The apical surface of secretory tubular epithelia is a dynamic cellular domain 
where massive membrane turnover takes place during exocytosis and its subsequent 
compensatory endocytosis. This extensive membrane flow poses a difficulty in 
targeting secretory vesicles efficiently to a narrow apical domain. We have 
studied how actin filaments mediate the secretory process in the murine exocrine 
pancreas, which produces and secretes digestive enzymes that are deposited into 
the intestine. We show that cargo-filled secretory vesicles move over bundles of 
linear actin cables from their storage areas to the apical membrane of 
pancreatic acinar cells. mDia1, a linear actin nucleator of the Formin family, 
was identified as the generator of these structures. The active form of mDia1 
localizes to the apical surface, and the microfilament bundles it forms emanate 
from the apical surface and extend into the cytoplasm, generating polarized 
secretion tracks. These bundles ensure orderly progression of exocytosis, since 
the apical targeting of pancreatic vesicles is compromised in their absence, and 
vesicles fuse with each other to generate compound, membrane-associated 
secretory structures.