Torosaurus is not Triceratops: ontogeny in chasmosaurine ceratopsids as a case study in dinosaur taxonomy

Background

In horned dinosaurs, taxonomy is complicated by the fact that the cranial ornament that distinguishes species changes with age. Based on this observation, it has been proposed that the genera Triceratops and Torosaurus are in fact synonymous, with specimens identified as Torosaurus representing the adult form of Triceratops. The hypothesis of synonymy makes three testable predictions: 1) the species in question should have similar geographic and stratigraphic distributions, 2) specimens assigned to Torosaurus should be more mature than those assigned to Triceratops, and 3) intermediates should exist that combine features of Triceratops and Torosaurus. The first condition appears to be met, but it remains unclear whether the other predictions are borne out by the fossil evidence.

Methodology/Principal Findings

We assessed the relative maturity of Torosaurus and Triceratops specimens by coding skulls for characters that vary with maturity, and then using a clustering analysis to arrange them into a growth series. We found that a well-defined sequence of changes exists in horned dinosaurs: development of cranial ornament occurs in juveniles, followed by fusion of the skull roof in subadults, and finally, the epoccipitals, epijugals, and rostral fuse to the skull in adults. Using this scheme, we identified mature and immature individuals of both Torosaurus and Triceratops. Furthermore, we describe the ventral depressions on the frill of Triceratops, and show that they differ in shape and position from the parietal fenestrae of Torosaurus. Thus, we conclude that these structures are not intermediates between the solid frill of Triceratops and the fenestrated frill of Torosaurus.

Conclusions/Significance

Torosaurus is a distinct genus of horned dinosaur, not the adult of Triceratops. Our method provides a framework for assessing the hypothesis of synonymy through ontogeny in the fossil record.     

Is Torosaurus Triceratops? Geometric Morphometric Evidence of Late Maastrichtian Ceratopsid Dinosaurs

Background

Recent assessments of morphological changes in the frill during ontogeny hypothesized that the late Maastrichtian horned dinosaur Torosaurus represents the “old adult” of Triceratops, although acceptance of this finding has been disputed on several lines of evidence.

Methodology/Principal Findings

Examining the cranial morphology of 28 skulls in lateral view and 36 squamosals of Nedoceratops hatcheri, Triceratops spp. and Torosaurus spp. by means of landmark-based geometric morphometrics, we compared ontogenetic trajectories among these taxa. Principal Component Analysis and cluster analysis confirmed different cranial morphologies. Torosaurus shape space is well separated from Triceratops, whereas Triceratops horridus and Triceratops prorsus partially overlap within Triceratops shape space. Linear regressions between shape and size suggest different ontogenetic trajectories among these taxa. Results support the “traditional” taxonomic status of Torosaurus. We hypothesize that ontogeny drives cranial morphology with different patterns between Torosaurus and Triceratops.

Conclusions/Significance

Torosaurus is a distinct and valid taxon. Whether looking at entire skulls, skulls without the frill, frills alone, or squamosals, Torosaurus has different morphologies and distinct allometric trajectories compared to Triceratops. This new approach confirms the taxonomic status of Torosaurus as well as the comparatively low diversity of ceratopsids at the end of the Maastrichtian in North America.     

Evidence of Combat in Triceratops

Background

The horns and frill of Triceratops and other ceratopsids (horned dinosaurs) are interpreted variously as display structures or as weapons against conspecifics and predators. Lesions (in the form of periosteal reactive bone, healing fractures, and alleged punctures) on Triceratops skulls have been used as anecdotal support of intraspecific combat similar to that in modern horned and antlered animals. If ceratopsids with different cranial morphologies used their horns in such combat, this should be reflected in the rates of lesion occurrence across the skull.

Methodology/Principal Findings

We used a G-test of independence to compare incidence rates of lesions in Triceratops (which possesses two large brow horns and a smaller nasal horn) and the related ceratopsid Centrosaurus (with a large nasal horn and small brow horns), for the nasal, jugal, squamosal, and parietal bones of the skull. The two taxa differ significantly in the occurrence of lesions on the squamosal bone of the frill (P = 0.002), but not in other cranial bones (P>0.20).

Conclusions/Significance

This pattern is consistent with Triceratops using its horns in combat and the frill being adapted as a protective structure for this taxon. Lower pathology rates in Centrosaurus may indicate visual rather than physical use of cranial ornamentation in this genus, or a form of combat focused on the body rather than the head.     

Anatomy and taxonomic status of the chasmosaurine ceratopsid Nedoceratops hatcheri from the upper Cretaceous Lance Formation of Wyoming, U.S.A

Background

The validity of Nedoceratops hatcheri, a chasmosaurine ceratopsid dinosaur known from a single skull recovered in the Lance Formation of eastern Wyoming, U.S.A., has been debated for over a century. Some have argued that the taxon is an aberrant Triceratops, and most recently it was proposed that N. hatcheri represents an intermediate ontogenetic stage between “young adult” and “old adult” forms of a single taxon previously split into Triceratops and Torosaurus.

Methodology/Principal Findings

The holotype skull of Nedoceratops hatcheri was reexamined in order to map reconstructed areas and compare the specimen with other ceratopsids. Although squamosal fenestrae are almost certainly not of taxonomic significance, some other features are unique to N. hatcheri. These include a nasal lacking a recognizable horn, nearly vertical postorbital horncores, and relatively small parietal fenestrae. Thus, N. hatcheri is tentatively considered valid, and closely related to Triceratops spp. The holotype of N. hatcheri probably represents an “old adult,” based upon bone surface texture and the shape of the horns and epiossifications on the frill. In this study, Torosaurus is maintained as a genus distinct from Triceratops and Nedoceratops. Synonymy of the three genera as ontogenetic stages of a single taxon would require cranial changes otherwise unknown in ceratopsids, including additions of ossifications to the frill and repeated alternation of bone surface texture between juvenile and adult morphotypes.

Conclusions/Significance

Triceratops, Torosaurus, and likely Nedoceratops, are all distinct taxa, indicating that species richness for chasmosaurine ceratopsids in the Lance Formation just prior to the Cretaceous-Paleocene extinction was roughly equivalent to that earlier in the Cretaceous.     

Modularity and heterochrony in the evolution of the ceratopsian dinosaur frill

The fossil record provides compelling examples of heterochrony at macroevolutionary scales such as the peramorphic giant antlers of the Irish elk. Heterochrony has also been invoked in the evolution of the distinctive cranial frill of ceratopsian dinosaurs such as Triceratops. Although ceratopsian frills vary in size, shape, and ornamentation, quantitative analyses that would allow for testing hypotheses of heterochrony are lacking. Here, we use geometric morphometrics to examine frill shape variation across ceratopsian diversity and within four species preserving growth series. We then test whether the frill constitutes an evolvable module both across and within species, and compare growth trajectories of taxa with ontogenetic growth series to identify heterochronic processes. Evolution of the ceratopsian frill consisted primarily of progressive expansion of its caudal and caudolateral margins, with morphospace occupation following taxonomic groups. Although taphonomic distortion represents a complicating factor, our data support modularity both across and within species. Peramorphosis played an important role in frill evolution, with acceleration operating early in neoceratopsian evolution followed by progenesis in later diverging cornosaurian ceratopsians. Peramorphic evolution of the ceratopsian frill may have been facilitated by the decoupling of this structure from the jaw musculature, an inference that predicts an expansion of morphospace occupation and higher evolutionary rates among ceratopsids as indeed borne out by our data. However, denser sampling of the meager record of early‐diverging taxa is required to test this further.     

Skull ontogeny in Arrhinoceratops brachyops (Ornithischia: Ceratopsidae) and other horned dinosaurs

Disentangling ontogenetic from interspecific variation is key to understanding biodiversity in the fossil record, yet information on growth in the ceratopsid subfamily Chasmosaurinae is sparse. Here, we describe the partial skull of a juvenile chasmosaurine, attributed to Arrhinoceratops brachyops, within the context of more mature specimens of this species, to better understand the ontogenetic transformations therein. We show that as A. brachyops matured, the postorbital horncores became longer and shifted from a posterior to an anterior inclination, the delta‐shaped frill epiossifications became lower and fused to the underlying frill, and the face became more elongate. In these respects, A. brachyops closely resembled Triceratops, suggesting that these ontogenetic changes may have been common to all long‐horned chasmosaurines. However, an event‐paired cladistic analysis of Chasmosaurinae using a standardized matrix of 24 developmental characters reveals that the relative timing of ontogenetic events in Arrhinoceratops was more like that of Chasmosaurus, particularly in the relatively late reduction in scalloping around the frill margins. Thus, the ontogenetic similarities between Arrhinoceratops and Triceratops appear to be plesiomorphic, partly related to the retention of the elongate postorbital horncores, which are primitive for Ceratopsidae. This study elucidates the otherwise contentious evolutionary relationships of Arrhinoceratops, and highlights the importance of ontogenetic data for resolving phylogenies when morphological data from adults alone are inadequate. © 2015 The Linnean Society of London     

Dinosaur census reveals abundant Tyrannosaurus and rare ontogenetic stages in the Upper Cretaceous Hell Creek Formation (Maastrichtian), Montana, USA

Background

A dinosaur census recorded during the Hell Creek Project (1999–2009) incorporates multiple lines of evidence from geography, taphohistory, stratigraphy, phylogeny and ontogeny to investigate the relative abundance of large dinosaurs preserved in the Upper Cretaceous Hell Creek Formation of northeastern Montana, USA. Overall, the dinosaur skeletal assemblages in the Hell Creek Formation (excluding lag-influenced records) consist primarily of subadult or small adult size individuals. Small juveniles and large adults are both extremely rare, whereas subadult individuals are relatively common. We propose that mature individuals of at least some dinosaur taxa either lived in a separate geographic locale analogous to younger individuals inhabiting an upland environment where sedimentation rates were relatively less, or these taxa experienced high mortality before reaching terminal size where late stage and often extreme cranial morphology is expressed.

Methodology/Principal Findings

Tyrannosaurus skeletons are as abundant as Edmontosaurus, an herbivore, in the upper Hell Creek Formation and nearly twice as common in the lower third of the formation. Smaller, predatory dinosaurs (e.g., Troodon and dromaeosaurids) are primarily represented by teeth found in microvertebrate localities and their skeletons or identifiable lag specimens were conspicuously absent. This relative abundance suggests Tyrannosaurus was not a typical predator and likely benefited from much wider food choice opportunities than exclusively live prey and/or specific taxa. Tyrannosaurus adults may not have competed with Tyrannosaurus juveniles if the potential for selecting carrion increased with size during ontogeny.

Conclusions/Significance

Triceratops is the most common dinosaur and isolated skulls contribute to a significant portion of this census. Associated specimens of Triceratops consisting of both cranial and postcranial elements remain relatively rare. This rarity may be explained by a historical collecting bias influenced by facies and taphonomic factors. The limited discovery of postcranial elements may also depend on how extensive a fossil quarry is expanded after a skull is collected.     

Three-dimensional geometric morphometric analysis of the skull of Protoceratops andrewsi supports a socio-sexual signalling role for the ceratopsian frill

Socio-sexual selection is predicted to be an important driver of evolution, influencing speciation, extinction and adaptation. The fossil record provides a means of testing these predictions, but detecting its signature from morphological data alone is difficult. There are, nonetheless, some specific patterns of growth and variation which are expected of traits under socio-sexual selection. The distinctive parietal-squamosal frill of ceratopsian dinosaurs has previously been suggested as a socio-sexual display trait, but evidence for this has been limited. Here, we perform a whole-skull shape analysis of an unprecedentedly large sample of specimens of Protoceratops andrewsi using a high-density landmark-based geometric morphometric approach to test four predictions regarding a potential socio-sexual signalling role for the frill. Three predictions—low integration with the rest of the skull, significantly higher rate of change in size and shape during ontogeny, and higher morphological variance than other skull regions—are supported. One prediction, sexual dimorphism in shape, is not supported, suggesting that sexual differences in P. andrewsi are likely to be small. Together, these findings are consistent with mutual mate choice or selection for signalling quality in more general social interactions, and support the hypothesis that the frill functioned as a socio-sexual signal in ceratopsian dinosaurs.     

Anatomical and Histological Investigations on the Caudal Skeleton of Apteronotus leptorhynchus (Apteronotidae,Gymnotoidei)

The caudal skeleton of Apteronotus leptorhynchus was studied at various stages from hatching to the adult stage using anatomical and histological techniques. The caudal skeleton that supports the lepidotrichia is reduced to a rhomboid caudal plate (caudal cartilage) that extends the vertebral axis. This cartilage appears for the first time in 8 day old fish, postero-ventral to the notochord. During its growth, perichondral and endochondral ossification occurs, beginning at the anterior end of the cartilage. Comparison with the anatomy and ontogeny of the typical caudal skeleton of teleosts allows us to interpret the caudal cartilage of A. leptorhynchus as an hypuro-opisthural component that is homologous to the cartilage that occurs at the tip of the axial skeleton in Eigenmannia virescens.     

New Horned Dinosaurs from Utah Provide Evidence for Intracontinental Dinosaur Endemism

Background

During much of the Late Cretaceous, a shallow, epeiric sea divided North America into eastern and western landmasses. The western landmass, known as Laramidia, although diminutive in size, witnessed a major evolutionary radiation of dinosaurs. Other than hadrosaurs (duck-billed dinosaurs), the most common dinosaurs were ceratopsids (large-bodied horned dinosaurs), currently known only from Laramidia and Asia. Remarkably, previous studies have postulated the occurrence of latitudinally arrayed dinosaur “provinces,” or “biomes,” on Laramidia. Yet this hypothesis has been challenged on multiple fronts and has remained poorly tested.

Methodology/Principal Findings

Here we describe two new, co-occurring ceratopsids from the Upper Cretaceous Kaiparowits Formation of Utah that provide the strongest support to date for the dinosaur provincialism hypothesis. Both pertain to the clade of ceratopsids known as Chasmosaurinae, dramatically increasing representation of this group from the southern portion of the Western Interior Basin of North America. Utahceratops gettyi gen. et sp. nov.—characterized by short, rounded, laterally projecting supraorbital horncores and an elongate frill with a deep median embayment—is recovered as the sister taxon to Pentaceratops sternbergii from the late Campanian of New Mexico. Kosmoceratops richardsoni gen. et sp. nov.—characterized by elongate, laterally projecting supraorbital horncores and a short, broad frill adorned with ten well developed hooks—has the most ornate skull of any known dinosaur and is closely allied to Chasmosaurus irvinensis from the late Campanian of Alberta.

Conclusions/Significance

Considered in unison, the phylogenetic, stratigraphic, and biogeographic evidence documents distinct, co-occurring chasmosaurine taxa north and south on the diminutive landmass of Laramidia. The famous Triceratops and all other, more nested chasmosaurines are postulated as descendants of forms previously restricted to the southern portion of Laramidia. Results further suggest the presence of latitudinally arrayed evolutionary centers of endemism within chasmosaurine ceratopsids during the late Campanian, the first documented occurrence of intracontinental endemism within dinosaurs.     

Ontogenetic changes in long-bone histology of the cryptobranchid Eoscapherpeton asiaticum (Amphibia: Caudata) from the Late Cretaceous of Uzbekistan

This paper presents histological data on the long bones of different size (age) individuals of the basal cryptobranchid salamander Eoscapherpeton asiaticum from the Upper Cretaceous (Turonian) of Uzbekistan. E. asiaticum is similar to modern members of Cryptobranchidae in being relatively large (estimated body length up to 50–60 cm), aquatic, and neotenic. The analysis of growth series of femora demonstrates a significant histological maturation during ontogeny, expressed by the progressive appearance of highly organized parallel-fibred bone in the peripheral part of the periosteal cortex, appearance and increasing number of bone remodeling features, progressive resorption of calcified cartilage in the diaphyseal areas and formation of endochondral bone lining the erosion cavities in the calcified cartilage, progressive thickening of endosteal inner circumferential layer and increasing of vascularity and appearance of vascular network of longitudinal and oblique canals in the cortex. These ontogenetic changes in the long-bone histology of E. asiaticum generally correspond to those of other salamanders, except the appearance of the vascular network in the periosteal cortex — the feature that is characteristic for cryptobranchids and connected with their large body size. According to new data, the large Cenozoic cryptobranchids appear to have attained their larger size by extending the skeletal growth period.     

Extreme Cranial Ontogeny in the Upper Cretaceous Dinosaur Pachycephalosaurus

Background

Extended neoteny and late stage allometric growth increase morphological disparity between growth stages in at least some dinosaurs. Coupled with relatively low dinosaur density in the Upper Cretaceous of North America, ontogenetic transformational representatives are often difficult to distinguish. For example, many hadrosaurids previously reported to represent relatively small lambeosaurine species were demonstrated to be juveniles of the larger taxa. Marginocephalians (pachycephalosaurids + ceratopsids) undergo comparable and extreme cranial morphological change during ontogeny.

Methodology/Principal Findings

Cranial histology, morphology and computer tomography reveal patterns of internal skull development that show the purported diagnostic characters for the pachycephalosaurids Dracorex hogwartsia and Stygimoloch spinifer are ontogenetically derived features. Coronal histological sections of the frontoparietal dome of an adult Pachycephalosaurus wyomingensis reveal a dense structure composed of metaplastic bone with a variety of extremely fibrous and acellular tissue. Coronal histological sections and computer tomography of a skull and frontoparietal dome of Stygimoloch spinifer reveal an open intrafrontal suture indicative of a subadult stage of development. These dinosaurs employed metaplasia to rapidly grow and change the size and shape of their horns, cranial ornaments and frontoparietal domes, resulting in extreme cranial alterations during late stages of growth. We propose that Dracorex hogwartsia, Stygimoloch spinifer and Pachycephalosaurus wyomingensis are the same taxon and represent an ontogenetic series united by shared morphology and increasing skull length.

Conclusions/Significance

Dracorex hogwartsia (juvenile) and Stygimoloch spinifer (subadult) are reinterpreted as younger growth stages of Pachycephalosaurus wyomingensis (adult). This synonymy reduces the number of pachycephalosaurid taxa from the Upper Cretaceous of North America and demonstrates the importance of cranial ontogeny in evaluating dinosaur diversity and taxonomy. These growth stages reflect a continuum rather than specific developmental steps defined by “known” terminal morphologies.     

Organization of selective attention during preparation for the recognition of global and local characteristics of a visual stimulus in children with different levels of maturity of the regulatory brain systems

Event-related potentials (ERPs) evoked by key stimuli informing a subject about the forthcoming recognition of the global or local level of a hierarchical test figure were analyzed in 7-year-old children with different levels of maturity of the regulatory brain systems. Differences in both the initial ERP components P1, N1, and P2 (which reflect the analysis of the sensory characteristics and significance of a key stimulus) and the late components N3, Pc, and Nc (which reflect the preparation for the recognition of a subsequent test figure) were found. It was shown that, in children with frontal-thalamic regulatory system immaturity (FTRSI), the amplitude of the ERP component N1 is decreased in the caudal areas. In children with an immature bottom-up activation system, a decrease in the amplitude of initial ERP components in the caudal areas was observed in a broader time interval in components P1, N1, and P2. As compared to the control groups of children, in children with immature frontal-thalamic structures, components N3, Pc, and Nc were different in both the caudal and precentral areas. In children with immature lower brainstem activation structures, the late ERP components were different, predominantly, in the parietal and temporo-parieto-occipital areas. Comparison of ERPs in response to global and local key stimuli in children of the control group demonstrated a clear-cut temporal and topographical organization in the period of preparation for subsequent recognition of a prescribed level of the test stimulus: the earlier preparation stages were associated with component N3 in the parietal and temporo-parieto-occipital areas, whereas later stages were associated with Pc changes in the frontal areas. In children with FTRSI, changes in the late components in the caudal areas were poorly expressed and their topographical organization (characteristic of the control group) was absent; the involvement of the frontal areas in the late stages of the key stimulus analysis was restricted. These findings may give grounds to suggest the significance of the frontal-thalamic system in the organization of the response to an expected stimulus. In children with immature lower brainstem activation structures, the type of the key stimulus was reflected in the late ERP components in a diffuse way.     

Floral ontogeny in Passiflora lobata (Malpighiales,Passifloraceae) reveals a rare pattern in petal formation and provides new evidence for interpretation of the tendril and corona

Passiflora lobata differs from most other passion flowers in that it has a tetramerous gynoecium and dorsiventral flowers. A detailed ontogenetic analysis using scanning electron microscopy revealed the following characters: tendril formation starts late, indicating an axial nature. The paired flowers show mirror symmetry, which is manifested very early in ontogeny. Five sepals initiate in a spiral followed by five petals, which are formed successively adjacent to each other. This is a rare pattern and the first report in Passifloraceae. Frequently a sixth petal primordium was found, which never develops and which could be interpreted as the first outgrowth or frill of the corona (which therefore might be interpreted as derived from the perianth). The abaxial carpel forms always in front of the first-formed sepal. The remaining three carpels are alternate with the stamens. This means that a positional change took place from the typical trimerous ovary with two carpels in front of stamens to only one antestaminal carpel in P. lobata. This shift might have opened up space for a fourth carpel. Together with the analysis of other tetramerous Passifloraceae, this study will foster the understanding of flower morphology in this family and its systematic relationships among Malpighiales.     

Agolohymena aspidocauda nov. gen., nov. spec., a histophagous freshwater tetrahymenid ciliate in the family Deltopylidae (Ciliophora,Hymenostomatia), from Idaho (northwest U.S.A.): Morphology,ontogenesis and molecular phylogeny

Morphology, ontogeny and the molecular phylogeny of Agolohymena aspidocauda nov. gen., nov. spec., a new freshwater tetrahymenid ciliate from Idaho, U.S.A, are described. The ontogeny and histophagous mode of nutrition are similar to those of Deltopylum rhabdoides Fauré-Fremiet and Mugard, 1946. The new genus is placed with Deltopylum in the resurrected family Deltopylidae Song and Wilbert, 1989. We emend the diagnostic features of the family to include division by polytomy, right and left somatic kineties extending into the preoral suture, crook-shaped or sigmoid adoral membranelles 1 and 2, markedly reduced adoral membranelle 3 and a tetrahymenid silverline pattern. The main diagnostic features of the new genus are a disc-shaped caudal ciliary array and formation of two types of resting cysts, one smooth and the other bearing tangled tubular or cylindrical lepidosomes. Nuclear small subunit ribosomal RNA gene and mitochondrial cytochrome oxidase subunit 1 gene sequences place the new genus basal within the order Tetrahymenida, well separated from members of the family Tetrahymenidae (Lambornella and Tetrahymena) and also from other tetrahymenids (Colpidium, Dexiostoma, Glaucoma). The genetic divergences between this species and other genera in Tetrahymenida are large enough to suggest placement of the new genus in a separate family. This corroborates the morphological data, since the elaborate caudal ciliary array and the lepidosome-covered resting cyst of this species are not found in other Tetrahymenidae.     

Histopathology of Nomuraea rileyi in Plathypena scabra larvae

Green cloverworm larvae. Plathypena scabra, were inoculated with Nomuraea rileyi by “tumbling” larvae in a vial of conidia. The ontogeny of the pathogen was followed by using standard histological techniques. N. rileyi conidia germinated on green cloverworm integument within 12 hr after inoculation. Germ tubes penetrated larval cuticle 36 hr after inoculation, then grew parallel to endocuticular laminae. After hyphal penetration of the epidermis ca. 4.5 days after inoculation, hyphal bodies were produced and were transported throughout the hemocoel. Hyphal bodies and hemocytes cohabited the hemocoel, but gut epithelial and muscle tissues were not invaded by Day 5. Hemocytes lysed and mycelia completely ramified throughout all larval tissues by 7 days after inoculation. Death of larvae was followed by conidiogenesis ca. 7.5 days after inoculation.     

Spleen of the snake (Elaphe climacophora) and intrasplenic vascular architecture

The spleen of the blue-green snake (Elaphe climacophora) lies at the head of the pancreas and is separated from it by a fibrous capsule. A hilus is not clearly distinguished. Arteries and veins enter or leave the spleen through the capsule, but no collateral relationship is observed between these vessels. Histologically, the spleen consists of the capsule-septal tissue, lymphoid tissue (the white pulp), and a fibrous zone (the perilymphoid fibrous zone: PLFZ) around the lymphoid tissue that includes many small veins. The PLFZ probably represents the involuted red pulp. The border between the white pulp and PLFZ is unclear in routine histological sections due to diffuse infiltration of lymphocytes into the latter region, but the border could be distinguished clearly in silver-impregnated specimens. Arteries enter the spleen, run within the septa, and divide into terminal arteries in the lymphoid tissue that form end branches. There are no ellipsoids around the terminal arteries. The end branches communicate with veins of the PLFZ through transitional vessels within the lymphoid tissue (closed circulation). The veins of the PLFZ anastomose with drainage veins in the capsule. The snake spleen retains the basic histological characteristics of a spleen and is morphologically distinguishable from a lymph node. It may represent an extreme example of a spleen modified by the remodelling of the intrasplenic vasculatures during evolution. © 1995 Wiley-Liss, Inc.     

Porphyromonas gingivalis Stimulates Bone Resorption by Enhancing RANKL (Receptor Activator of NF-κB Ligand) through Activation of Toll-like Receptor 2 in Osteoblasts

Periodontitis has been associated with rheumatoid arthritis. In experimental arthritis, concomitant periodontitis caused by oral infection with Porphyromonas gingivalis enhances articular bone loss. The aim of this study was to investigate how lipopolysaccharide (LPS) from P. gingivalis stimulates bone resorption. The effects by LPS P. gingivalis and four other TLR2 ligands on bone resorption, osteoclast formation, and gene expression in wild type and Tlr2-deficient mice were assessed in ex vivo cultures of mouse parietal bones and in an in vivo model in which TLR2 agonists were injected subcutaneously over the skull bones. LPS P. gingivalis stimulated mineral release and matrix degradation in the parietal bone organ cultures by increasing differentiation and formation of mature osteoclasts, a response dependent on increased RANKL (receptor activator of NF-κB ligand). LPS P. gingivalis stimulated RANKL in parietal osteoblasts dependent on the presence of TLR2 and through a MyD88 and NF-κB-mediated mechanism. Similarly, the TLR2 agonists HKLM, FSL1, Pam2, and Pam3 stimulated RANKL in osteoblasts and parietal bone resorption. LPS P. gingivalis and Pam2 robustly enhanced osteoclast formation in periosteal/endosteal cell cultures by increasing RANKL. LPS P. gingivalis and Pam2 also up-regulated RANKL and osteoclastic genes in vivo, resulting in an increased number of periosteal osteoclasts and immense bone loss in wild type mice but not in Tlr2-deficient mice. These data demonstrate that LPS P. gingivalis stimulates periosteal osteoclast formation and bone resorption by stimulating RANKL in osteoblasts via TLR2. This effect might be important for periodontal bone loss and for the enhanced bone loss seen in rheumatoid arthritis patients with concomitant periodontal disease.