<P> However, it is important to note that pathologists do not consider endothelium and mesothelium (both derived from mesoderm) to be true epithelium . This is because such tissues present very different pathology . For that reason, pathologists label cancers in endothelium and mesothelium sarcomas, whereas true epithelial cancers are called carcinomas . Also, the filaments that support these mesoderm - derived tissues are very distinct . Outside of the field of pathology, it is, in general, accepted that the epithelium arises from all three germ layers . </P> <P> Epithelial tissues have as their primary functions: </P> <Ol> <Li> to protect the tissues that lie beneath from radiation, desiccation, toxins, invasion by pathogens, and physical trauma </Li> <Li> the regulation and exchange of chemicals between the underlying tissues and a body cavity </Li> <Li> the secretion of hormones into the blood vascular system, and / or the secretion of sweat, mucus, enzymes, and other products that are delivered by ducts </Li> <Li> to provide sensation </Li> </Ol> <Li> to protect the tissues that lie beneath from radiation, desiccation, toxins, invasion by pathogens, and physical trauma </Li>

Name four of the main functions of epithelia tissues
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