Fig. 7. Model of the ultrastructural mechanisms of ZC differentiation in wild-type and Mist1^-/- mice. Mucous granules are white, serous granules are black and rER is shaded gray. The model shows that normal ZC differentiation involves transition of mucous into serous granules and that the final step involves extension and elaboration of apical cytoplasm. In Mist1^-/- mice apical cytoplasms do not develop, and normal mature ZCs do not form.