Bonghan ducts (BHDs), and their associated Bonghan corpuscles (BHCs), which are novel threadlike structures, were recently observed in rats and rabbits by using various methods. As further support for the putative circulatory function of the novel threadlike structures (NTS), we investigated the presence and the origin of the endothelial cells within these structures. We immunostained the NTS with anti-CD146, an endothelial cell marker, and with anti-podoplanin, a lymphatic cell marker. Positive expression of CD146 in the BHDs was obtained, and the distribution of endothelial cells showed that the inner boundaries of the channels in the subducts branched from the BHDs and curled around, in a complicated manner, inside a BHCs. The negative expression of podoplanin implies that the endothelial cells in the BHDs are likely to be of vascular and not of lymphatic origin.