title: Novel cell types, neurosecretory cells, and body plan of the early-diverging metazoan Trichoplax adhaerens.
abstract: BACKGROUND
  Trichoplax adhaerens is the best-known member of the phylum Placozoa, one of the earliest-diverging metazoan phyla. It is a small disk-shaped animal that glides on surfaces in warm oceans to feed on algae. Prior anatomical studies of Trichoplax revealed that it has a simple three-layered organization with four somatic cell types.
  
  
  RESULTS
  We reinvestigate the cellular organization of Trichoplax using advanced freezing and microscopy techniques to identify localize and count cells. Six somatic cell types are deployed in stereotyped positions. A thick ventral plate, comprising the majority of the cells, includes ciliated epithelial cells, newly identified lipophil cells packed with large lipid granules, and gland cells. Lipophils project deep into the interior, where they alternate with regularly spaced fiber cells whose branches contact all other cell types, including cells of the dorsal and ventral epithelium. Crystal cells, each containing a birefringent crystal, are arrayed around the rim. Gland cells express several proteins typical of neurosecretory cells, and a subset of them, around the rim, also expresses an FMRFamide-like neuropeptide.
  
  
  CONCLUSIONS
  Structural analysis of Trichoplax with significantly improved techniques provides an advance in understanding its cell types and their distributions. We find two previously undetected cell types, lipohil and crystal cells, and an organized body plan in which different cell types are arranged in distinct patterns. The composition of gland cells suggests that they are neurosecretory cells and could control locomotor and feeding behavior.
authors: National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892, USA. Electronic address: smithca@ninds.nih.gov.
authors: Carolyn L
authors: CL
authors: Smith
authors: Department of Molecular Neurobiology, Max Planck Institute of Experimental Medicine, Hermann-Rein-Strasse 3, 37075 Göttingen, Germany; Department of Fundamental Neurosciences, University of Lausanne, Rue du Bugnon 9, 1005 Lausanne, Switzerland.
authors: Frédérique
authors: F
authors: Varoqueaux
authors: Department of Molecular Neurobiology, Max Planck Institute of Experimental Medicine, Hermann-Rein-Strasse 3, 37075 Göttingen, Germany.
authors: Maike
authors: M
authors: Kittelmann
authors: National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892, USA.
authors: Rita N
authors: RN
authors: Azzam
authors: Department of Molecular Neurobiology, Max Planck Institute of Experimental Medicine, Hermann-Rein-Strasse 3, 37075 Göttingen, Germany.
authors: Benjamin
authors: B
authors: Cooper
authors: National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892, USA.
authors: Christine A
authors: CA
authors: Winters
authors: Ecology and Evolution, Institut für Tierökologie und Zellbiologie, TiHo Hannover, Buenteweg 17d, 30559 Hannover, Germany.
authors: Michael
authors: M
authors: Eitel
authors: Department of Fundamental Neurosciences, University of Lausanne, Rue du Bugnon 9, 1005 Lausanne, Switzerland. Electronic address: dirk.fasshauer@unil.ch.
authors: Dirk
authors: D
authors: Fasshauer
authors: National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892, USA.
authors: Thomas S
authors: TS
authors: Reese
mesh_terms: D000818:Animals; D003594:Cytoplasmic Granules / Q000378:metabolism*; D004847:Epithelial Cells / Q000145:classification / Q000378:metabolism*; D004848:Epithelium / Q000378:metabolism; D009474:Neurons / Q000145:classification / Q000378:metabolism*; D009489:Neurosecretion / Q000502:physiology*; D054909:Placozoa / Q000033:anatomy & histology* / Q000166:cytology*