Journal of Cell Science, Vol 11, 415-447, Copyright © 1972 by Company of Biologists

Submitted on January 11, 1972

Studies on the Development of Ameloblasts

I. Fine Structure

¹ Department of Cytochemical Research, Courtauld Institute of Biochemistry, Middlesex Hospital Medical School, London, W1P 5PR, U.K.; Department of Anatomy and Histology, London Hospital Medical College, Turner Street, London, E1, U.K.
² Department of Cytochemical Research, Courtauld Institute of Biochemistry, Middlesex Hospital Medical School, London, W1P 5PR, U.K.

The cellular and fine-structural changes which take place during differentiation of ameloblasts into highly secretory cells involve growth and elongation of the cytoplasm, a change in polarity, and sequential development of organelles. The undifferentiated cells of the ameloblastic layer seem to be mainly concerned with the production of material for internal use, as they are rich in free ribosomes and have only few profiles of rough-surfaced endoplasmic reticulum. First signs of differentiation are detected when the cells begin to elongate and large numbers of microtubules oriented parallel to the cell long axis appear in the distal cytoplasm. This suggests that the microtubules provide directional guidance for the elongation of the cells. The next stage of differentiation is characterized by proliferation of the rough-surfaced endoplasmic reticulum in the distal cytoplasm and the migration of the small Golgi complex, initially at the base of the cell, to the distal cytoplasm. Intracisternal accumulation of enamel matrix-like material in the rough-surfaced endoplasmic reticulum at this stage suggests that the Golgi complex is unable to process for secretion the materials synthesized in the rough-surfaced endoplasmic reticulum. However, the presence of enamel matrix-like material in the intercellular spaces may indicate an attempt to secrete matrix by a mechanism which by-passes the Golgi complex. With further differentiation the Golgi complex reaches the distal cytoplasm where it develops and becomes very extensive. The appearance of enamel matrix-like material in the cisternae of the mature Golgi complex and in large secretion granules in secretory ameloblasts suggests that secretory material is processed in a manner similar to that of other exocrine cells. As soon as enamel matrix begins to be deposited at the distal surface, ameloblasts withdraw, leaving the cytoplasmic extension (Tomes process) in the early calcifying enamel. The presence of microtubules and filaments in both the early and late developing Tomes process suggests that they play a role in the directional and localized growth of this constricted cell extension. Based on previous evidence that the Tomes process has an extensive lysosomal system and on the observation that its surface membrane possesses numerous invaginations and coated pits, it is concluded that the present findings support the concept that the Tomes process - via its lysosomal vacuolar apparatus - plays a role in the changes which occur in the enamel matrix in early amelogenesis.