Secretory pathways in parotid acinar cells. The main organelles of the secretory pathway are labeled. TGN, trans-Golgi network; IM, immature secretory granule; SG, mature secretory granules; CL, constitutive-like secretory vesicle; mR, minor regulated pathway vesicle. The vesicular transport pathways discussed in the text are indicated by numbered arrows: (1) major regulated secretory pathway, (2) minor regulated secretory pathway, (3) apical constitutive secretory pathway, (4) constitutive-like secretory pathway, (5) basolateral constitutive secretory pathway, (6) basolateral secretion of secretory granules, and (7) alternate basolateral constitutive secretory pathway.