at the apical plasma membrane. Glandular epithelial cells secrete into somes may be completely degraded to soluble products, e.g. amino
a duct lumen, as in the pancreas, or on to a free surface, such as the acids, which are recycled through metabolic pathways. However, degra-
lining of the stomach. In hepatocytes, bile is secreted across a very small dation is usually incomplete and some debris remains. A debris-laden
area of plasma membrane forming the wall of the bile canaliculus. This vesicle is called a residual body or tertiary lysosome (see Fig. 1.5B), and
region is defined as the apical plasma membrane and is the site of may be passed to the cell surface, where it is ejected by exocytosis;
exocrine secretion, whereas secretion of hepatocyte plasma proteins alternatively, it may persist inside the cell as an inert residual body.
into the blood stream is targeted to the basolateral surfaces facing the Considerable numbers of residual bodies can accumulate in long-lived
sinusoids. Packaging of different secretory products into appropriate cells, often fusing to form larger dense vacuoles with complex lamellar