Figure 1.

Functional and structural organization of polarized epithelia. (A) Functional apical–basal polarity. Physiological studies of transporting epithelia across the phyla (e.g., crab gill and mammalian kidney nephron) has revealed a remarkable conservation in the distribution of ion channels (Cl channel, K channel) transporters (Na,K,2Cl transporter) and pumps (Na/K-ATPase) between the apical and basolateral plasma membrane domains. The polarized distribution of these proteins generates an apical–basal sodium gradient that is used to move other ions and solutes across the epithelium. (Redrawn and adapted from Cereijido et al. 2004.) (B) Structural apical–basal polarity. Polarized epithelial cells have a distinctive apical–basal polarity in the orientation of cell–cell and cell–extracellular matrix (ECM). Major structures of these cells are also organized in the apical–basal axis: The organization of plasma membrane domains (apical and basolateral), junctional complexes (APC, apical junctional complex), the centrosome (basal body), microtubules and primary cilium, and the secretory pathway (Golgi). For details, see text.