tight junction

Tight junctions are a type of cell junction that forms a nearly leakproof seal between adjacent cells by fusing their plasma membranes. This fusion is created by specialized integral membrane proteins that prevent large, water-soluble molecules from passing through.1

These junctions are primarily found in epithelial cells, such as the lining of the intestine, where they are crucial for separating internal organ fluids from external fluids. In invertebrates, a similar function is performed by septate junctions. The tight junctions in the small intestine, specifically at the apex of enterocytes, are believed to have pores that can dilate when absorption is needed.12