en·do·the·li·um 
(ĕn′dō-thēlē-əm)
(ĕn′dō-thēlē-əm)n. pl. en·do·the·li·a (-lē-ə)
A thin layer of flat epithelial cells that lines serous cavities, lymph vessels, and blood vessels.
[New Latin endothēlium : ENDO- + Greek thēlē, nipple; see dhē(i)- in the Appendix of Indo-European roots.]
en′do·theli·al, en′do·theli·oid′ adj.