The Ontogeny of Molar Pattern

Abstract

SUMMARY The crown pattern of the tooth is essentially that of the surface of the dentine (dentine‐enamel junction), modified by the deposition of enamel which may be uneven in thickness. The dentine‐enamel junction preserves in the completed tooth the form of the membrana praeformativa, the basement membrane of the inner enamel epithelium of the enamel organ. Folding of this membrane creates the crown pattern. The inner enamel epithelium is subjected to pressure from both sides. On the basal side there is the rapidly growing mesenchyme of the dental papilla, and on the occlusal side there is the stellate reticulum, which swells by the accumulation of fluid. The stellate reticulum prevents distortion of the epithelium by growth of the papilla, and thus ensures that folding of the epithelium is due to its intrinsic growth pattern. This makes for more accurate control of the crown pattern, the details of which are of importance in the function of chewing. The enamel knot is a region of the inner enamel epithelium from which cells are contributed to the stellate reticulum. It represents the tip of the primary cusp. The enamel cord (‘enamel septum’) which consists of cells which are in process of transforming into stellate reticulum, has been confused with two other structures that develop later: a cleavage septum, preparatory to the formation of crown cementum, and an epithelial septum, found in marsupials and crocodilians. The epithelial nodules of monotremes are probably degenerate relics of an epithelial septum. The inner enamel epithelium is a diaphragm passing across the interior of the dental follicle, and folding to adapt its increased area to a confined space. The cement organ, which in some mammals develops from the follicle, probably plays no part in the deformation of the epithelium, but the follicle as a whole may be subject to compression by adjacent follicles. A cusp is a centre of precocious maturation of the cells of the inner enamel epithelium. Here growth ceases (perhaps after a transitory burst of mitosis) and eventually the hard tissues are deposited. The process starts at the tip of the cusp and extends basally, so that growth continues longest in the valleys, intensifying the crown relief. Dens in dente is due to retarded maturation of an area of the enamel epithelium. Throughout the development of the crown there is a marginal zona cingularis, where growth continues. The crown pattern depends upon the position and the stage of growth in which cusps are differentiated from the zona cingularis, by accelerated maturation of groups of cells. Cusps which appear late in development stand low on the crown and frequently form part of a cingulum. Changes in the timing of cusp formation play an important part in serial modifications of pattern, as well as in phylogeny. Ridges are probably produced by tensions set up in the epithelium by the relative movement of cusps, owing to unequal growth or to changes in the shape of the follicle. They form in areas where growth has slowed down but the apposition of ...

Mechanical constraint from growing jaw facilitates mammalian dental diversity

“…The self-regulatory aspect of teeth is further supported by the reiterative use of the same signaling pathways and signaling centers, called enamel knots, present within each growing tooth (2). Experiments on growing teeth have revealed how an activator-inhibitor balance of signaling molecules within a developing tooth and between adjacent teeth regulate dental patterning (6-12).Despite the evidence from mice indicating self-regulation of teeth, several observations suggest an intriguing link between jaw bone and tooth crown formation (3,13,14). In humans, abnormal crown patterns in patients with congenital syphilis have been ascribed to result from infection of the tooth follicle (3).…”

“…Furthermore, tooth development, similar to organ development in general, results from sequential events that appear to be relatively independent from the development of other organs. In particular, mammalian teeth have been considered highly selfregulatory, as they form inside the dental follicle buffered from the surrounding environment (3)(4)(5). The self-regulatory aspect of teeth is further supported by the reiterative use of the same signaling pathways and signaling centers, called enamel knots, present within each growing tooth (2).…”

“…Despite the evidence from mice indicating self-regulation of teeth, several observations suggest an intriguing link between jaw bone and tooth crown formation (3,13,14). In humans, abnormal crown patterns in patients with congenital syphilis have been ascribed to result from infection of the tooth follicle (3).…”

Is the deciduous/permanent molar enamel thickness ratio a taxon-specific indicator in extant and extinct hominids?

“…in Bailey et al, 2014Bailey et al, , 2016; see also Evans et al, 2016), i.e., they are meristic elements with a similar and serially repeated structure within the same organism (Butler, 1956(Butler, , 1967Kraus and Jordan, 1965). In this study on some extant and fossil hominids, we thus investigate the degree of covariation in enamel proportions between the dm2 and the M1 (for the extant human condition, see Gantt et al, 2001;Grine, 2005;Huszár, 1972;Mahoney, 2010;Rossi et al, 1999).…”

Evolutionary studies: Genetics, development, and palaeontology interlock

“…Genetic analysis was, until the 1990s, limited to a few measures of heritability in tooth size. Developmental biologists showed more interest, especially in the patterning of tooth types in the dentition (the field and clone theories) and how the interlocking morphology is controlled in occluding teeth, a morphology that arises before the teeth erupt and come into contact (Butler, 1956). But the tools to link molecular genetics, development, and tooth morphology were lacking.…”