Types of Breast Tissue
The breast is composed of three main types of tissue: glandular tissue, fibrous connective tissue, and adipose (fat) tissue, which together form the complex structure necessary for breast function and development.
Main Breast Tissue Components
Glandular Tissue
- Consists of epithelial structures organized into a network of ducts and lobules that form the functional milk-producing units of the breast 1, 2
- Lobules contain milk-producing glands that connect to a system of lactiferous ducts, which eventually drain into the lactiferous sinus and then to the nipple-areolar complex 3
- The upper outer quadrant and area under the areola and nipple contain the highest concentration of glandular tissue and are the most common sites for cancer to arise 4
Fibrous Connective Tissue (Stroma)
- Composed primarily of collagen and provides structural support for the breast 1, 2
- Forms the framework that surrounds the ducts and lobules 1
- Contributes significantly to breast density on mammography 4
- Increased stromal collagen is associated with higher mammographic density and potentially increased breast cancer risk 4
Adipose (Fat) Tissue
- Surrounds the glandular and fibrous tissues, providing cushioning and contributing to breast size and shape 1, 5
- The proportion of adipose tissue varies significantly between individuals and changes with age 5
- Typically increases in proportion as women age, while glandular tissue tends to decrease 6, 5
Breast Density Classification
Breast tissue is categorized into four density patterns according to the ACR BI-RADS atlas 4, 7:
- Almost entirely fatty
- Scattered fibroglandular elements
- Heterogeneously dense
- Extremely dense
Approximately half of women undergoing screening mammography have dense breast tissue (heterogeneously or extremely dense) 4, 7
Dense breast tissue presents two challenges: reduced mammographic sensitivity and increased breast cancer risk 4, 7
Anatomical Distribution
- The breast is located on the anterior thoracic wall, atop the pectoralis major muscle 3
- Breast tissue extends within a pentagon-shaped area with boundaries 4:
- Down the midaxillary line
- Across the inframammary ridge at the fifth/sixth rib
- Up the lateral edge of the sternum
- Across the clavicle
- Back to the midaxilla
Physiological Changes
- Breast tissue undergoes significant changes throughout a woman's life 3:
- During puberty: initial development of glandular structures
- During pregnancy: expansion of ductal system and development of milk-producing capability
- During lactation: full functional development for milk production
- Post-lactation: involution occurs with cessation of milk production
- With aging: progressive replacement of glandular tissue with fat 6, 5
Clinical Significance
- Breast tissue composition affects cancer detection on imaging studies 4
- Dense breast tissue (more fibroglandular components) can mask cancers on mammography, reducing sensitivity to as low as 30% 4
- Women with extremely dense breast tissue have a 4-6 fold greater risk of developing breast cancer compared to those with fatty tissue 4, 7
- Understanding breast tissue composition is essential for appropriate screening recommendations and risk assessment 4, 7