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Placozoa is a phylum of non-bilaterian marine animals. These small, flat organisms adhere to the substrate via their densely ciliated ventral epithelium, which mediates mucociliary locomotion and nutrient uptake. They have only six morphological cell types, including one, fiber cells, for which functional data is lacking. Fiber cells are non-epithelial cells with multiple processes. We used electron and light microscopic approaches to unravel the roles of fiber cells in Trichoplax adhaerens, a representative member of the phylum. Three-dimensional reconstructions of serial sections of Trichoplax showed that each fiber cell is in contact with several other cells. Examination of fiber cells in thin sections and observations of live dissociated fiber cells demonstrated that they phagocytose cell debris and bacteria. In situ hybridization confirmed that fiber cells express genes involved in phagocytic activity. Fiber cells also are involved in wound healing as evidenced from microsurgery experiments. Based on these observations we conclude that fiber cells are multi-purpose macrophage-like cells. Macrophage-like cells have been described in Porifera, Ctenophora, and Cnidaria and are widespread among Bilateria, but our study is the first to show that Placozoa possesses this cell type. The phylogenetic distribution of macrophage-like cells suggests that they appeared early in metazoan evolution., (© 2021. The Author(s).)

Trichoplax adhaerens is an enigmatic animal with an extraordinarily simple morphology and a cellular organization, which are the focus of current research. Protocols outlined here provide detailed descriptions of advanced techniques for light and electron microscopic studies of Trichoplax. Studies using these techniques have enhanced our understanding of cell type diversity and function in placozoans and have provided insight into the evolution, development, and physiology of this little understood group.

Trichoplax adhaerens is a small, ciliated marine animal that glides on surfaces grazing upon algae, which it digests externally. It has no muscles or nervous system and only six cell types, all but two of which are embedded in its epithelium. The epithelial cells are joined by apical adherens junctions; neither tight junctions nor gap junctions are present. Monociliated epithelial cells on the lower surface propel gliding. The cilia beat regularly, but asynchronously, and transiently contact the substrate with each stroke. The animal moves in random directions in the absence of food. We show here that it exhibits chemotaxis, moving preferentially toward algae embedded in a disk of agar. We present a mathematical model to explain how coherent, directional movements could arise from the collective actions of a set of ciliated epithelial cells, each independently sensing and responding to a chemoattractant gradient. The model incorporates realistic values for viscoelastic properties of cells and produces coordinated movements and changes in body shape that resemble the actual movements of the animal. The model demonstrates that an animal can move coherently in search of food without any need for chemical signaling between cells and introduces a different approach to modeling behavior in primitive multicellular organisms., (Copyright © 2019 the Author(s). Published by PNAS.)

Trichoplax adhaerens has only six cell types. The function as well as the structure of crystal cells, the least numerous cell type, presented an enigma. Crystal cells are arrayed around the perimeter of the animal and each contains a birefringent crystal. Crystal cells resemble lithocytes in other animals so we looked for evidence they are gravity sensors. Confocal microscopy showed that their cup-shaped nuclei are oriented toward the edge of the animal, and that the crystal shifts downward under the influence of gravity. Some animals spontaneously lack crystal cells and these animals behaved differently upon being tilted vertically than animals with a typical number of crystal cells. EM revealed crystal cell contacts with fiber cells and epithelial cells but these contacts lacked features of synapses. EM spectroscopic analyses showed that crystals consist of the aragonite form of calcium carbonate. We thus provide behavioral evidence that Trichoplax are able to sense gravity, and that crystal cells are likely to be their gravity receptors. Moreover, because placozoans are thought to have evolved during Ediacaran or Cryogenian eras associated with aragonite seas, and their crystals are made of aragonite, they may have acquired gravity sensors during this early era.

Trichoplax adhaerens is a flat, millimeter-sized marine animal that adheres to surfaces and grazes on algae. Trichoplax displays a repertoire of different feeding behaviors despite the apparent absence of a true nervous system with electrical or chemical synapses. It glides along surfaces to find food, propelled by beating cilia on cells at its ventral surface, and pauses during feeding by arresting ciliary beating. We found that when endomorphin-like peptides are applied to an animal, ciliary beating is arrested, mimicking natural feeding pauses. Antibodies against these neuropeptides label cells that express the neurosecretory proteins and voltage-gated calcium channels implicated in regulated secretion. These cells are embedded in the ventral epithelium, where they comprise only 4% of the total, and are concentrated around the edge of the animal. Each bears a cilium likely to be chemosensory and used to detect algae. Trichoplax pausing during feeding or spontaneously in the absence of food often induce their neighbors to pause as well, even neighbors not in direct contact. Pausing behavior propagates from animal to animal across distances much greater than the signal that diffuses from just one animal, so we presume that the peptides secreted from one animal elicit secretion from nearby animals. Signal amplification by peptide-induced peptide secretion explains how a small number of sensory secretory cells lacking processes and synapses can evoke a wave of peptide secretion across the entire animal to globally arrest ciliary beating and allow pausing during feeding., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2017. Published by The Company of Biologists Ltd.)

Trichoplax adhaerens is the sole named member of Placozoa, an ancient metazoan phylum. This coin-shaped animal glides on ventral cilia to find and digest algae on the substrate. It has only six cell types, all but two of which are incorporated into the epithelium that encloses it. The upper epithelium is thin, composed of a pavement of relatively large polygonal disks, each bearing a cilium. The lower epithelium is thick and composed primarily of narrow ciliated cells that power locomotion. Interspersed among these cells are two different secretory cells: one containing large lipophilic granules that, when released, lyse algae under the animal; the other, less abundant, is replete with smaller secretory granules containing neuropeptides. All cells within both epithelia are joined by adherens junctions that are stabilized by apical actin networks. Cells are held in place during shape changes or under osmotic stress, but dissociate in low calcium. Neither tight, septate, nor gap junctions are evident, leaving only the adherens junction to control the permeability of the epithelium. Small (<4 kDa) fluorescent dextrans introduced into artificial seawater readily penetrate into the animal between the cells. Larger dextrans enter slowly, except in animals treated with reduced calcium, indicating that the adherens junctions form a circumferential belt around each cell that impedes diffusion into the animal. During feeding, the limited permeability of the adherens junctions helps to confine material released from lysed algae within the narrow space under the animal, where it is absorbed by endocytosis.

Trichoplax is a small disk-shaped marine metazoan that adheres to substrates and locomotes by ciliary gliding. Despite having only six cell types and lacking synapses Trichoplax coordinates a complex sequence of behaviors culminating in external digestion of algae. We combine live cell imaging with electron microscopy to show how this is accomplished. When Trichoplax glides over a patch of algae, its cilia stop beating so it ceases moving. A subset of one of the cell types, lipophils, simultaneously secretes granules whose content rapidly lyses algae. This secretion is accurately targeted, as only lipophils located near algae release granules. The animal pauses while the algal content is ingested, and then resumes gliding. Global control of gliding is coordinated with precise local control of lipophil secretion suggesting the presence of mechanisms for cellular communication and integration.

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., (Copyright © 2014 Elsevier Ltd. All rights reserved.)