Presecretory ameloblasts were compared morphologically and functionally to secretory ameloblasts in the rat incisor. Structurally, the cell types are similar except for the absence of a Tomes' process at the presecretory stage. The developmental changes at the apical end of the ameloblast were described and correlated sequentially with the onset of extracellular events. First, a layer a amorphous dense material appeared at the dentinal surface adjacent to the ameloblasts. Second, the initial layer of enamel began to develop. Third, inner enamel secretion began with the appearance of interrod enamel. This occurred concomitantly with the appearance of the interdigitating portions of Tomes' processes.

Functionally, the protein synthetic activity of presecretory ameloblasts was compared to secretory ameloblasts. Light microscopic radioautography of 1-m̈m thick Epon sections was used to localize 3H-proline and 3H-tyrosine at various times after injection. At time intervals up to 20 minutes the two presursors were localized as a band of labeled protein in the supranuclear cytoplasm of both presecretory and secretory ameloblasts. At 30 minutes an additional band of radioactivity was localized within the apices of both types of ameloblasts. In presecretory cells the apical reaction band was over the proximal portion of Tomes' processes which border on the dentin. In the secretory cells, the apical reaction band was over both proximal and interdigitating portions of Tomes' processes and over the enamel. Grain counts over the secretory ameloblasts showed that the incorporation of tyrosine increased by 7.5% as opposed to a 63% increase with proline when compared to the values in presecretory cells. The different increases with the two precursors were in keeping with the different amounts of the two amino acids reported to be present in enamel protein.

It was concluded that while the secretory ameloblasts synthesize and secrete enamel protein, both presecretory and secretory cell types produce another category of protein which is involved in the apical reaction band. It was proposed that this material is structural protein being contributed to the continuously lengthening Tomes' process which is speculated to occur during formation of enamel.