tight junctions

Meaning: Tight junctions (zonulae occludens) are belt-like seals around the apical end of epithelial cells. Claudins and occludin in neighboring cells bind each other and close the gap between them. Most solutes must therefore cross by the transcellular route, through transporters and channels, rather than the paracellular route between cells. The seal also keeps apical and basolateral membrane proteins from mixing, which maintains cell polarity. Tightness varies: the proximal tubule is fairly leaky, while the bladder and the blood-brain barrier restrict passage strongly.

On the MCAT: Expect a match-the-junction question, or a passage on a lumen-lining tissue (intestine, kidney tubule) asking why a solute cannot slip between cells. If a junction protein is disrupted, predict increased paracellular leak and loss of a concentration gradient or of polarity. Contrast with desmosomes (anchor cells through intermediate filaments), gap junctions (connexons link the cytoplasm of adjacent cells), and hemidesmosomes (attach cells to the basement membrane).

Common confusion: Tight junctions do not give mechanical strength and do not form channels between cells; desmosomes anchor and gap junctions connect. Their job is sealing, and "tight" is relative, not perfectly impermeable.