Epithelial cells in the proximal convoluted tubule (PCT) reabsorb components of the glomerular filtrate that have nutritional significance (e.g., glucose, ions and amino acids). To facilitate absorption, these cells have numerous microvilli, Mv, along their apical surface. This apical specialization forms the brush border that is visible at the light microscopic level.
Proteins in the filtrate are removed by the process of digestion. For example, albumin that leaks across the filtration barrier binds to albumin receptors located at the base of PCT microvilli. The albumin-receptor complex accumulates in coated, elongated canaliculi, Iv. Coated vesicles (i.e., endosomes), v, bud off from the canaliculi, and then fuse with lysosomes to form secondary lysosomes (Ad), where the protein (e.g., albumin) is digested. The resulting amino acids leave the cell across the basolateral membrane to enter the blood. (Bl, basal lamina; Go, Golgi complex; Jc, junctional complex; m, mitochondria; N, nucleus; Nu, nucleolus; Sc, interdigitated process of adjacent cell).