Search for: All records

Abstract Tusks are ever‐growing teeth present in mammals of the clade Paenungulata. Unlike the perpetually growing incisors of rodents, tusks are not used in mastication, and in at least some paenungulatans, the tusk is composed of dentin alone in adults. Few studies have provided tissue‐level information on tusks of adult paenungulatans with embedding techniques that identify epithelial and other soft tissues. In order to examine the mineralized tissues as well as the cells that form teeth, we studied a single, subadult rock hyrax (Procavia capensis) using microCT and paraffin histology with traditional staining as well as RUNX2 immunohistochemistry, and compared its teeth to scans of adult hyraxes. Three‐dimensional reconstructions from microCT volumes revealed that the tusk of this specimen is the only fully erupted replacement tooth, the first adult premolar (P1) is starting to erupt, and the first permanent molar (M1) is fully erupted, whereas all other replacement teeth and M2 remain in crypts. The tusk has a thin layer of enamel on its dorsal side; this is confirmed by histology. All deciduous premolars still possess roots that are in the process of resorption. Amelogenesis has progressed to maturation or nearly so in P1–P3. Notable histological characteristics of replacement premolars include the lack of a stellate reticulum in all except P4, and expression of RUNX2 in ameloblasts, a marker which is expressed by ameloblasts at all stages of amelogenesis. Since the pulp chambers of replacement premolars are relatively large compared to adults, a lengthy time in crypts may be important for dentin production. The results confirm that the hyrax has thin enamel on tusks, supporting the hypothesis that enamel is of limited importance for non‐feeding behaviors. 

Abstract Cranial synchondroses are cartilaginous joints between basicranial bones or between basicranial bones and septal cartilage, and have been implicated as having a potential active role in determining craniofacial form. However, few studies have examined them histologically. Using histological and immunohistochemical methods, we examined all basicranial joints in serial sagittal sections of newborn heads from nine genera of primates (five anthropoids, four strepsirrhines). Each synchondrosis was examined for characteristics of active growth centers, including a zonal distribution of proliferating and hypertrophic chondrocytes, as well as corresponding changes in matrix characteristics (i.e., density and organization of Type II collagen). Results reveal three midline and three bilateral synchondroses possess attributes of active growth centers in all species (sphenooccipital, intrasphenoidal, presphenoseptal). One midline synchondrosis (ethmoseptal) and one bilateral synchondrosis (alibasisphenoidal synchondrosis [ABS]) are active growth centers in some but not all newborn primates. ABS is oriented more anteriorly in monkeys compared to lemurs and bushbabies. The sphenoethmoidal synchondrosis (SES) varies at birth: in monkeys, it is a suture‐like joint (i.e., fibrous tissue between the two bones); however, in strepsirrhines, the jugum sphenoidale is ossified while the mesethmoid remains cartilaginous. No species possesses an SES that has the organization of a growth plate. Overall, our findings demonstrate that only four midline synchondroses have the potential to actively affect basicranial angularity and facial orientation during the perinatal timeframe, while the SES of anthropoids essentially transitions toward a “suture‐like” function, permitting passive growth postnatally. Loss of cartilaginous continuity at SES and reorientation of ABS distinguish monkeys from strepsirrhines.