymph system travels away from the areola
  • Majority of lymph flow out of the breast occurs through the axillary, internal mammary and intercostal lymphatic chains
  • Lymphatic vessels contain valves to assist in the flow of lymphatic fluid
  • Lymphatic vessels are not usually visible Sonographically
  • Lymphatic vessels do not easily dilate when flow abnormalities are present and they are not easily compressible when dilated
Lymphatic Chains Draining the Breast:
  1. External Mammary: segment that follows the course of the lateral thoracic vessels
  2. Scapular: segment that follows the course of the subscapular vessels
  3. Axillary: lymph nodes and vessels that follow the course of the lateral axillary vessels; consists of 30-40 lymph nodes responsible for 75% of lymph drainage; #1 site for lymph node metastasis from breast cancer
  4. Subclavicular: segment that follows the course of the subclavian vessels
  5. Central: segment located within fatty tissues medial to the axillary vessels
  6. Rotter’s (interpectoral): segment located between the pectoral major and minor muscles

Axillary Node Classification:
  • Used for staging and developing the surgical approach
  • Nodes are assessed for metastasis
Level I - found in nodes lateral to pectoralis minor muscle; pectoral, subscapular, lateral, and paramammary nodes
Level II - found in nodes deep to the pectoralis minor muscle; interpectoral and central nodes
Level III - found in nodes medial to the pectoralis minor muscle; apical nodes

Accessory Node Chains (25%)
  • Internal Mammary: lymph nodes and vessels that follow the internal mammary vessels; involved in metastatic drainage most commonly with medial cancer formation
  • Intercostal/Parasternal: found adjacent to the course of the internal thoracic artery and vein
  • Supraclavicular - found adjacent to the internal jugular vein and the subclavian vein

Breast Physiology:
  • Exocrine gland - secretes substance carried by a ductal system (milk)
  • Main function is the production and secretion of milk
  • Acini cells are the smallest functional unit of the breast
  • Several hundred acini cells in each breast produce milk into a separate duct (terminal duct)
  • Milk funneled into the ductal system toward the nipple
  • Estrogen - produced by the ovaries; levels rise during the first half of the menstrual cycle and stimulate ductal proliferation
  • Progesterone - produced by the ovaries; levels rise with ovulation and stimulate lobular proliferation and growth which can lead to "PMS" symptoms in breasts
  • Prolactin inhibitors - produced by the hypothalamus; prevents lactation until pregnancy
  • Prolactin - produced by the pituitary gland; stimulates milk production with pregnancy
  • Oxytocin - produced by the pituitary gland; causes ductal contraction with lactation
  • Estrogen and progesterone levels decrease significantly after birth and prolactin levels increase to stimulate milk production
  • Milk production usually begins within 2-3 days post-partum
Breast Tissue Changes:
  • Puberty: estrogen and progesterone stimulate breast development
  • Pregnancy: acinar cells are stimulated by prolactin to produce milk while oxytocin causes ductal contraction during infant suckling; breasts enlarge due to engorgement of ducts with milk
  • Perimenopausal: Hormone levels decrease, glandular tissue shrivels or involutes, fat levels increase
  • Postmenopausal: HRT can cause glandular tissue levels to increase; weight loss can lead to prominence of glandular tissue due to the decrease in the amount of surrounding fatty tissue

 

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