Enamel ultrastructure and masticatory function in molars of the American opossum, Didelphis virginiana

Abstract

Maxillary and mandibular molars of the American opossum, Didelphis virginiana L., were viewed in the scanning electron microscope (SEM) after acid‐etching or after cutting and acid‐etching. Observations were made on enamel prism patterns as they relate to functional properties of the tooth at a particular site. Molars at different stages of wear were also observed under a dissecting microscope; worn surfaces were correlated with function and enamel ultrastructure. Pounding surfaces of molar cusps wear more rapidly than near‐vertical shearing surfaces or crushing basins (i.e. the trigon and talonid basin). Pounding surfaces are subjected to abrasion by food and arc not normally involved in tooth‐tooth contact. Near‐vertical shearing surfaces and basins used for crushing do experience tooth‐tooth contact, but are surprisingly more resistant to wear. Prisms at pounding sites approach the occlusal surface at a near 90° angle and are surrounded with very thick interprismatic (IP) enamel parallel to the occlusal surface of the tooth. The pounding pattern is present at tips of cusps and at occlusal surfaces of ridges of the tooth. At near‐vertical shearing surfaces, the prisms approach the outer surface obliquely and are surrounded with IP crystals which are perpendicular to the vertical surface. The angle between prismatic and IP enamel in these patterns is 60–90° in a cervical to occlusal direction. In basins of the tooth used principally for crushing and some shearing, IP enamel is perpendicular to the changing slope of the basin and the prisms are usually at a 55–65° angle to the IP enamel. When the pounding and shearing‐crushing patterns meet at a ridge, a distinct seam is observed. Pounding forces occur parallel to the long axis of the prisms and perpendicular to the thick IP enamel (i.e. perpendicular to the long axis of the IP crystals) lying on either side of the prisms. Shearing and crushing forces occur at an oblique angle to the prism, and interprismatic enamel is more evenly distributed about the prism. A spiral pattern is found at the bottoms of the trigon and talonid basins, but not at the bottom of the trigonid which is a non‐occluding basin. It is concluded that the differential rates of wear of the enamel surfaces are necessary in maintaining the sharp cutting edges and effective crushing basins of the tribosphenic molar, and the ultrastructural arrangements of the enamel prisms are of functional significance.

Enamel microstructure and molar wear in the greater galago, Otolemur crassicaudatus (mammalia, primates)

“…In contrast, Stern et al (1989) suggested that the much more extensive molar cusp wear in Didelphis is the result of abrasion by food particles driven parallel to the surface. It should be noted, however, that if adhesion between opposing surfaces accounts for formation of small microwear pits (Teaford and Runestad, 1992), then surface-parallel force also may be a factor in galago cuspal abrasion.…”

“…Cusps of Didelphis molars are much more extensively worn than those of galago molars. Unlike the blunted cusps and ridges of worn galago molars, worn Didelphis molar cusps and ridges form sharp cutting edges, a result of the rapid wear of apices of cusps and crests in conjunction with wear resistance of wear crest and cusp slopes (Stern et al, 1989). The different wear patterns in these two animals are a general indication of differences in molar function.…”

“…It has been suggested here that differences in molar form imply differences in orientation of crystallites relative to wear surfaces, and thus differences in resistance to wear. The recent study of enamel structure and function in Didelphis by Stern et al (1989) allows a comparison of the relationship of microstructure and wear between an animal with primitive tribosphenic molars (Didelphis) and the galago.…”

“…Enamel structure influences the wear properties of teeth (e.g., Boyde, 1984a,b) and studies of a variety of mammals have demonstrated that the relationship between structure and wear can be important in maintaining dental functional efficiency (e.g., Boyde and Fortelius, 1986;Ferreira et al, 1989;Rensberger and Koenigswald, 1980;Stern et al, 1989;Walker, 1984;Young et al, 1987). This study of enamel microstructure and dental wear in the largeeared greater galago, Otolemur crassicaudatus', represents a first look at the relationship between enamel structure and molar function in the primates.…”

Hunter-Schreger Band patterns and their implications for clinical dentistry

“… mammalian teeth have evolved such that individual prisms are optimally orientated to maximally resist abrasion, even if this means that prism distribution within a tooth is asymmetric (14, 15); the angle at which an individual prism meets the tooth surface will determine its resistance to wear (16, 17). …”