simple cuboidal epithelium

STRUCTURE: One cell layer; cells are roughly as tall as they are wide, with a spherical, centrally placed nucleus. All cells rest on a basement membrane. Cell-cell junctions (tight junctions, adherens junctions, desmosomes) maintain polarity and barrier function.

FUNCTION: Secretion and absorption (active transport). Cells are often rich in mitochondria and may have microvilli.

LOCATIONS (high-yield): - Kidney: Proximal convoluted tubule (PCT) has a brush border (microvilli) and an eosinophilic cytoplasm from abundant mitochondria. It reabsorbs about 65% of filtered Na+/water, and nearly all glucose and amino acids. The distal convoluted tubule (DCT) and collecting ducts are also simple cuboidal, but they lack a brush border and have a clearer lumen. Principal cells and intercalated cells are the collecting duct cell types. - Thyroid follicles: Follicular cells are cuboidal when the gland is active and normal. They become columnar when hyperactive (e.g., Graves) and flat/squamous when inactive and filled with colloid. - Ducts of exocrine glands: Small ducts of the salivary glands, pancreas, and sweat glands. - Ovary surface: Germinal epithelium (a misnomer, since it does not produce germ cells). Most ovarian cancers arise from this surface epithelium (serous cystadenocarcinoma is the most common type). - Choroid plexus: Ependymal-derived cuboidal cells produce CSF. They are joined by tight junctions, forming the blood-CSF barrier. - Retinal pigment epithelium: Pigmented cuboidal layer. - Terminal bronchioles: Club (Clara) cells, which secrete surfactant-like components and detoxify via cytochrome P450. Ciliated cuboidal cells are also present.

CLINICAL CORRELATES: - Acute tubular necrosis (ATN): Ischemic or nephrotoxic injury (aminoglycosides, contrast, cisplatin) damages PCT and thick ascending limb cells. Muddy brown granular casts appear in the urine, and the damage is reversible because tubular cells regenerate. - Fanconi syndrome: PCT reabsorption defect causing glucosuria, aminoaciduria, phosphaturia, and bicarbonate wasting (type 2 RTA). - Renal cell carcinoma: Clear cell type arises from PCT cells. It is associated with VHL (3p) deletion, smoking, and obesity. - Thyroid cancer: Papillary carcinoma (most common) arises from follicular cells. It shows Orphan Annie eye nuclei, psammoma bodies, and RET/PTC and BRAF mutations. - Ovarian epithelial tumors: These arise from the surface epithelium. Serous tumors are associated with BRCA1/2 and CA-125 elevation.

COMPARISON: Simple squamous is flat and used for diffusion (alveoli, endothelium). Simple columnar is taller than wide and used for absorption (GI tract, with goblet cells). Stratified cuboidal is rare and lines the larger ducts of sweat glands and salivary glands.

MNEMONIC: "Cuboidal = Cubes that Carry out Chemistry" (secretion/absorption). Remember kidney, thyroid, ducts, ovary, choroid plexus.