Crossword clue: the thin epithelium lining the blood vessels, lymphatics, and serous cavities. see epithelium.
1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).
Possible answers
an epithelium of mesoblastic origin; a thin layer of flattened cells that lines the inside of some body cavities