Embryonic mesenchyme, mesenchymal tumors and mesenchymal stem cells: need for clarification of cell types and standardization of biomedical terminology

In vertebrate embryos, cellular compartments are formed with epithelial and mesenchymal arrangement of cells. During differentiation, cells can switch between these compartments several times (epithelial-mesenchymal transition; mesenchymal-epithelial transition). All germ layers produce epithelial and mesenchymal cells, so that the mesenchyme contains precursors for a large number of different cell types. Very unfortunately, practically all textbooks on anatomy and histology, and all online databases, refer to the early mesenchyme as 'embryonic connective tissue'. Also, there is constant confusion with the term mesoderm. This has obviously influenced the definition of the terms 'mesenchymal tumors' and 'mesenchymal stem cells' in such a way that the original meaning of mesenchyme is no longer recognizable. The detection of a bona fide mesenchymal stem cell in adult tissue does not appear to have been successful to date, and is not to be expected, as such a stem cell does not exist in the embryo. With the aim of more precise oncological diagnostics and therapy, the heterogeneity of the cells of the connective tissue and stromal compartments in particular must be better characterized. The identification of sustentacular cells, innate lymphoid cells and immunosuppressive stromal cells is an important step in this direction. Here we discuss the term mesenchyme and argue for a more precise characterization of cell types and standardization of terminology of cells in mesenchymal tissue assemblies.