Douglas Harper's Etymology Dictionary
histo-
medical word-forming element, from Greek histos "warp, web," literally "anything set upright," from histasthai "to stand," from PIE *sta- "to stand" (see stet). Taken by 19c. medical writers as the best Greek root from which to form terminology for "tissue."
Wiktionary
histo-
pre. (context biology English) Combining form with the meaning of ''tissue''.