tissue mesenchyme mesenchyma portion of mesenchyme tissue portion of mesenchymal tissue Portion of tissue composed of mesenchymal cells (motile cells that develop from epthelia via an epithelial to mesenchymal transition) and surrounding extracellular material. Mesenchyme has different embryological origins in different metazoan taxa - in many invertebrates it derives in whole or part from ectoderm. In vertebrates it derives largely from mesoderm, and sometimes the terms are used interchangeably, e.g. lateral plate mesoderm/mesenchyme. mesenchyme tissue mesenchymal tissue