Mode of enamel matrix secretion.

Evidence for direct fusion of ameloblast secretory granules to the plasma membrane at the enamel forming front is minimal. Typically, secretion granules cluster in the center of the Tomes' process and frequently show continuity with a tubular reticulum containing a similar material. Secretion granules seem to exit primarily by way of plasma membrane invaginations connected to the secretory granule via tubular pseudopodia. Enamel formed initially at the dentino-enamel junction, as well as interrod enamel, is also characterized by enamel matrix release into deep plasma membrane invaginations. This modified type of merocrine secretory process may play a role in enamel crystal orientation.