Some aspects of the growth and differentiation of the molar tooth germ in the mouse (Mus musculus)
The present work Is concerned with two aspects: I. A description is given of the distribution of mitoses in the inner enamel epithelium of the mouse molar teeth during ontogeny, from the aspect of the establishment of the crown pattern. This distribution is correlated with cusp formation. Within limits of experimental error during reconstruction and plotting the mitoses, it has been shown that mitosis ceases at an early stage in areas which will eventually lie at the cusp tips, but continues between them, so that the valleys grow downwards leaving the cusps in a more elevated position. In other words, the relative positions of the cusps (mitoses-free areas), and the valleys (areas of intense mitoses) on the crown have been shown to be mapped out (predetermined) early in development. The theory of 'insinuation' of the stratum intermedium among the inner enamel epithelium has been rejected. II. A description of the cytology of the ameloblasts andodontoblasts is given with the aim of adding more detail to earlier accounts. In particular, it is shown that the Golgi apparatus changes polarity and varies in form during the life cycle of the ameloblasts, and an attempt is made toarrive at a more concrete idea of its role during the life of ameloblasts. The Aoyama method is used for the first time for demonstrating the Golgi apparatus in teeth. Also, Powers' method for nerve fibres, is used for the first time to show the Golgi apparatus, a method which has not been used for this purpose in teeth or any other tissue. The cytologic activity of the formative cells has been correlated with the morphologic changes of the Golgi apparatus. This correlation has been used to illustrate important phases of amelogenesis and dentinogenesis. The presence of the kionoblasts" among the ameloblasts and the 'radial cells' among the odontoblasts is denied.