Relationships Between Enamel Prism Decussation and Organization of the Ameloblast Layer in Rodent Incisors

Abstract

Rodent enamel microstructure has been extensively investigated, primarily on the basis of 2D electronic microscopy data. The nature and dynamics of the ameloblasts (the enamel‐secreting cells) have also been well studied. However, critical issues still remain surrounding exactly how the ameloblasts produce the astonishing microstructural complexity of enamel, and how this subtle architecture evolved through time. In this article, we used a new methodology based on confocal laser microscopy to reconstruct the enamel microstructure of rodent incisors in three dimensions (3D) with the ameloblasts in situ. We proposed interpretations regarding the possible relationships between the workings of the ameloblasts and the resulting enamel prisms, especially how the phenomenon of decussation is generated. Finally, we were able to represent the two main types of modern rodent incisor microstructures (uniserial and multiserial decussations), as a set of parameters that have been entered into the 3D enamel simulation software Simulenam to generate 3D models that can be digitally manipulated. Associating 2D data of incisor enamel microstructure of fossil rodents and Simulenam, it was then possible to better understand how the various decussation parameters evolved through time and gave rise to the two modern microstructure types from the same ancestral type (pauciserial). This study also confirmed that rodent and artiodactyl enamel do not share the same mechanism of decussation formation. Anat Rec, 302:1195–1209, 2019. © 2018 Wiley Periodicals, Inc.

Characteristics of the transverse 2D uniserial arrangement of rows of decussating enamel rods in the inner enamel layer of mouse mandibular incisors

“…The data from this investigation suggest that the reason why spaces do not develop because of physical sliding apart of two adjacent decussating rows of ameloblasts in rat and mouse incisors (Boyde, ; Warshawsky, ; Alloing‐Séguier et al. ) is based on the manner in which row formation is induced and extended from the central region into the mesial and lateral regions at the level of the DEJ. This idea is schematically illustrated in Fig.…”

“…There has been speculation that some enamel rods in rat and mouse incisor enamel may not extend the entire thickness of the enamel layer, but evidence for this has been weak (Alloing‐Séguier et al. ). The origin of the focal stack arrangement may, however, be easier to explain.…”

“…The straightening out of the initial wavy rows of ameloblasts and of the ameloblasts themselves changing from irregular polygonal shape to a more rectangular shape at the distal terminal web provides two very simple physical methods, not requiring cell proliferation, to extend the rows outwards in the transverse plane (mesial‐to‐lateral direction) traditionally associated with gain in row length needed to prevent interrow spaces from developing internally as decussating rows move away from one another as the enamel rods are formed (Alloing‐Séguier et al. ).…”

“…Enamel rods originate near the dentinoenamel junction (DEJ) and travel toward the outer surface of the enamel layer, usually along some nonlinear path that may follow an angularly segmented, sigmoid or wavy/spiral trajectory (Boyde, ; Warshawsky, , ; Sahni, ; von Koenigswald & Clemens, ; Alloing‐Séguier et al. , ). Enamel rods traverse the thickness of the enamel layer usually in groups that follow the same nonlinear path (Alloing‐Séguier et al.…”

“…Enamel rods traverse the thickness of the enamel layer usually in groups that follow the same nonlinear path (Alloing‐Séguier et al. , ). Immediately adjacent groups of enamel rods follow different nonlinear paths, often at an opposite angle relative to neighboring groups of rods.…”