Studies in the 1970s and 1980s (e.g. [1,2]) have shown that Trichoplax adhaerens lacks both, a basal lamina and an extracellular matrix. No symmetry of any kind is seen in Trichoplax, and nothing like an oral–aboral or even a dorso-ventral polarity exists. The only polarity present results from the fact that the lower (nutritive) epithelium faces the substrate while the upper (protective) epithelium faces the open water. The unique bauplan is based on a simple, irregular sandwich organization. The two epithelia enclose a loose network of contractile so-called fiber cells (see Fig. 1a). All these simple bauplan characteristics, together with only five somatic cell types, make Trichoplax adhaerens more similar to protozoans than to any other metazoan. At present only one fiber cell type has been described, arranged in one layer. We have recently identified several sub-types of fiber cells that are arranged in at least three layers [3]. This discovery leads to a revised bauplan of the Placozoa (Fig. 1b, c). In our current research we focus on characterizing the bauplan using state-of-the-art ultrastructural analyzes (including high-pressure freezing methods).

legend: Fig. 1. Schematic cross section of placozoans. The traditional drawing is shown in [a]. Here one layer of fiber cells is sandwiched between the upper and the lower epithelium. We have recently shown, however, that fiber cells are arranged in three layers [b, c]. Two different mode of shiny sphere production are also  shown: Shiny spheres are produced either in the interspace and then transferred to the upper epithelium [b] or they are produced in the fiber cell layer directly underneath the upper epithelium [c]. From here the shiny spheres are directly integrated into the upper epithelium through extensions of the fiber cells. Please see Guidi et al., 2011 [3].

In addition to vegetative reproduction, placozoans reproduce bisexually, i.e. by producing female and male gametes [4-6]. However, sperm cells have not been unequivocally identified yet (but see Fig. 2). Oocytes appear in small numbers in cultured placozoans when triggered by environmental factors. Oocytes are comparatively huge (up to 120 µm in diameter). Beyond early cleavage stages up to 128 cells, no embryonic development has been observed (see Fig. 2; [4]). Field specimens have never shown signs of sexual reproduction, but genetic evidence suggests the presence of recent events of sexual reproduction in nature [7]. The Placozoa are currently the only model system for which the genome was published before the life cycle could be completed. We currently investigate sexual reproduction and embryonic development also in the field and optimize culture conditions to complete the placozoan life cycle in the laboratory.

legend: Fig. 2. Oogenesis and early embryonic development in the Placozoa using the Placozoa sp. H2 as an example. Typically, a single oocyte (o) with a large nucleus (n) grows in a large and healthy animal [a]. Fluorescent nuclei staining (DAPI) of the huge nucleus in the oocyte (arrow in [b]). After fertilization a protective eggshell (=fertilization membrane) surrounds the embryo, which starts cleavage (zygote and 8-cell stage in [c] and [d], respectively). DAPI staining reveals distinct signals for blastomers [e, f]. TEM images [g] reveal a three-layered ultrastructure of the fertilization membrane (fm), consisting of a ‘striped layer’ (sl), the ‘ground substance’ (gs) and the ‘dense ground substance’ (dgs). A putative sperm cell (sc) in an adult animal is shown in [h]. See Eitel et al. 2011 [4].

References:
1. Grell KG, Benwitz G (1971) Die Ultrastruktur von Trichoplax adhaerens F.E. Schulze. Cytobiologie 4: 216-240.
2. Grell KG, Ruthmann A (1991) Placozoa. In: Harrison FW, Westfall, J.A., editor. Microscopic Anatomy of Invertebrates, Placozoa, Porifera, Cnidaria, and Ctenophora. New York: Wiley-Liss. pp. 13-28.
3. Guidi L, Eitel M, Cesarini E, Schierwater B, Balsamo M (2011) Ultrastructural analyses support different morphological lineages in the Placozoa, Grell 1971. Journal of Morphology 272: 371-378.
4. Eitel M, Guidi L, Hadrys H, Balsamo M, Schierwater B (2011) New insights into placozoan sexual reproduction and development. PLoS One in press.
5. Grell KG (1984) Reproduction of Placozoa. In: Engels W, editor. Advances in Invertebrate Reproduction: Elsevier. pp. 541-546.
6. Grell KG, Benwitz G (1974) Elektronenmikroskopische Beobachtungen über das Wachstum der Eizelle und die Bildung der "Befruchtungsmembran" von Trichoplax adhaerens F.E.Schulze (Placozoa). Zeitschrift für Morphologie der Tiere 79: 295-310.
7. Signorovitch AY, Dellaporta SL, Buss LW (2005) Molecular signatures for sex in the Placozoa. Proceedings of the National Academy of Sciences USA 102: 15518-15522. 

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