What is "mesothelium"

Mesothelium \Mes`o*the"li*um\, n. [NL., fr. Gr. me`sos middle + E. epithelium.] (Biol.) Epithelial mesoderm; a layer of cuboidal epithelium cells, formed from a portion of the mesoderm during the differetiation of the germ layers. It constitutes the boundary of the c[oe]lum.

n. (context biology English) A membrane of flat cells that lines the body cavity of embryos and forms the squamous cells of the peritoneum, pericardium, and pleura

n. epithelium originating in the embryonic mesoderm; lines the primordial body cavity

The mesothelium is a membrane composed of simple squamous epithelial cells that forms the lining of several body cavities: the pleura (thoracic cavity), peritoneum (abdominal cavity including the mesentery), mediastinum and pericardium ( heart sac). Mesothelial tissue also surrounds the male internal reproductive organs (the tunica vaginalis testis) and covers the internal reproductive organs of women (the tunica serosa uteri). Mesothelium that covers the internal organs is called visceral mesothelium, while the layer that covers the body walls is called the parietal mesothelium. Mesothelium is the epithelial component of serosa.