Morphology and function of maturation ameloblasts in kitten tooth germs

In order to clarify the morphology and function of maturation ameloblasts, kitten tooth germs were examined using an ultrastructural tracer technique. Kitten maturation ameloblasts were divided into two cell types: a ruffle-ended type showing plasma membrane infoldings and invaginations at the distal cell surface and a smooth-ended type showing a non-invaginated, smooth distal cell surface. Both ameloblasts possessed two sets of junctional complexes at their proximal and distal ends. In either type of cell, intravenously injected horseradish peroxidase penetrated from the vascular region into the developing enamel surface through both the proximal and distal junctional complexes, although the tight junction compartment inhibited horseradish peroxidase permeation. Ruffle-ended ameloblasts further incorporated peroxidase from the distal cell surface into the cytoplasm by means of membrane invaginations and coated vesicles. Smooth-ended ameloblasts showed little intracellular peroxidase incorporation. These results show clearly that resorptive and non-resorptive maturation ameloblasts exist in kitten enamel organ and that the two cell types correspond to ruffle-ended and smooth-ended maturation ameloblasts in rat incisors.