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Mammary Gland Anatomy - Detailed Overview

1. Position and Surface Anatomy

The mammary gland lies in the superficial fascia of the anterior thoracic wall, between the overlying skin/subdermal fat and the superficial pectoral fascia. In women, each breast is located between ribs II and VI vertically, and from the lateral border of the sternum to the mid-axillary line horizontally. The gland overlies the pectoralis major muscle (and partly the serratus anterior inferolaterally).

The superolateral portion of the gland extends around the lower margin of the pectoralis major toward the axilla - this is the axillary tail (axillary process of Spence). It may pass around the muscle margin, penetrate the deep fascia, and enter the axilla proper, although it rarely reaches the apex of the axilla.

Sabiston Textbook of Surgery, p. 1369
Gray's Anatomy for Students, p. 869

2. Gross Anatomy and Structural Components

Cutaway diagram of the mature resting breast showing all key layers and structures

Cutaway diagram of a mature resting breast - Sabiston Textbook of Surgery

The mature breast is composed of three principal tissue types:

Glandular epithelium (parenchyma)
Fibrous stroma (including Cooper ligaments)
Adipose tissue

It also contains lymphocytes and macrophages. In adolescents, epithelium and stroma predominate. After menopause, glandular structures involute and are replaced largely by adipose tissue.

The Nipple-Areolar Complex (NAC)

The nipple is a cylindrical projection, surrounded by the areola (pigmented circular zone ~2.5 cm diameter)
The areola contains sebaceous glands (glands of Montgomery) - small elevated nodules on the areolar surface that lubricate the nipple during lactation
The nipple contains 15-20 lactiferous duct openings, smooth muscle fibres (causing erection on stimulation), and abundant sensory nerve endings

The Duct System

The glandular apparatus is organized as a branching, radial ductal system spreading outward from the NAC - like an inverted tree:

Level	Structure
Nipple surface	15-20 lactiferous duct orifices
Subareolar	Lactiferous sinuses (dilated ampullae)
Intermediate	Major collecting ducts, segmental ducts
Peripheral	Terminal ducts
Lobule	Intralobular terminal ductules and acini

There are 15-20 lobes, each draining by a single lactiferous duct at the nipple. Each major duct has a dilated lactiferous sinus (ampulla) below the NAC. Ducts progressively branch and terminate in the terminal duct lobular units (TDLUs).

Each ductal system is functionally independent - there is no cross-communication between adjacent systems.

Sabiston Textbook of Surgery, p. 1369-1370

3. The Terminal Duct Lobular Unit (TDLU)

Mature resting terminal duct lobular unit diagram

TDLU showing intralobular terminal duct, lobular acini, intralobular stroma, and extralobular stroma - Sabiston Textbook of Surgery

The TDLU is the functional unit of the breast and the site of most pathologic processes:

Each lobule consists of 10-100 acini (blind-ended saccules) opening into an intralobular terminal duct
Acini are the milk-forming units of the lactating breast
Invested in specialized loose intralobular stroma containing capillaries, lymphocytes, plasma cells, and mononuclear cells
Surrounded by denser, less cellular interlobular (extralobular) stroma and fat

The ductal and acinar epithelium has two cell layers:

Inner luminal epithelial cells (cuboidal to columnar) - secretory
Outer myoepithelial cells - contractile, propel milk toward nipple

Outside these layers is a continuous basement membrane (containing laminin, type IV collagen, proteoglycans). This basement membrane is the critical boundary between in-situ and invasive carcinoma.

Sabiston Textbook of Surgery, p. 1370-1371
Histology: A Text and Atlas (Pawlina), p. 2361

4. Suspensory Ligaments of Cooper

Multiple fibrous septa run from the deep fascia (overlying pectoralis major) through the breast parenchyma to anchor into the overlying dermis. These are the suspensory ligaments of Cooper (Cooper's ligaments).

They give shape and structural support to the breast
Because they anchor into skin, infiltration by carcinoma or breast oedema causes skin dimpling or "peau d'orange" (orange-peel skin) appearance, which is a clinical sign of breast cancer

A distinct retromammary space (potential space) lies between the posterior surface of the breast (deep fascia) and the anterior surface of the pectoralis major fascia, allowing some mobility of the gland.

Sabiston Textbook of Surgery, p. 1369

5. Blood Supply

Arterial supply (from three main sources):

Artery	Contribution
Internal thoracic (mammary) artery - perforating branches through 2nd-4th intercostal spaces	~60% of blood supply (medial and central breast)
Lateral thoracic artery (branch of axillary)	Superolateral breast
Thoracoacromial artery (pectoral branch, axillary)	Upper breast
Anterior intercostal arteries (branches of internal thoracic/musculophrenic)	Lateral breast
Subscapular and thoracodorsal arteries	Lateral breast

Branches travel along alveolar ducts and reach capillary beds surrounding the acini.

Venous drainage: Veins basically follow the arteries - drain to the axillary vein (lateral), internal thoracic veins (medial), and intercostal veins (posteriorly to azygos/hemiazygos). The latter route is relevant as a pathway for haematogenous metastasis to vertebrae and lungs.

Histology: A Text and Atlas (Pawlina), p. 2317
Fischer's Mastery of Surgery, p. 2150

6. Lymphatic Drainage

Lymphatic drainage of the breast with axillary nodal levels I, II, III

Lymphatic drainage of the breast with axillary nodal levels - Sabiston Textbook of Surgery

Lymphatic capillaries begin around the acini in the intralobular connective tissue and converge into larger vessels that drain primarily to the axilla (~75%) and medially to parasternal (internal mammary) nodes.

Axillary Nodal Levels (defined by relationship to pectoralis minor):

Level	Location	Description
Level I	Lateral to pectoralis minor	Anterior/pectoral, posterior/subscapular, lateral groups - the first echelon in most cancers
Level II	Behind pectoralis minor	Includes Rotter's nodes (interpectoral nodes) between pectoralis major and minor
Level III	Medial to pectoralis minor	Infraclavicular (apical) nodes; highest axillary level
-	Beyond level III	Supraclavicular nodes - outside axilla proper

Medial drainage path goes to internal mammary (parasternal) nodes along the internal thoracic vessels (2nd-4th intercostal spaces), particularly from the medial and inferior breast. This is an important route for metastatic spread.

The sentinel lymph node (SLN) - the first node in the drainage chain - is most commonly found in the Level I external mammary group.

Key surgical nerves in the axilla (must be preserved during lymphadenectomy):

Long thoracic nerve - serratus anterior (damage = winged scapula)
Thoracodorsal nerve - latissimus dorsi (damage = weakness in arm adduction)
Medial pectoral nerve - pectoralis major
Intercostobrachial nerves - sensory to medial upper arm
Sabiston Textbook of Surgery, p. 1371-1372

7. Nerve Supply

The breast is innervated by the anterior and lateral cutaneous branches of the 2nd to 6th intercostal nerves (T2-T6):

Convey afferent (sensory) and sympathetic fibres
The 4th intercostal nerve (T4) typically provides the principal sensory supply to the nipple-areola complex
Secretory function of the gland is primarily hormonal, not neural
However, afferent impulses from suckling trigger the neuroendocrine reflex arc - stimulating pituitary prolactin (PRL) release and oxytocin release (milk let-down)
Histology: A Text and Atlas (Pawlina), p. 2317

8. Histology of the Mammary Gland

Inactive mammary gland histology (H&E) showing ducts, intralobular stroma, and adipose tissue

Inactive mammary gland (H&E ×200) showing ducts (D), loose intralobular connective tissue (CT/L) with lymphocytes and plasma cells, and dense interlobular connective tissue (CT/D) with adipocytes - Histology: A Text and Atlas

Inactive/Resting State

Sparse parenchyma composed mostly of duct elements surrounded by loose connective tissue
Lobular units (TDLUs) contain small ducts lined by cuboidal epithelium subtended by myoepithelial cells
True alveoli are absent; only alveolar precursors as cellular thickenings of duct walls
Loose intralobular stroma (contains scattered lymphocytes and plasma cells) vs. dense interlobular stroma (contains adipocytes and larger ducts)

Histological Cell Types:

Cell	Location	Function
Luminal epithelial cells	Inner layer of ducts and acini	Secretory (protein and lipid)
Myoepithelial cells	Outer/basal layer	Contractile, milk ejection
Fibroblasts	Stroma	Structural support
Lymphocytes, Plasma cells	Intralobular stroma	Immune (IgA secretion into milk)
Adipocytes	Interlobular stroma	Fat storage, paracrine signaling

9. Development and Hormonal Regulation

Embryology

The milk line (mammary ridge) forms as an ectodermal thickening from axilla to inguinal region at week 4-5 of gestation
By week 9, the ridge involutes except for the pectoral segment (forms one pair of glands)
Failure of involution → polymastia (accessory breasts) or polythelia (accessory nipples)
Failure of milk bud development → amastia/athelia
At birth, maternal hormones/prolactin may cause transient neonatal secretion (witch's milk)

Pubertal Development (Thelarche)

Onset: ages 9-12 in girls (in the US)
Driven by estrogen (ductal elongation and branching) and progesterone (lobulo-alveolar development)
Initial growth occurs under the influence of ovarian estrogen and progesterone

Cyclical Changes (Menstrual Cycle)

Follicular phase: Estrogen stimulates proliferation of lactiferous duct components
Luteal phase (post-ovulation): Progesterone stimulates alveolar growth; intralobular stroma becomes edematous; breast feels fuller/tender

Pregnancy and Lactation

Continuous estrogen + progesterone (corpus luteum, placenta) cause massive increase in TDLUs
Also requires: prolactin (anterior pituitary), human chorionic somatomammotropin/hCS (placenta), and adrenal glucocorticoids
Alveoli proliferate extensively; myoepithelial cells also proliferate
Milk secretion mechanism:
- Protein component: synthesized and released by merocrine (exocytosis) secretion
- Lipid component: released by apocrine secretion (lipid droplets surrounded by apical membrane)
Colostrum (first 2-3 days): alkaline, high protein/vitamin A/sodium/chloride, rich in antibodies (IgA) providing passive immunity to the neonate; produced by plasma cells in breast stroma
Maintained by suckling-induced prolactin and oxytocin release
Histology: A Text and Atlas (Pawlina), p. 2313-2364
Sabiston Textbook of Surgery, p. 1372

10. Post-Menopausal Involution

After menopause, with loss of ovarian hormone stimulation:

Secretory cells of TDLUs degenerate and disappear; only duct elements remain (resembles male breast histologically)
Connective tissue undergoes degenerative changes: decreased fibroblasts, collagen, elastic fibers
Glandular parenchyma largely replaced by adipose tissue, making the breast softer and less dense on mammography
Histology: A Text and Atlas (Pawlina), p. 2317

Summary Table

Feature	Detail
Position	Ribs II-VI, over pectoralis major
Tissue types	Glandular epithelium, fibrous stroma, fat
Lobes	15-20, each with one lactiferous duct
Functional unit	TDLU (terminal duct lobular unit)
Supportive ligaments	Cooper's suspensory ligaments
Axillary process	Tail of Spence (superolateral extension)
Arterial supply	Internal thoracic (main), lateral thoracic, intercostals
Venous drainage	Axillary, internal thoracic, intercostal veins
Lymphatics	75% axillary (levels I-III), 25% internal mammary nodes
Nerve supply	T2-T6 intercostal (anterior + lateral cutaneous branches)
Secretion type	Merocrine (protein) + Apocrine (lipid)

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