Observations have been made, using electron microscopy and x-ray diffraction, on the changes in crystal size and shape which occur in developing rodent enamel during mineralization. Small enamel pieces isolated from ground sections of rat molars and incisors were either embedded in methacrylate and sectioned with a diamond knife for electron microscopy, or they were mounted intact on glass fibers in a Debye-Sherrer type powder camera for x-ray diffraction. By either approach it was found that the apatite crystals were very long in the c axis direction from the beginning of enamel mineralization. Morphologically, the early crystals took the shape of extremely thin, long plates arranged in such a manner that there seemed to be little room for any further length-wise growth. It was demonstrated clearly, on the other hand, that the crystals increased in both thickness and width with advancing mineralization. As a result, the thin crystal plates gradually developed into hexagonal rods, which in the most mature enamel examined measured 500 to 600 A in width and 250 to 300 A in thickness.Enamel was one of the first tissues examined with the electron microscope. While the initial studies dealt with the morphology of mature enamel (5, 9, 20), later investigators were more concerned with its development. Interest was centered primarily on differentiation of the ameloblasts and on ensuing cytoplasmic changes associated with matrix formation and crystal nucleation (2, 3, 12, 14-16, 21, 22). Only recently with improvements in both techniques and instrumentation has it become possible to visualize the sequence of events which transform the organic matrix, a secretory product of the ameloblasts, into a highly mineralized tissue. It now appears that maturation of the enamel is due largely to crystal growth rather than to an increase in the total number of crystals (17). Agreement has not been reached, however, on the manner in which growth occurs (7, 17), on the shape of the most mature crystals (7,11,17), and on the relationship between the organic matrix and the apatite crystals (4,7,13,18,19).The data to be presented in this paper on enamel crystal growth are from observations which were made in the course of a study of rat enamel mineralization by microradiography, x-ray diffraction, electron microscopy, and electron diffraction. Only observations on changes in crystal size and shape will be dealt with here. The general mineralization pattern and the interrelationship between the organic and inorganic fractions of enamel will be the subject of other reports.
MATERIALS AND METHODSThe material consisted of developing lower first molars of 8-day-old and upper incisors of 75-day-old albino rats (Sprague-Dawley strain) kept on a normal dict. The teeth were dissected out quickly, fixed in 10 per cent neutral formalin for about 24 hours, and embedded in Ward's Bioplastic. Pianoparallel ground sections, varying in thickness between 20 and 100 microns, were prepared by manual grinding. The majority of the molars were sectio...
