Rebecca E. Fisher

Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix

A recently improved understanding of gut immunity has merged with current thinking in biological and medical science, pointing to an apparent function of the mammalian cecal appendix as a safe‐house for symbiotic gut microbes, preserving the flora during times of gastrointestinal infection in societies without modern medicine. This function is potentially a selective force for the evolution and maintenance of the appendix, and provides an impetus for reassessment of the evolution of the appendix. A comparative anatomical approach reveals three apparent morphotypes of the cecal appendix, as well as appendix‐like structures in some species that lack a true cecal appendix. Cladistic analyses indicate that the appendix has evolved independently at least twice (at least once in diprotodont marsupials and at least once in Euarchontoglires), shows a highly significant (P < 0.0001) phylogenetic signal in its distribution, and has been maintained in mammalian evolution for 80 million years or longer.

A Histological Comparison of the Original and Regenerated Tail in the Green Anole, Anolis carolinensis

This study provides a histological comparison of the mature regenerated and original tail of the lizard Anolis carolinensis. These data will provide a framework for future studies of this emerging model organism whose genome was recently published. This study demonstrated that the cartilage skeleton of the regenerated tail enclosed a spinal cord with an ependymal core, but there was no evidence that dorsal root ganglia or peripheral nerves are regenerated. The cartilage tube contained foramina that allowed the vasculature to cross, but was otherwise a rigid structure. The original tail has muscle groups arranged in quadrants in a regular pattern that attach to the vertebral column. The regenerated tail has irregular muscle bundles of variable number that form unusual attachments to each other and to the cartilage tube. Furthermore, the data show that there was increased connective tissue within the muscle bundles. Implications for functionality of the regenerated tail and for future biomechanical studies are discussed. Anat Rec, 2012.

Transcriptomic Analysis of Tail Regeneration in the Lizard Anolis carolinensis Reveals Activation of Conserved Vertebrate Developmental and Repair Mechanisms

Lizards, which are amniote vertebrates like humans, are able to lose and regenerate a functional tail. Understanding the molecular basis of this process would advance regenerative approaches in amniotes, including humans. We have carried out the first transcriptomic analysis of tail regeneration in a lizard, the green anole Anolis carolinensis, which revealed 326 differentially expressed genes activating multiple developmental and repair mechanisms. Specifically, genes involved in wound response, hormonal regulation, musculoskeletal development, and the Wnt and MAPK/FGF pathways were differentially expressed along the regenerating tail axis. Furthermore, we identified 2 microRNA precursor families, 22 unclassified non-coding RNAs, and 3 novel protein-coding genes significantly enriched in the regenerating tail. However, high levels of progenitor/stem cell markers were not observed in any region of the regenerating tail. Furthermore, we observed multiple tissue-type specific clusters of proliferating cells along the regenerating tail, not localized to the tail tip. These findings predict a different mechanism of regeneration in the lizard than the blastema model described in the salamander and the zebrafish, which are anamniote vertebrates. Thus, lizard tail regrowth involves the activation of conserved developmental and wound response pathways, which are potential targets for regenerative medical therapies.

The phylogeny of the red panda (Ailurus fulgens): evidence from the forelimb.

Within the order Carnivora, the phylogeny of the red panda (Ailurus fulgens) is contentious, with morphological and molecular studies supporting a wide range of possible relationships, including close ties to procyonids, ursids, mustelids and mephitids. This study provides additional morphological data, including muscle maps, for the forelimb of Ailurus, based on the dissection of four cadavers from the National Zoological Park, Washington, DC, USA. The red panda forelimb is characterized by a number of primitive features, including the lack of m. rhomboideus profundus, a humeral insertion for m. cleidobrachialis, the presence of mm. brachioradialis, articularis humeri and coracobrachialis, a single muscle belly for m. extensor digitorum lateralis with tendons to digits III–V, four mm. lumbricales, and the presence of mm. flexor digitorum brevis manus, adductores digiti I, II and V, and abductor digiti I and V. Red pandas resemble Ailuropoda, mustelids and some procyonids in possessing a soft tissue origin of m. flexor digitorum superficialis. In addition, red pandas are similar to ursids and procyonids in having a variable presence of m. biceps brachii caput breve. Furthermore, Ailurus and some ursids lack m. rhomboideus capitis. The forelimb muscle maps from this study represent a valuable resource for analyzing the functional anatomy of fossil ailurids and some notes on the Miocene ailurid, Simocyon batalleri, are presented.

Evolving between land and water: key questions on the emergence and history of the Hippopotamidae (Hippopotamoidea, Cetancodonta, Cetartiodactyla)

The fossil record of the Hippopotamidae can shed light on three major issues in mammalian evolution. First, as the Hippopotamidae are the extant sister group of Cetacea, gaining a better understanding of the origin of the Hippopotamidae and of their Paleogene ancestors will be instrumental in clarifying phylogenetic relationships within Cetartiodactyla. Unfortunately, the data relevant to hippopotamid origins have generally been ignored in phylogenetic analyses of cetartiodactyls. In order to obtain better resolution, future analyses should consider hypotheses of hippopotamid Paleogene relationships. Notably, an emergence of the Hippopotamidae from within anthracotheriids has received growing support, leading to reconciliation between genetic and morphological evidence for the clade Cetancodonta (Hippopotamidae + Cetacea). Secondly, full account needs to be taken of the Hippopotamidae when studying the impact of environmental change on faunal evolution. This group of semi‐aquatic large herbivores has a clear and distinct ecological role and a diverse and abundant fossil record, particularly in the African Neogene. We examine three major phases of hippopotamid evolution, namely the sudden appearance of hippopotamines in the late Miocene (the “Hippopotamine Event”), the subsequent rampant endemism in African basins, and the Pleistocene expansion of Hippopotamus. Each may have been influenced by multiple factors, including: late Miocene grass expansion, African hydrographical network disruption, and a unique set of adaptations that allowed Hippopotamus to respond efficiently to early Pleistocene environmental change. Thirdly, the fossil record of the Hippopotamidae documents the independent emergence of adaptive character complexes in relation to semiaquatic habits and in response to insular isolation. The semiaquatic specializations of fossil hippopotamids are particularly useful in interpreting the functional morphology and ecology of other, extinct groups of large semiaquatic herbivores. Hippopotamids can also serve as models to elucidate the evolutionary dynamics of island mammals.

The primate appendix: a reassessment.

The presence of a vermiform appendix is often cited as a shared, derived character uniting the Hominoidea (apes and humans). However, appendix‐like structures have been reported for many other primate taxa. A review of the literature reveals that the confusion arises because several different, and sometimes contradictory, criteria are enlisted to distinguish an appendix. The measures most frequently used to define this structure are gross shape and certain aspects of histology (e.g., lymphoid concentration). Unfortunately, descriptions of shape lack quantification, and histological thin‐sections have not been studied for many primate taxa. In addition, although lymphoid concentration in the human appendix is known to vary considerably with age, this information is rarely reported in the primate literature. Given these complications, additional studies on the morphology and ontogeny of this region are warranted. This research will lead to a more accurate definition of the vermiform appendix. Most authors currently describe this feature as a narrow diverticulum of the cecum with thick walls and concentrated lymphoid tissue. However, the presence of thick mucosal layers and appreciable lymphoid tissue in taxa lacking appendices (e.g., Saguinus, Cercocebus) suggests that these features may be primitive primate traits. If so, wall thickness and lymphoid concentration cannot be used to define the vermiform appendix. These results suggest that a more rigorous definition of the appendix is requisite for this feature to be used in primate systematics. Anat Rec (New Anat) 261:228–236, 2000.

The Gross Anatomy of the Original and Regenerated Tail in the Green Anole (Anolis carolinensis)

This study investigates the gross anatomy of the original and the regenerated tail in the green anole (Anolis carolinensis). Dissections were conducted on 24 original and 13 regenerated tails. While the extrinsic muscles of the original tail in A. carolinensis are similar to those in other known Anolis lizard species, the extent of the origins of m. caudofemoralis longus and m. caudofemoralis brevis is more restricted. These differences may underlie variation in locomotor performance among anole ecomorphs. The intrinsic muscles of the original tail are also described, confirming previous findings and documenting new details, including muscle origins and insertions and the range of intraspecific variation. A comparison of the intrinsic muscles of the original tail and the regenerated tail muscles reveals key differences, such as the lack of interdigitating muscle segments and intramuscular septa in the regenerated tail. These findings, along with the replacement of interlocking vertebrae with a stiff, cartilaginous rod, suggest that important functional differences exist between the original and regenerated tail. In particular, the regenerated tail is predicted to be less capable of coordinated, fine movements. Studies of the physical properties and range of motion of the original and regenerated tail are required to test this hypothesis. This atlas of tail anatomy in A. carolinensis represents a key resource for developmental and genetic studies of tail regeneration in lizards, as well as studies of anole evolution and biomechanics. Anat Rec,, 2012.

The phylogeny of the red panda (Ailurus fulgens): evidence from the hindlimb: Red panda hindlimb, R. E. Fisher et al.

The red panda (Ailurus fulgens) is an endangered carnivore living in the temperate forests of the Himalayas and southern China. The phylogeny of the red panda has been the subject of much debate. Morphological and molecular studies have supported a wide range of possible relationships, including close ties to procyonids, ursids, mustelids, and mephitids. This study provides additional morphological data, including muscle maps, for Ailurus. The hindlimbs of four cadavers from the National Zoological Park were dissected. Red pandas retain a number of muscles lost in other carnivore groups, including muscles and tendons related to their robust and weight‐bearing hallux. Three features, including a single‐bellied m. sartorius, a proximal insertion for m. abductor digiti V, and an absent m. articularis coxae, are found in all terrestrial arctoids, including Ailurus. In addition, red pandas are similar to ursids and canids in lacking a caudal belly of m. semitendinosus, while they resemble procyonids and mustelids in the degree of fusion observed between mm. gluteus medius and piriformis. Furthermore, Ailurus and procyonids are characterized by numerous subdivisions within the adductor compartment, while red pandas and raccoons share a variable m. semimembranosus, composed of one, two, or three bellies. Lastly, a deep plantar muscle inserting onto the metatarsophalangeal joint of the hallux is described for Ailurus. This muscle has not been previously described and is given the name m. flexor hallucis profundus. Additional dissections of the forelimb and axial musculature of red pandas may shed further light on the phylogeny of this species. In addition, the muscle maps presented here offer a valuable resource for interpreting the functional anatomy of fossil ailurids.

Forelimb myology of the pygmy hippopotamus (Choeropsis liberiensis)

Based on morphological analyses, hippos have traditionally been classified as Suiformes, along with pigs and peccaries. However, molecular data indicate hippos and cetaceans are sister taxa (see review in Uhen, 2007 , this issue). This study analyzes soft tissue characters of the pygmy hippo forelimb to elucidate the functional anatomy and evolutionary relationships of hippos within Artiodactyla. Two specimens from the National Zoological Park in Washington, D.C. were dissected, revealing several adaptations to an aquatic lifestyle. However, these adaptations differ functionally from most aquatic mammals as hippos walk along river or lake bottoms, rather than swim. Several findings highlight a robust mechanism for propelling the trunk forward through the water. For example, mm. pectoralis superficialis and profundus demonstrate broad sites of origin, while the long flexor tendons serve each of the digits, reflecting the fact that all toes are weight‐bearing. Pygmy hippos also have eight mm. interossei and a well‐developed m. lumbricalis IV. Retention of intrinsic adductors functions to prevent splaying of the toes, an advantageous arrangement in an animal walking on muddy substrates. Published descriptions indicate common hippos share all of these features. Hippo and ruminant forelimbs share several traits; however, hippos are unique among artiodactyls in retaining several primitive muscles (e.g., mm. palmaris longus and flexor digitorum brevis). These findings are consistent with the hypothesis that hippos diverged from other Artiodactyla early in the history of this group. Additional analyses of hindlimb and axial muscles may help determine whether this trajectory was closely allied to that of Cetacea. Anat Rec, 290:673–693, 2007.

Scoliosis and segmentation defects of the vertebrae.

The vertebral column derives from somites, which are transient paired segments of mesoderm that surround the neural tube in the early embryo. Somites are formed by a genetic mechanism that is regulated by cyclical expression of genes in the Notch, Wnt, and fibroblast growth factor (FGF) signaling pathways. These oscillators together with signaling gradients within the presomitic mesoderm help to set somitic boundaries and rostral–caudal polarity that are essential for the precise patterning of the vertebral column. Disruption of this mechanism has been identified as the cause of severe segmentation defects of the vertebrae in humans. These segmentation defects are part of a spectrum of spinal disorders affecting the skeletal elements and musculature of the spine, resulting in curvatures such as scoliosis, kyphosis, and lordosis. While the etiology of most disorders with spinal curvatures is still unknown, genetic and developmental studies of somitogenesis and patterning of the axial skeleton and musculature are yielding insights into the causes of these diseases. WIREs Dev Biol 2012, 1:401–423. doi: 10.1002/wdev.34