Hidden cell diversity in Placozoa: ultrastructural insights from Hoilungia hongkongensis

• Published in: Cell and tissue research Vol. 385; no. 3; pp. 623 - 637
• Main Authors: Romanova, Daria Y., Varoqueaux, Frédérique, Daraspe, Jean, Nikitin, Mikhail A., Eitel, Michael, Fasshauer, Dirk, Moroz, Leonid L.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.09.2021; Springer; Springer Nature B.V
• Subjects: Animals; Biology; Biomedical and Life Sciences; Biomedicine; Epithelium; Evolution; Genomes; Geobiology; Human Genetics; Microscopy; Molecular Medicine; Morphology; Neurophysiology; Neurosciences; Peptides; Phylogeny; Placozoa - ultrastructure; Proteomics; Regular Article; Signaling; Physiology; Functional anatomy; Cell types; Morphology; Placozoa; Hoilungia hongkongensis; Trichoplax adhaerens
• ISSN: 0302-766X; 1432-0878
• DOI: 10.1007/s00441-021-03459-y

From a morphological point of view, placozoans are among the most simple free-living animals. This enigmatic phylum is critical for our understanding of the evolution of animals and their cell types. Their millimeter-sized, disc-like bodies consist of only three cell layers that are shaped by roughly seven major cell types. Placozoans lack muscle cells and neurons but are able to move using their ciliated lower surface and take up food in a highly coordinated manner. Intriguingly, the genome of Trichoplax adhaerens , the founding member of the enigmatic phylum, has disclosed a surprising level of genetic complexity. Moreover, recent molecular and functional investigations have uncovered a much larger, so-far hidden cell-type diversity. Here, we have extended the microanatomical characterization of a recently described placozoan species— Hoilungia   hongkongensis . In H. hongkongensis , we recognized the established canonical three-layered placozoan body plan but also came across several morphologically distinct and potentially novel cell types, among them novel gland cells and “shiny spheres”-bearing cells at the upper epithelium. Thus, the diversity of cell types in placozoans is indeed higher than anticipated.