Figure 7.

Distribution of of enteric phagocytes and macrophages among animals. Simplistic phylogenetic tree is presented at upper left. Schematic cross sections of sponge (bottom center), cnidarian (bottom right) and coelomate bilaterian (top right) are grouped around the phylogenetic tree. Different cell types of endodermal digestive epithelium are represented in different shapes and colors, as explained in box at bottom right of figure. In sponges, motile macrophages and stationary phagocytes (choanocytes) may represent different stages in the same cell lineage; one stage is able to “transdifferentiate” into the other (Nakanishi et al. 2014; Sogabe et al. 2016). In cnidarians, the endodermal gastrodermis contains several differentiated cell types, among them enteric phagocytes. Motile macrophages (amoebocytes) with immune functions have been described (Menzel et al. 2015); it is possible that they are derived from the endoderm. In bilateria, mesodermal and endodermal cell lineages are separated early in development. Enteric phagocytes continue to develop from the endoderm; macrophages, along with other motile blood cells (hemocytes) are generated in specific hematopoietic centers within the mesoderm.