Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix

Authors

  • H. F. SMITH,

    1. Department of Basic Medical Sciences, University of Arizona College of Medicine-Phoenix, in Partnership with Arizona State University, Phoenix, AZ, USA
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    • Present address: Department of Anatomy, Arizona College of Osteopathic Medicine, Midwestern University, Glendale, AZ, USA.

  • R. E. FISHER,

    1. Department of Basic Medical Sciences, University of Arizona College of Medicine-Phoenix, in Partnership with Arizona State University, Phoenix, AZ, USA
    2. School of Life Sciences, Arizona State University, Tempe, AZ, USA
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  • M. L. EVERETT,

    1. Department of Surgery, Duke University Medical Center, Durham, NC, USA
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  • A. D. THOMAS,

    1. Department of Surgery, Duke University Medical Center, Durham, NC, USA
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  • R. RANDAL BOLLINGER,

    1. Department of Surgery, Duke University Medical Center, Durham, NC, USA
    2. Department of Immunology, Duke University, Durham, NC, USA
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  • W. PARKER

    1. Department of Surgery, Duke University Medical Center, Durham, NC, USA
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William Parker, Department of Surgery, Duke University Medical Center, Box 2605, Durham, NC 27710, USA.
Tel.: 919 681 3886; fax: 919 681 7263; e-mail: bparker@duke.edu

Abstract

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.

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