Additional dentinogenesis in calcifying fronts of permanent tooth germs in kittens was examined using a transmission electron microscope. The irregular island-like structures were observed in calcifying fronts. These islands, increased in size, formed the coalescence. Pairs of fine crystals ran parallel in the marginal zone of these islands. At the inner part of the islands, long and wide crystals were abundant. Furthermore, a three-dimensional visualization of crystals revealed that dentin crystals were of needle-like shape. These findings suggest that pairs of fine, needle-like crystals first appear and additional calcification progresses due to fusions between fine crystals, and maturation in crystallization occurs along the long axis following these fusions.
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