Symphysis Menti of the Rabbit: Anatomy, Histology, and Postnatal Development

Journal of Experimental Biology

Kunwar

Stock

et al. 2007

151

SUMMARY Excessive, repetitive and altered loading have been implicated in the initiation of a series of soft- and hard-tissue responses or `functional adaptations' of masticatory and locomotor elements. Such adaptive plasticity in tissue types appears designed to maintain a sufficient safety factor, and thus the integrity of given element or system, for a predominant loading environment(s). Employing a mammalian species for which considerable in vivo data on masticatory behaviors are available, genetically similar domestic white rabbits were raised on diets of different mechanical properties so as to develop an experimental model of joint function in a normal range of physiological loads. These integrative experiments are used to unravel the dynamic inter-relationships among mechanical loading, tissue adaptive plasticity, norms of reaction and performance in two cranial joint systems:the mandibular symphysis and temporomandibular joint (TMJ). Here, we argue that a critical component of current and future research on adaptive plasticity in the skull, and especially cranial joints, should employ a multifaceted characterization of a functional system, one that incorporates data on myriad tissues so as to evaluate the role of altered load versus differential tissue response on the anatomical, cellular and molecular processes that contribute to the strength of such composite structures. Our study also suggests that the short-term duration of earlier analyses of cranial joint tissues may offer a limited notion of the complex process of developmental plasticity, especially as it relates to the effects of long-term variation in mechanical loads, when a joint is increasingly characterized by adaptive and degradative changes in tissue structure and composition. Indeed, it is likely that a component of the adaptive increases in rabbit TMJ and symphyseal proportions and biomineralization represent a compensatory mechanism to cartilage degradation that serves to maintain the overall functional integrity of each joint system. Therefore, while variation in cranial joint anatomy and performance among sister taxa is, in part, an epiphenomenon of interspecific differences in diet-induced masticatory stresses characterizing the individual ontogenies of the members of a species,this behavioral signal may be increasingly mitigated in over-loaded and perhaps older organisms by the interplay between adaptive and degradative tissue responses.

Section: Methodsmentioning

confidence: 99%

Section: Histology and Immunohistochemistry Of Cartilage Compositionmentioning

confidence: 99%

Pushing the limit: masticatory stress and adaptive plasticity in mammalian craniomandibular joints

Journal of Experimental Biology

Kunwar

Stock

et al. 2007

151

“…Once weaned, juvenile mammals soon ingest foods similar to adults [39] and develop adultlike jaw adductor recruitment patterns [40][41][42][43][44][45] and corresponding morphological responses to adult symphyseal loading regimes [32][33][34][46][47][48][49][50][51]. During mastication, the symphysis experiences lateral transverse bending.…”

Section: Functional Significance Of Symphyseal Character Statesmentioning

confidence: 99%

Anthropoid Origins and the Modern Symphysis

1999

IJFP

103

To highlight adaptive transformations in craniomandibular form during anthropoid origins, symphyseal character states and underlying masticatory loading regimes were investigated vis-à-vis shifts in diet and body size. A study of fossil anthropoids is possible because variation in symphyseal fusion is continuous and directly proportional to the amount of symphyseal stress and because such variation can be considered a series of discrete character states each with unique functional underpinnings. Using recent systematic renderings of Eocene and Oligocene taxa as a template with which to assess character evolution, this analysis indicates when, and in which clade(s), specific masticatory features became fixed and thus diagnostic. A general trend throughout early anthropoid evolution is for descendent taxa to be progressively larger than ancestral forms. Coupled with this pattern is the tendency for larger-bodied fossil anthropoids to have ingested tougher diets variably consisting of thick-coated, unripe fruits and/or leaves. Mastication of mechanically tougher foods entails greater repetitive loading of the mandible and requires relatively larger amounts of balancing-side muscle force, thus resulting in correspondingly greater symphyseal fusion due to elevated dorsoventral shear. With a single exception, these adaptive transformations characterize the evolutionary pathway leading both to parapithecines and a catarrhine:platyrrhine clade (crown anthropoids). While the ancestor of crown anthropoids would have possessed a body size, diet and masticatory adaptations similar to parapithecines, such a common suite of features evolved independently. Moreover, the evolution of an early-fusing symphysis and associated wishboning loading regime of catarrhines and platyrrhines is unique among all anthropoids. Lastly, the apparent lack of reversals in symphyseal fusion indicates the improbability of phylogenetic hypotheses in which a relationship is proposed between ‘ancestral’ taxa with a greater degree of symphyseal fusion and ‘descendent’ anthropoids with a lesser degree of ossification.

“…2 and 3). Similar morphologies in extant prosimians and other mammals are generally associated with the presence of ligaments spanning the symphyseal joint (Scapino, 1965(Scapino, , 1981Trevisan and Scapino, 1976a,b;Beecher, 1977Beecher, , 1979Hirschfeld et al, 1977). The upper lingual border of the symphyseal surface of the superior transverse torus (near the central incisor) also possesses a series of bony elevations, which are less organized and perhaps best termed rugosities ( Fig.…”

Section: Morphology Of the Unfused Symphysismentioning

confidence: 93%

“…2A). In those mammals studied to date, this is typically the site of a fibrocartilaginous pad interposed between dentaries (Scapino, 1965(Scapino, ,1981Trevisan and Scapino, 1976a,b;Beecher, 1977Beecher, ,1979Hirschfeld et al, 1977), and perhaps this was also the condition for juvenile Archaeolemur. Among some specimens ofArchaeolemur this region has a series of foramina situated among the rugosities, much like that noted for other mammals with unfused symphyses.…”

Section: Morphology Of the Unfused Symphysismentioning

confidence: 99%

Mandibular growth and function in Archaeolemur

American J Phys Anthropol

Simons

1994

Ontogenetic changes in the morphology of the mandibular symphysis are described in Archaeolemur so as to infer the functional significance of symphyseal fusion in this subfossil Malagasy lemur. The first regions of the symphysis to show a more complex morphology were the lower and anterior borders of the joint and, to a lesser extent, the lingual borders of the superior and inferior transverse tori. During growth, these regions became increasingly rugose and encroached upon a centrally located, smooth, "oval" region, which may have been a principal pathway for neurovascular structures communicating with the unfused joint. In subadults, the symphysis was completely fused except for the lingual surface of the inferior transverse torus, where a patent suture and potential space were present between dentaries. Thus, in Archaeolemur there was an age- and size-related pattern of increased symphyseal ossification or fusion that was complete by adulthood. The morphology of the interlocking bony processes and the sequence of ossification in the symphysis suggest that increased dorsoventral shear stress during mastication was the most likely determinant of symphyseal fusion in Archaeolemur. The allometric pattern of greater symphyseal fusion may be linked to the presence of relatively greater dorsoventral shear in adults due to an increased recruitment of balancing-side jaw-muscle force. There is little indication that the symphysis of juvenile Archaeolemur was buttressed to resist forces associated with "wishboning" during mastication or vertical bending during incision. Our observations, as well as those of others, suggest that symphyseal fusion in primates occurs initially as a response to increased dorsoventral shear during mastication. Therefore, wishboning stress might only become a major determinant of symphyseal form and function in those taxa that develop a fused symphysis to counter increased dorsoventral shear.