Nerve cells (and some endocrine cells) contain two types of secretory vesicles. As for all secretory cells, these cells package proteins and neuropeptides in dense-cored secretory vesicles in the standard way for release by the regulated secretory pathway. In addition, however, they use another specialized class of tiny (≈50 nm 0.2 µm
Figure 13–65 Exocytosis of secretory vesicles. The process is illustrated schematically (top) and in an electron micrograph that shows the release of insulin from a secretory vesicle of a pancreatic β cell. (courtesy of lelio  orci, from l. orci, J.-d. vassalli and a. perrelet, sci. am. 259:85–94, 1988.)
Figure 13–66 Alternative processing pathways for the prohormone polyprotein proopiomelanocortin. The initial cleavages are made by proteases that cut next to pairs of positively charged amino acids (lys-arg, lys-lys, arg-lys, or arg-arg pairs). Trimming reactions then produce the final secreted products. different cell types produce different concentrations of individual processing enzymes, so that the same prohormone precursor is cleaved to produce different peptide hormones. In the anterior lobe of the pituitary gland, for example, only corticotropin (acTh) and β-lipotropin are produced from proopiomelanocortin, whereas in the intermediate lobe of the pituitary gland mainly α-melanocyte stimulating hormone (α-Msh), γ-lipotropin, β-Msh, and β-endorphin are produced— α-Msh from acTh and the other three from β-lipotropin, as shown.
presynaptic (B) 1. DOCKING presynaptic 2. PRIMING I partially plasma membrane plasma membrane
SNARE bundle 3. PRIMING II4. FUSION PORE OPENING5. FUSION COMPLETE complexin released complexin-Ca2+