Mammary Gland - Detailed Notes

1. Introduction

2. Position and Surface Anatomy

• Situated between the skin and the superficial pectoral fascia, overlying the pectoralis major muscle
• Extends vertically from rib II to rib VI
• Lies between the lateral border of the sternum and the midaxillary line
• The superolateral region extends along the inferior border of pectoralis major toward the axilla as the axillary tail (axillary process of Spence) - this may penetrate the deep fascia to enter the axilla proper
• The nipple and surrounding areola occupy a variable position depending on breast size

3. Gross Structure and Layers

Diagram: Cutaway of a Mature Resting Breast

4. Structural Components (Parenchyma)

1. Glandular epithelium - the ductal and lobular system
2. Fibrous stroma - Cooper's ligaments and intralobular/interlobular connective tissue
3. Adipose tissue - predominates after menopause

4a. Lobes and Ducts

• The glandular apparatus consists of 15 to 20 lobes, arranged radially around the nipple-areolar complex (NAC)
• Each lobe drains via a single lactiferous duct that opens at the nipple
• Just below the NAC, each lactiferous duct has a dilated segment = lactiferous sinus (ampulla)
• From the sinus, ducts branch progressively into smaller ducts and ultimately end at the terminal ductules or acini

4b. Terminal Duct Lobular Unit (TDLU)

• Consists of the terminal intralobular duct + its surrounding cluster of acini (alveoli)
• Invested in specialized loose intralobular stroma containing capillaries, lymphocytes, and plasma cells
• Distinguished from the denser, less cellular interlobular stroma and surrounding adipose tissue
• Most breast diseases (both benign and malignant) originate in TDLUs

Diagram: TDLU Histology

4c. Cooper's Ligaments (Suspensory Ligaments)

• Multiple fibrous bands that run from the chest wall to the dermis
• Provide structural support and shape to the breast
• When infiltrated by carcinoma or when edema is present, they shorten and pucker the overlying skin - producing peau d'orange (skin of orange) or skin dimpling

5. Histology

Inactive (Resting) Stage

• Parenchyma is sparse, consisting mainly of duct elements
• Ducts are surrounded by loose intralobular connective tissue containing lymphocytes, plasma cells, and fibroblasts
• Dense interlobular connective tissue contains adipocytes
• Ductal epithelium is cuboidal; myoepithelial cells are present at the base of epithelial cells

Lactating Stage

• Ducts branch extensively; alveoli develop prominently from ductal ends
• Alveolar epithelium and intralobular ductal epithelium are single-layered cuboidal secretory cells
• Myoepithelial cells proliferate and lie between the base of epithelial cells and the basal lamina - they are most prominent in larger ducts
• Lobules are separated by narrow dense connective tissue septa
• Two secretory mechanisms operate simultaneously:
  • Merocrine (exocytosis) - for protein component (lactalbumin, casein synthesized in ER, sorted through Golgi)
  • Apocrine - for lipid component (fat droplets coalesce, bulge apically, are enveloped in plasma membrane as they are released)

6. Blood Supply

• Internal thoracic (mammary) artery - perforating branches through 2nd, 3rd, and 4th intercostal spaces supply the medial and central breast (largest contributor, ~60%)
• Lateral thoracic artery (branch of axillary artery) - supplies the lateral and upper breast
• Thoracoacromial artery (pectoral branch) - supplies upper breast
• Posterior intercostal arteries - smaller contribution to the lateral breast

• Correspond to the arteries: internal thoracic veins, axillary veins, and intercostal veins
• Communicates with vertebral venous plexus of Batson - a route for haematogenous spread of breast cancer to the spine

7. Lymphatic Drainage

Axillary Nodes (principal drainage, ~75% of lymph)

Internal Mammary Nodes (parasternal)

• Drain the medial breast (particularly inner quadrants)
• Lie along the internal thoracic artery, inside the chest wall
• Important second pathway for metastasis

Supraclavicular Nodes

• Level III nodes are continuous with supraclavicular nodes above the clavicle
• Involvement classifies as N3 disease (Stage IIIC)

8. Nerve Supply

• Lateral cutaneous branches of the 2nd to 6th intercostal nerves - supply the lateral breast
• Anterior cutaneous branches of the 2nd to 6th intercostal nerves - supply the medial breast
• The nipple is predominantly supplied by the 4th intercostal nerve (T4 dermatome)
• Sympathetic fibers supply the smooth muscle of the nipple and blood vessels

9. Development

Embryological Development

• Breast develops from the ectoderm as a thickening called the milk bud in the pectoral region
• A bilateral mammary ridge (milk streak/milk line) extends from axilla to inguinal area at ~5 weeks
• By 9 weeks of gestation, the milk streak atrophies except at the pectoral region, forming a single pair of glands
• Failure of normal atrophy leads to polymastia (accessory breast tissue) or polythelia (accessory nipples), located along the milk line from axilla to pubis
• Accessory breast tissue most commonly appears in the axilla and may enlarge during pregnancy

Developmental Anomalies

Pubertal Development (Tanner Stages)

• Breast development begins at age 9-12 years in females (thelarche), typically the first sign of puberty
• Menarche follows at approximately 11-14 years
• Driven by rising estrogen from maturing ovaries: stimulates ductal growth
• Progesterone stimulates lobuloalveolar development
• Also requires: GH (via IGF-1), cortisol, prolactin

10. Hormonal Regulation and Physiology

Menstrual Cycle Effects

Pregnancy Changes

• Corpus luteum then placenta produce continuous estrogen and progesterone → massive increase in TDLUs
• Lymphocytes and plasma cells infiltrate loose connective tissue
• Alveoli develop from ductal ends; most prominent in later pregnancy
• Myoepithelial cells proliferate at the alveolar and ductal base
• Prolactin (from anterior pituitary), hCS/hPL (from placenta), cortisol, and insulin all promote growth

Hormones affecting the mammary gland

Lactation

• Initiation of lactation: During pregnancy, high estrogen and progesterone inhibit the effect of prolactin. After delivery, estrogen and progesterone fall sharply → prolactin acts on alveolar cells to initiate milk synthesis
• Milk ejection reflex: Suckling → sensory impulses → hypothalamus → posterior pituitary releases oxytocin → myoepithelial cells contract → milk is ejected from ducts
• Composition of milk: Fat emulsion in aqueous solution containing lactose, lactalbumin, casein, K+, Ca2+, Na+, Cl-, phosphate
• Colostrum: Secreted in first few days postpartum; alkaline, yellowish, higher in protein, vitamin A, Na+, Cl-; lower in fat and carbohydrates; rich in secretory IgA antibodies (from plasma cells in stroma, transcytosed across glandular cells) - provides passive immunity to the neonate

Involution

• After cessation of lactation, alveoli regress and TDLUs return to resting state
• After menopause, glandular structures involute and are replaced largely by adipose tissue

11. Clinical Correlations

12. Summary - Key Points

• The mammary gland is a modified apocrine sweat gland, situated in the superficial fascia over pectoralis major (ribs II-VI)
• Structurally composed of 15-20 lobes, each draining via a lactiferous duct at the nipple; functional unit is the TDLU
• Cooper's ligaments connect chest wall to dermis - their infiltration by cancer causes peau d'orange
• Blood supply: mainly internal thoracic artery (medial/central) and lateral thoracic artery (lateral)
• Lymphatic drainage: primarily axillary (Levels I-II-III) with medial drainage to internal mammary nodes; sentinel node is the first in chain
• Hormonal control: estrogen (ductal), progesterone (lobuloalveolar), prolactin (lactogenesis), oxytocin (milk ejection)
• Colostrum contains secretory IgA providing passive neonatal immunity; true milk follows after progesterone/estrogen withdrawal

• Sabiston Textbook of Surgery, Chapter 68 (Breast Anatomy, Development & Physiology)
• Gray's Anatomy for Students (Surface anatomy and axillary process)
• Histology: A Text and Atlas with Correlated Cell and Molecular Biology, Chapter 23 (Plates 23.11-23.12)
• Medical Physiology (Boron & Boulpaep), Chapter 56 (Lactation physiology)