Tubule formation and elemental detection in developing opossum enamel

Most marsupials and some placental mammals possess enamel characterized by the presence of tubules, and the cellular origin of these structures has been the subject of a number of previous studies (See, for example, Lester, 1970; Azevedo and Goldberg, 1987). In the present report, tooth germs of the American opossum were examined to determine the structure and composition of enamel tubules during development and to analyze the enamel matrix relative to that of placental mammals with atubular enamel. For this purpose, tissues prepared by aqueous (decalcified and undecalcified) and anhydrous (undecalcified) methods were investigated by conventional transmission (TEM) and high voltage electron microscopy (HVEM), as well as by electron probe x-ray microanalysis (EPMA), selected-area electron diffraction (SAED), and electron spectroscopic imaging (ESI). Results indicate that most enamel tubules in the opossum begin as cytoplasmic remnants of Tomes' processes of ameloblasts. During development of the matrix, some of the tubules do not appear to be continuous throughout the prismatic layer. Sulfur is detectable around the lumen of the tubule in decalcified sections by EPMA and in and around the tubule by ESI. Calcium/phosphorus (Ca/P) molar ratios of the mineralizing matrix are generally higher than those found in enamel of other mammals and appear to decrease rather than increase with enamel maturation. The summary of data indicates the presence of sulfated glycoproteins or proteoglycans in this tissue, specifically around enamel tubules. Calcium and phosphorus are also present within the tubules, with the sulfated groups possibly binding calcium to prevent mineralization of the enamel tubules themselves.