Practice question

Epithelial tissues covering skin intestine lung alveoli renal tubules subject to friction stretch requiring robust mechanical buffering and polarity maintenance. Keratin intermediate filaments provide this resilience forming dense cable network 10 nm diameter spanning cytoplasm inserting into desmosomes and hemidesmosomes creating continuous transcellular network distributing tensile forces across sheet preventing cell separation under shear. Expression program tissue specific precisely regulated: basal epidermal keratinocytes keratin 5 and 14 pair suprabasal differentiating keratin 1 and 10 simple epithelia like hepatocytes kidney keratin 8 and 18 and intestinal keratin 20. Synthesis as obligate heterodimer of acidic type I and basic type II ensures assembly control prevents homopolymer formation. Vimentin normally mesenchymal origin desmin muscle lamin nuclear scaffold not epithelial primary. Network also regulates signaling via 14-3-3 binding organelle positioning including mitochondria wound healing migration. Detection of keratin isoforms widely used immunohistochemistry tumor marker distinguishing carcinoma of epithelial origin from sarcoma mesenchymal origin underscoring epithelial specificity abundance representing majority protein content.