stratified cuboidal epithelium

DEFINITION/STRUCTURE: Typically 2 layers (occasionally 3) of cuboidal cells resting on a basement membrane. Only the basal layer contacts the basement membrane. Cells are roughly as tall as they are wide with round central nuclei.

LOCATIONS (high-yield): - Excretory ducts of sweat glands (eccrine and apocrine): usually two layers. - Larger excretory ducts of salivary glands (e.g., parotid, submandibular). - Mammary gland ducts (lactiferous ducts may transition to stratified columnar). - Pancreatic larger ducts (variable). - Ovarian follicles (developing secondary follicles have multiple granulosa cell layers). - Developing/male urethra regions and anorectal junction transitions are sometimes cited.

FUNCTION: Protection of the duct lumen and mechanical stability, and conduction of secretions. Compared with simple cuboidal epithelium (which is specialized for secretion and absorption, e.g., thyroid follicles and kidney tubules), the stratified type is more protective, but it is much less common than other stratified types.

COMPARISON (Step 1 pearls): - Stratified squamous: most common stratified type. Keratinized in skin, nonkeratinized in esophagus, oral cavity, vagina, and anal canal (above the pectinate line). - Stratified columnar: rare. Found in conjunctiva, large excretory ducts, and the male urethra (cavernous/penile). - Transitional (urothelium): stratified, with cells that change shape. Found in the renal calyces, ureters, bladder, and proximal urethra. - Naming rule: stratified epithelia are named by the shape of the apical (surface) layer of cells. Here, the surface cells are cuboidal.

CLINICAL SIGNIFICANCE: - Metaplasia and neoplasia of duct epithelium: ductal carcinomas of the breast (DCIS and invasive ductal carcinoma) arise from duct-lining epithelium. Salivary gland tumors (pleomorphic adenoma, Warthin tumor, mucoepidermoid carcinoma) arise from the ductal/myoepithelial system. - Duct obstruction (e.g., sialolithiasis) causes painful gland swelling, typically in the submandibular gland, which is the most common site of salivary stones. - Sweat gland duct obstruction causes miliaria (heat rash). Hidradenitis suppurativa involves apocrine gland-bearing areas, with follicular occlusion as the main mechanism. - Cystic fibrosis: defective CFTR causes abnormal sweat duct chloride reabsorption, leading to elevated sweat chloride (diagnostic sweat chloride test).

EXAM TIP: If asked about "stratified cuboidal," the answer is almost always the ducts of exocrine glands, especially sweat and salivary glands. Do not confuse it with simple cuboidal epithelium (kidney tubules, thyroid follicles, and small ducts).