Cranial sensory neuron development in the absence of brain-derived neurotrophic factor in BDNF/Bax double null mice

To investigate the role of brain-derived neurotrophic factor (BDNF) in differentiation of cranial sensory neurons in vivo, we analyzed development of nodose (NG), petrosal (PG), and vestibular (VG) ganglion cells in genetically engineered mice carrying null mutations in the genes encoding BDNF and the proapoptotic Bcl-2 homolog Bax. In bax(-/-) mutants, ganglion cell numbers were increased significantly compared to wild-type animals, indicating that naturally occurring cell death in these ganglia is regulated by Bax signaling. Analysis of bdnf(-/-)bax(-/-) mutants revealed that, although the Bax null mutation completely rescued cell loss in the absence of BDNF, it did not rescue the lethality of the BDNF null phenotype. Moreover, despite rescue of BDNF-dependent neurons by the bax null mutation, sensory target innervation was abnormal in double null mutants. Vagal sensory innervation to baroreceptor regions of the cardiac outflow tract was completely absent, and the density of vestibular sensory innervation to the cristae organs was markedly decreased, compared to wild-type controls. Moreover, vestibular afferents failed to selectively innervate their hair cell targets within the cristae organs in the double mutants. These innervation failures occurred despite successful navigation of sensory fibers to the peripheral field, demonstrating that BDNF is required locally for afferent ingrowth into target tissues. In addition, the bax null mutation failed to rescue expression of the dopaminergic phenotype in a subset of NG and PG neurons. These data demonstrate that BDNF signaling is required not only to support survival of cranial sensory neurons, but also to regulate local growth of afferent fibers into target tissues and, in some cells, transmitter phenotypic expression is required.     

The primate subarcuate fossa and its relationship to the semicircular canals part II: adult interspecific variation

Studies have reported an empirical link between the size of the semicircular canals and locomotor agility across adult primates. In this paper, we investigate the possibility that this relationship does not follow from the function of the semicircular canals to sense head rotations, but rather reflects spatial constraints imposed by the subarcuate fossa. The latter sits among the three canals and contains the petrosal lobule of the cerebellar paraflocculus, a structure involved in neural processing of locomotion-related eye movements. Hence, it is feasible that agility-related variations of lobule and fossa size affect the arc size of the surrounding semicircular canals. The present study tests such hypothetical correlations by evaluating canal size, fossa size, and agility among extant adult primates. Phylogenetically informed multivariate regression analyses show that, after controlling for body mass, the size of the subarcuate fossa has a significant positive effect on the overall size of the anterior canal and the width of the posterior canal. Multivariate regressions involving the height of the posterior canal and overall size of the lateral canal are not significant. Further bivariate analyses confirm that fossa size is unlikely to play a role in the previously reported link between agility and the size of the posterior and lateral canals. However, fossa size, especially its opening though the arc of the anterior canal, cannot be excluded as a factor that influences the size of the anterior canal more than agility. The findings show that the most reliable functional signals pertaining to locomotion in species that possess a patent subarcuate fossa are likely to come from the lateral canal and are least likely to come from the anterior canal.     

The primate subarcuate fossa and its relationship to the semicircular canals part I: prenatal growth

Studies have reported a functional link between the arc size of the semicircular canals and locomotor agility across adult primates. However, canal size is spatially interlinked with the subarcuate fossa. This fossa can house the petrosal lobule of the paraflocculus, which also plays a role in coordinating head and eye movements. Consequently, it could be that it is the size of the petrosal lobule and fossa that are directly associated with locomotor agility, and not canal arc size. The apparent association of the latter would only follow from the spatial requirement of the canals to accommodate a suitably enlarged subarcuate fossa and petrosal lobule. This study aims to test the ontogenetic basis of this argument by examining high-resolution magnetic resonance images of fetal samples of Homo sapiens, Macaca nemestrina, and Alouatta caraya. Falsifiable null hypotheses examined are (1) that development of the subarcuate fossa is initiated by growth of the petrosal lobule, and (2) that growth of the semicircular canals and of the subarcuate fossa are independent. The findings confirm that the subarcuate fossa forms independently of a petrosal lobule in all three species, thereby falsifying the first hypothesis. Significant correlations were observed between size variables of the semicircular canals and the subarcuate fossa, particularly between the anterior canal and the opening of the fossa. These results falsify the hypothesis that the canals and fossa grow entirely independently. In the human sample, canal growth outpaces fossa growth, possibly because no petrosal lobule is present in humans. In the other two species, the subarcuate fossa simply seems to fill the space made available by canal growth. However, fossa enlargement cannot be excluded as an influence on size increase in the canals. Nevertheless, taken together, the results suggest that canal size is unlikely to be determined primarily by the spatial requirements of the subarcuate fossa and petrosal lobule, rather than by sensory demands reflected in the empirically established link with locomotor agility.     

Homoplasy in the ear region of Tethytheria and the systematic position of Embrithopoda (Mammalia,Afrotheria)

The ear region of mammals has long been considered as morphologically very conservative and accordingly, phylogenetically useful. In this study, the anatomy of the petrosal and bony labyrinth (osseous inner ear) of Numidotherium (Proboscidea) and Arsinoitherium (Embrithopoda) are investigated and compared in order to assess the evolution of ear region characters in proboscideans and embrithopods. Using a cladistic analysis across Paenungulata based on ear region characters only, we found that Arsinoitherium is surprisingly best placed as a crown proboscideans to the exclusion of Numidotherium and Phosphatherium, which results in the paraphyly of proboscidean. The clade Proboscidea is actually well supported by dental and post-cranial characters, and we propose that this result underlines the great amount of morphological convergences in the ear region of Embrithopoda and Proboscidea, possibly due to convergent evolution of capabilities toward infrasonic hearing.     

The Ear Region of the Pen-tailed Treeshrew, Ptilocercus lowii Gray, 1848 (Placentalia, Scandentia, Ptilocercidae)

The ear region of the pen-tailed treeshrew, Ptilocercus lowii Gray, 1848 (Scandentia, Ptilocercidae), is described and illustrated in detail based on five museum specimens from the National Museum of Natural History, two with the auditory bulla removed exposing the intratympanic surfaces. Soft tissues (arteries, veins, nerves, and muscles) are reconstructed onto the adult skulls based on published reports of these elements in a fetal P. lowii. Comparisons are made with four specimens of the common treeshrew, the tupaiid Tupaia glis (Diard, 1820), from the Carnegie Museum of Natural History, including one with the auditory bulla removed. The mammalian ear region widely is regarded to be a rich source of characters for phylogenetic analysis. This study supports this view by identifying numerous features that are shared between the two treeshrews as well as numerous features that distinguish them. Several features used in the past to distinguish tupaiid treeshrews from primates are found to differ between P. lowii and T. glis: the composition of the bony tubes for the internal carotid artery and the composition of the intrabullar septa and spaces. Despite the compositional differences, it seems likely that the bony carotid tubes and intrabullar septa and spaces shared by P. lowii and T. glis occurred in their common ancestor. Evaluating the utility of these and other ear region features awaits future phylogenetic analysis of treeshrews and related Euarchontoglires.     

The subarcuate fossa and cerebellum of extant primates: comparative study of a skull-brain interface

The subarcuate fossa of the petrosal bone houses the petrosal lobule of the cerebellar paraflocculus. Although the subarcuate fossa can be extensive, little is known about its relative size and distribution in primates. Studies indicate parafloccular involvement with cerebellar areas coordinating vestibular, visual, auditory, and locomotor systems. Hypotheses have proposed a role for the paraflocculus in vestibular-oculomotor integration, caudal muscle control, autonomic function, and visual-manual predation. This study examines the morphology and relative extent of the subarcuate fossa/petrosal lobule in a range of living primates. Methods include study of postmortem specimens representing nine mammalian orders, and qualification of the volume of the subarcuate fossa and endocranial cavity in 155 dry primate crania of 36 genera. Results show that, in mammals, the size and morphology of the petrosal lobule is directly related to that of the subarcuate fossa. Craniometric analysis shows that the ratio of subarcuate fossa volume to endocranial volume is largest in lemuriforms. The largest ratio is in Microcebus and Hapalemur. Lorisids show a significant reduction in the size of the subarcuate fossa to almost 50% below the lemuriform mean. Tarsius is near the lemuriform mean. Among platyrrhines, the ratio is high, but significantly reduced compared to lemuiforms. The highest platyrrhine ratio is seen in Ateles, the lowest in Saimiri and Alouatta. Atelids are significantly elevated compared to cebids. In cercopithecids, the fossa is significantly reduced compared to platyrrhines. The trend toward reduction of the cercopithecid fossa is most pronounced in Theropithecus and least evident in Presbytis. In hominoids, the fossa is present only in Hylobates. In great apes and humans, other than Gorilla, the petromastoid canal occupies a similar location to the subarcuate fossa of other primates, but is not homologous to it. Neither the subarcuate fossa nor the petromastoid canal are present in Gorilla. A graded reduction of the subarcuate fossa/petrosal lobule is evident among primates which evolved later in time. The relative size of this cerebellar lobule within primates may reflect size-related factors and/or degree of neocortical evolution as these relate to usage of a specific sensory-mediated locomotor behavior. The subarcuate fossa may serve as an indicator to the differentiation of the petrosal lobule of the paraflocculus in fossil forms.     

Morphologische Untersuchungen am Mittelohr der Marsupialial

Morphological studies on the auditory bulla of marsupials The resent study on the comparative anatomy of the marsupial auditory bulla starts with that of the didelphids; ontogenetic data are included. It becomes clear from these comparisons, that the marsupial bulla is throughout composed of the same skeletal elements. Contrary to widespread opinion, there exists no good evidence for the existence of an entotympanic in any marsupial. The tympanic process of the petrosal develops relatively late in ontogeny as ‘additional bone’ (“Zuwachsknochen” of Starck ); the same holds true for the tympanic process of the alisphe-noid. Biometric study of the tymanic ring in didelhids reveals that the tympanic membrane does only slightly increase with skull size (pronounced negative allometry). It is surprising, however, that the data of the tympanic diameter do arrane at two distinct levels of regression. Several taxa of didelphids must have jumped at this level independently. The possible functional meanings of these data are briefly discussed.