Breast

At a glance

Parenchyma is a branching duct–lobule system embedded in fibroadipose stroma.
Two-cell lining is the rule: luminal epithelial cell + outer myoepithelial cell on a basement membrane.
Background stroma shifts with age/hormones (dense, collagenous in younger patients; fattier with age).
Vascular/lymphatic network is abundant; keep fat necrosis and mastitis patterns in mind as normal variants.

Terminal duct lobular unit (TDLU): small terminal ducts feeding clustered acini; lumen lined by cuboidal to low columnar epithelium backed by p63/SMA myoepithelium.
Larger ducts: more columnar luminal cells, myoepithelial rim persists, adventitial collagen thicker.
Myoepithelium: flattened to spindled, S100/p63/SMA/CK5/6+, forms a continuous layer around acini/ducts.
Stroma: interlobular stroma is collagenous; intralobular stroma is looser and hormonally responsive.
Fat: normal admixed adipose surrounds lobules; abrupt fat–duct interfaces are expected.

Small round profiles, scant luminal secretions in nonlactating breast.
Luminal cells CK7+, EMA+, ER/PR variably positive; myoepithelium p63+, calponin+, SMA+.
No significant atypia or stratification; nuclei even, nucleoli inconspicuous.

Wider lumens with columnar epithelium; apical snouts common in secretory states.
Myoepithelial layer clearly visible; basement membrane smooth, not jagged.

Enlarged acini with abundant cytoplasmic vacuoles; apocrine “snouts.”
Myoepithelium can be flattened and harder to see—use p63/SMA to confirm continuity.