A chorionic villus that provides structural support to the villous tree. A chorionic stem villus is characterized by a condensed fibrous stroma in which fetal arteries, veins, arterioles, and venules are embedded. It represents a distributing framework for fetal chorionic circulation, supplying the floating villi. It can exhibit a diameter ranging from approximately 3000 μm (trunci near the chorionic plate) to 80 μm (smallest ramuli chorii). [ http://www.ncbi.nlm.nih.gov/pubmed/9260835 doi:10.1007/978-3-030-84725-8 ]