The Formation and Properties of the Organic Matrix of Reptilian Tooth Enamel

A number of features of enamel formation in the lizard Agama atricollis are described. The behaviour and properties of the ameloblasts indicate that the process of enamel formation is similar to the corresponding process in mammals; the fibrous enamel matrix appears to be formed from outgrowths of the cytoplasm of these cells. Interprismatic material, as it is known in mammals, is not produced, so that reptilian matrix tends to be uniformly fibrous. Nevertheless, the fibres are initially arranged in groups corresponding to the ameloblasts. There is no distinct pre-enamel stage because matrix production is immediately followed by a limited influx of mineral in an elementary state, converting the matrix into an basiphil form. Striae of Retzius may be due to periodic pauses in the normal process of matrix production enabling the ameloblasts to assimilate and secrete mineral. Before the onset of final calcification, the matrix seems to undergo a modification rendering it capable of influencing the size and orientation of mineral crystallites. The organic matrix has a refractive index of 1-57 and has no intrinsic birefringence. However, in suitable liquids the parallel fibres produce a positive form birefringence. If paraffin wax is allowed to crystallize on the matrix, optically negative streaks are formed parallel with the fibres, perhaps as the result of crystal overgrowth. Evidence obtained indicates that this reptilian type of ectodermal enamel is a likely precursor of the mammalian prismatic type. The evolution from one to the other could have been achieved in a comparatively simple step.