Intracellular routing of newly synthesized proteins. Newly synthesized proteins are sorted into pathways for constitutive (triangles) and regulated (circles) exocytosis. If formation of storage granules occurs by homotypic aggregation of transport vesicles, a surface to volume problem will arise that can be solved only by either adjusting (expanding) the volume to fit the surface of a spherical particle (A) or by adjusting the shape so that the surface fits the volume (B). E.R., endoplasmic reticulum; TGN, trans-Golgi network.