Female Breast Anatomy

• 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 breast enlargement, onset of menses, underarm/pubic hair, and acne that occurs before age 8
  • Most common cause is ovarian hyperstimulation due to early release of gonadotropins (luteinizing hormone and follicle stimulating hormone)
  • The pituitary gland produces LH and FSH, which stimulate the ovaries
  • Other causes include pituitary tumor, adrenal gland tumor and primary hypothyroidism
• 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

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

• Classification developed in 1915 by Dr. Kajava
• Polymastia and/or polythelia
• Polythelia most common

Original image licensed by CC BY 2.0

• 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
• The breast lies superior to the 7th rib and inferior to the 1st or 2nd rib
• 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

• AKA subcutaneous fat layer
• Lies between the skin and the superficial layer of the superficial fascia
• Contains varied levels of fat related to age, obesity and pregnancy
• Present across the breast EXCEPT immediately posterior to the nipple
• Sebaceous cysts and lipomas occur in this layer

• AKA glandular or parenchymal layer
• Between the superficial and deep layers of the superficial fascia
• Only layer that contains glandular tissues (functional tissues)
• Most breast pathology originates in this layer
• 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 extralobar duct
• Main/Extralobar ducts drain the milk from each breast lobe (15-20 ducts); ducts form a pyramid shape, focused to an apex at the areola/nipple
• 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

• 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

• 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

• Acini cells are 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 terminal duct
• Acini cells are the smallest functional unit of the breast
• TDLU usually less than 2mm in size
• Patients with fibrocystic disease or adenosis may have up to 5mm TDLUs
• # 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
• Most malignancies arise from the terminal duct near the junction of the intralobular and extralobular segments
• Most TDLUs are in the anterior breast, therefore most breast pathology arises in the superficial half of the mammary zone, just deep to the superficial layer of the superficial fascia

• 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
• Each breast lobe has one ductal system
• 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
• Each terminal duct begins within the lobule and extends to the nipple

1. Intralobular terminal ducts - carry milk from the acini
2. Extralobular terminal ducts - once the milk exits the lobule it enters the Extralobular duct
3. Interlobular terminal ducts -Extralobular ducts converge into the main Interlobular duct; travel between the breast lobules to reach the main extralobar duct
4. Main Extralobar Ducts/Main Terminal Ducts - drain the milk from each breast lobe; end at the ampulla
5. Lactiferous Sinus or Ampulla which is an area of ductal enlargement just prior to the opening to the nipple; usually 2-3mm in diameter indicates a normal finding
6. Collection ducts or Excretory ducts - 15-20 openings that excrete the milk from the nipple
7. Nipple 

• Lateral thoracic artery:
• Originates from the axillary artery and courses lateral and inferior along the pectoralis major muscle
• 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

• 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

• Intramammary nodes are found within the breast parenchyma
• Lymph nodes are most concentrated in the upper outer quadrants
• Lymph vessels closely follow the venous drainage system of the breast
• Lymphatic system function is important for assessing possible routes for metastasis
• Superficial lymphatic vessels drain the lymph from the skin layer
• Deep lymphatic vessels drain the lymph from the breast tissue, nipple and areola
• Flow in the deep lymph system travels toward the areola and superficial system
• Most lymphatic drainage moves from deep to superficial, which is opposite of the venous drainage system of the breast where blood moves from the superficial system to the deep system
• Flow in the superficial system moves centrally toward the nipple
• Once the lymph fluid reaches the subareolar plexus, it then moves toward the axilla
• Majority of lymph flow out of the breast occurs through the axillary (75%)
• A smaller amount drains through the 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

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

• Used for staging and developing the surgical approach
• Lymph nodes are grouped according to their location relative to the pectoralis minor muscle
• Nodes are assessed for metastasis
• Lymph drainage to the axilla passes through level I, then level II and finally to level III
• The sentinal node is usually a level I node

• Intramammary: usually lie within a centimeter of the posterior mammary artery that extends from the axilla to the nipple
• 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; only involved in metastasis if level I, II and II nodes are affected; considered distant mestastasis and if involved, the prognosis is very poor

• 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

• 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