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1. A Comparison of the Cortical Structure of the Bowhead Whale ( Balaena mysticetus ), a Basal Mysticete, with Other Cetaceans

   https://doi.org/10.1002/ar.23991  

   Raghanti, Mary Ann ; Wicinski, Bridget ; Meierovich, Rachel ; Warda, Tahia ; Dickstein, Dara L. ; Reidenberg, Joy S. ; Tang, Cheuk Y. ; George, John C. ; Hans Thewissen, J. G. M. ; Butti, Camilla ; et al ( November 2018 , The Anatomical Record)

   Few studies exist of the bowhead whale brain and virtually nothing is known about its cortical cytoarchitecture or how it compares to other cetaceans. Bowhead whales are one of the least encephalized cetaceans and occupy a basal phylogenetic position among mysticetes. Therefore, the bowhead whale is an important specimen for understanding the evolutionary specializations of cetacean brains. Here, we present an overview of the structure and cytoarchitecture of the bowhead whale cerebral cortex gleaned from Nissl‐stained sections and magnetic resonance imaging (MRI) in comparison with other mysticetes and odontocetes. In general, the cytoarchitecture of cetacean cortex is consistent in displaying a thin cortex, a thick, prominent layer I, and absence of a granular layer IV. Cell density, composition, and width of layers III, V, and VI vary among cortical regions, and cetacean cortex is cell‐sparse relative to that of terrestrial mammals. Notably, all regions of the bowhead cortex possess high numbers of von Economo neurons and fork neurons, with the highest numbers observed at the apex of gyri. The bowhead whale is also distinctive in having a significantly reduced hippocampus that occupies a space below the corpus callosum within the lateral ventricle. Consistent with other balaenids, bowhead whales possess what appears to be a blunted temporal lobe, which is in contrast to the expansive temporal lobes that characterize most odontocetes. The present report demonstrates that many morphological and cytoarchitectural characteristics are conserved among cetaceans, while other features, such as a reduced temporal lobe, may characterize balaenids among mysticetes. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc. Anat Rec, 302:745–760, 2019. © 2018 Wiley Periodicals, Inc.

    

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2. DiceCT Analysis of the Extreme Gouging Adaptations Within the Masticatory Apparatus of the Aye‐Aye ( Daubentonia madagascariensis )

   https://doi.org/10.1002/ar.24303  

   Dickinson, Edwin ; Kolli, Shruti ; Schwenk, Alysa ; Davis, Cassidy E. ; Hartstone‐Rose, Adam ( November 2019 , The Anatomical Record)

   Relative to all other primates, the aye‐aye (Daubentonia madagascariensis) exists at the extremes of both morphology and behavior. Its specialized anatomy—which includes hypselodont incisors and highly derived manual digits—reflects a dietary niche, unique among primates, which combines tap‐foraging with gouging to locate and extract wood‐boring larvae. Here, we explore the impact of this extreme dietary ecology upon the masticatory musculature of this taxon with reference to a second, similarly sized but highly generalist lemuriform—the mongoose lemur (Eulemur mongoz). Using non‐destructive, high‐resolution diffusible iodine‐based contrast‐enhanced computed tomography techniques, we reconstruct the three‐dimensional volumes of eight masticatory muscles, and, for the first time in strepsirrhines, isolate and visualize their constituent muscle fasciclesin situand in three dimensions. Using these data, we report muscle volumes, forces, and fascicle lengths from each muscle portion, as well as their orientation relative to two standardized anatomical planes. Our findings demonstrate the overbuilt nature of the aye‐aye's masticatory apparatus, in which each muscle possesses an absolutely and relatively larger muscle volume and PCSA than its counterpart in the mongoose lemur. Likewise, for several adductor muscles, aye‐ayes also possess relatively greater fascicle lengths. Finally, we note several unusual features within the lateral pterygoid of the aye‐aye—the muscle most responsible for jaw protrusion—that relate to force maximization and reorientation. As this jaw motion is critical to gouging, we interpret these differences to reflect highly specific specializations that facilitate the aye‐aye's extreme subsistence strategy. Anat Rec, 2019. © 2019 American Association for Anatomy Anat Rec, 303:282–294, 2020. © 2019 American Association for Anatomy

    

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3. Vertical Clinging and Leaping Ahead: How Bamboo Has Shaped the Anatomy and Physiology of Hapalemur

   https://doi.org/10.1002/ar.24183  

   Hemingway, Holden W. ; Burrows, Anne M. ; Omstead, Kailey M. ; Zohdy, Sarah ; Pastor, Juan Francisco ; Muchlinski, Magdalena N. ( June 2019 , The Anatomical Record)

   Hapalemur sps. andProlemur simus(bamboo lemurs, collectively) stand out from the relatively homogeneous lemurids because they are bamboo feeders and vertical clingers and leapers. This unique diet presents equally unique challenges, like its verticality, toughness, and toxicity. The bamboo lemurs share the generalized anatomy of the other lemurids, but also display some well‐documented skeletal adaptations, perhaps to overcome the problems presented by their specialization. Soft‐tissue adaptations, however, remain largely unexplored. Explored here are possible soft‐tissue adaptations inHapalemur griseus. We compareH.griseuswith other lemurids,Propithecus,Galago,Tarsier, and a tree shrew. Based on the available anatomical and physiological data, we hypothesize thatHapalemurandProlemurspecies will have differences in hindlimb morphology when compared with other lemurids. We predict thatH.griseuswill have more hindlimb muscle mass and will amplify muscle mass differences with increased type II muscle fibers. Relative hindlimb muscle mass inH.griseusis less than other prosimians sampled, yet relative sural muscle mass is significantly heavier (P< 0.01) inH.griseus. Results show that the soleus muscle ofH.griseushas a higher amount of type II (fast) fibers in plantarflexors. These findings indicate althoughH.griseusshares some generalized lemurid morphology, its diet of bamboo may have pushed this generalized lemurid to an anatomical extreme. We suspect additional bamboo‐specific adaptations in their anatomy and physiology will be uncovered with further examination into the anatomy of the bamboo lemurs. Anat Rec, 2019. © 2019 Wiley Periodicals, Inc. Anat Rec, 303:295–307, 2020. © 2019 American Association for Anatomy

    

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4. Diversity of extracellular matrix morphology in vertebrate skeletal muscle

   https://doi.org/10.1002/jmor.21088  

   Sleboda, David A. ; Stover, Kristin K. ; Roberts, Thomas J. ( December 2019 , Journal of Morphology)

   Existing data suggest the extracellular matrix (ECM) of vertebrate skeletal muscle consists of several morphologically distinct layers: an endomysium, perimysium, and epimysium surrounding muscle fibers, fascicles, and whole muscles, respectively. These ECM layers are hypothesized to serve important functional roles within muscle, influencing passive mechanics, providing avenues for force transmission, and influencing dynamic shape changes during contraction. The morphology of the skeletal muscle ECM is well described in mammals and birds; however, ECM morphology in other vertebrate groups including amphibians, fish, and reptiles remains largely unexamined. It remains unclear whether a multilayered ECM is a common feature of vertebrate skeletal muscle, and whether functional roles attributed to the ECM should be considered in mechanical analyses of non‐mammalian and non‐avian muscle. To explore the prevalence of a multilayered ECM, we used a cell maceration and scanning electron microscopy technique to visualize the organization of ECM collagen in muscle from six vertebrates: bullfrogs (Lithobates catesbeianus), turkeys (Meleagris gallopavo), alligators (Alligator mississippiensis), cane toads (Rhinella marina), laboratory mice (Mus musculus), and carp (Cyprinus carpio). All muscles studied contained a collagen‐reinforced ECM with multiple morphologically distinct layers. An endomysium surrounding muscle fibers was apparent in all samples. A perimysium surrounding groups of muscle fibers was apparent in all but carp epaxial muscle; a muscle anatomically, functionally, and phylogenetically distinct from the others studied. An epimysium was apparent in all samples taken at the muscle periphery. These findings show that a multilayered ECM is a common feature of vertebrate muscle and suggest that a functionally relevant ECM should be considered in mechanical models of vertebrate muscle generally. It remains unclear whether cross‐species variations in ECM architecture are the result of phylogenetic, anatomical, or functional differences, but understanding the influence of such variation on muscle mechanics may prove a fruitful area for future research.

    

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5. Visualization and Quantification of Digitally Dissected Muscle Fascicles in the Masticatory Muscles of Callithrix jacchus Using Nondestructive DiceCT

   https://doi.org/10.1002/ar.24212  

   Dickinson, Edwin ; Basham, Colin ; Rana, Avadh ; Hartstone‐Rose, Adam ( July 2019 , The Anatomical Record)

   The organization and length of a muscle's fascicles imparts its contractile properties. Longer fascicles permit increased muscle excursion, whereas changes in fascicle orientation relate to the overall vector of contractile force. Collecting data on fascicle architecture has traditionally involved destructive and irreversible gross dissection. In recent years, however, new imaging modalities have permitted muscles and their fascicles to be visualized nondestructively. Here, we present data from a primate (Callithrix jacchus), in which, for the first time, individual muscle fascicles are digitally “dissected” (segmented and reconstructed) using nondestructive, high‐resolution diffusible iodine‐based contrast‐enhanced computed tomography (DiceCT) techniques. We also present quantitative data on the length and orientation of these fascicles within 10 muscle divisions of the jaw adductor and abductor musculature (superficial, deep, and zygomatic portions of temporalis and masseter; medial and lateral pterygoid; anterior and posterior digastric) and compare these digitally measured lengths to fascicular lengths measured using traditional gross and chemical dissection. Digitally derived fascicle lengths correspond well to their dissection‐derived counterparts. Moreover, our analyses of changes in fascicle orientation across the adductor complex enable us to visualize previously uncharacterized levels of detail and highlight significant variation between adjacent muscle layers within muscle groups (e.g., between superficial, deep, and zygomatic portions of masseter and temporalis). We conclude that this technique offers great potential to future research, particularly for questions centered around the visualization and quantification of obscured and often‐overlooked muscles such as the pterygoid and digastric muscles, and for deriving more accurate models of the masticatory system as a whole. Anat Rec, 302:1891–1900, 2019. © 2019 American Association for Anatomy

    

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