276 A. BoYDE 



We wish to thank Dr. Karl Schmid for the electrophoretic analyses, and Miss 

 Cathy Rutstein and Mr. Paul Day for technical assistance. 



References 



BoNAR, L. C, M. J. Glimcher, and G. L. Mechanic: The molecular structure of the neutral- 

 soluble proteins of embryonic bovine enamel in the solid state. J. Ultrastruct. Res. (in 

 press). 



Glimcher, M. J., L. C. Bonar, and E. J. Daniel: The molecular structure of the protein 

 matrix of bovine dental enamel. J. molec. Biol. 3, 541 (1961). 



— , G. L. Mechanic, L. C. Bonar, and E. J. Daniel: The amino acid composition of the orga- 

 nic matrix of decalcified fetal bovine dental enamel. J. biol. Chem. 236, 3210 (1961 b). 



— — , and U. A. Friberg: The amino acid composition of the organic matrix and the 

 neutral-soluble and acid-soluble components of embryonic bovine enamel. Biochem. J. 93, 

 198 (1964). 



The Development of Enamel Structure in Mammals 



A. BOYDE 

 The London Hospital Medical College, London, England ' 



Introduction 



A one to one ratio between the numbers of prisms and the numbers of ameloblasts 

 has been accepted by the great majority of those who have studied enamel develop- 

 ment. It has also usually been accepted that one ameloblast forms one prism, either 

 by its own transformation, or because its secretion remains separate from the secretion 

 of the other, surrounding ameloblasts for some reason. A separate and different 

 "interprismatic substance" has been supposed to originate from various sources. Two 

 particularly notable schemes were, 1. that it arose from the conversion or trans- 

 formation of the inner terminal bar apparatus of the ameloblasts and, 2. that It was 

 an Intercellular secretory substance whilst the prism substance was transformed cyto- 

 plasm of ameloblasts. 



The revelation that the whole of enamel is secreted (Fearnhead, 1960; Watson, 

 1960) followed shortly after the discovery that the principal (If not the only) 

 difference between "prism" and "Interprismatic" substances lay in the orientation of 

 the crystallites which these regions contained. It posed the problem of explaining how 

 two separate and different substances or two regions containing differently oriented 

 crystallites and the "prism sheaths" which separate them could be produced by the 

 same cells. The problem is simplified by the realisation that the prism sheaths In 

 developing enamel only exist as planes at which the crystallite orientation changes 

 quite suddenly between domains (prism or Interprism) in which It only changes 

 gradually; the higher organic content of the prism sheaths Is only acquired during 

 the increase In mineral content of the bulk of the enamel. The problem then remain- 

 ing Is purely one of crystallite orientation, that is of finding where the orientation Is 

 determined, what the orientation Is, and hence deducing what sort of controlling 

 factors might be Involved. 



An early and constant finding of this study was that the crystallites grow very 

 close to the ameloblasts in what Fearnhead (1960) called the mineralising front. The 



