endothelioid Resembling endothelium or the layer of epithelial cells that lines the cavities of the heart and of the blood and lymph vessels and the serous cavities of the body, originating from the mesoderm. The mesoderm is the middle of the three germ layers or the layers of cells comprising the early embryo; which gives rise to the musculoskelet... Found on http://www.wordinfo.info/words/index/info/view_unit/729/