Ruthmann, A., Behrendt G., Wahl, R. 1986. The ventral epithelium of Trichoplax adhaerens (Placozoa). Zoomorphology 106:115-122.

The basic anatomy of Placozoa is known, but not the structure and function of its cytoskeleton. Fixation in glutaraldehyde and osmium tetroxide does not properly preserve the outer layer of Trichoplax adhaerens. By using a new method, outer structures of these evolutionarily enigmatic animals may be seen more clearly.

The Placozoa are sister taxon to animals with true tissues, and therefore may show a primitive form of the 3-layered body plan. The outer layer is unique because it lacks a basal lamina, but otherwise closely resembles an epithelium. They are commonly considered an intermediate between sponges and higher animals with tissues. Based on functional-morphological evidence, there is only one species: Trichoplax adhaerens.

Ruthmann et al. stabilized whole Trichoplax using a new method, involving freeze-substitution, osmium tetroxide, and glutaraldehyde. When the samples were analyzed under an electron microscope, cilia in ciliary pits surrounded by dense rodlets and an apical belt desmosome that connects outer cells, were observed, instead of typical tight junctions. Large vesicles surrounded by small vesicles involved in receptor-mediated endocytosis after extracellular digestion were also noticed in ciliated cells, along with a bristly glycocalyx covering the cell's surface.

Ruthmann et al. conclude that ciliary movement is made possible by much of the terminal region of ciliary cells, the apical belt desmosome is conventionally organized, and the ciliated ventral surface appears to be important for endocytosis. This study uncovers many anatomical features and backs the significance of Placozoa as an evolutionary intermediate form.

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(posted Dec. 24 2015)