139 K.-S. Soh et al. (eds.), The Primo Vascular System: Its Role in Cancer and Regeneration, DOI 10.1007/978-1-4614-0601-3_20, © Springer Science+Business Media, LLC 2012 Abstract Anatomical and histological studies on acupuncture points (acupoints) and meridians have been one of the most important approaches to revealing the mechanism of acupuncture analgesia and therapeutics. However, researchers have not yet reached a conclusion on the exact anatomical structure of the acupuncture meridians. In this chapter, the network of primo-vessels and nodes in rat hypoder- mis is reported. Fluorescence imaging showed the network of the primo-vascular system on the layer of superficial fascia (stratum fibrosum) that existed around the location of the stomach meridian from the knee to the middle of the tibia about 24 h after the subcutaneous injection of fluorescent nanoparticles at the acupoint ST36. Its fine morphologies were analyzed by using confocal laser scanning microscopy and transmission electron microscopy. The primo-vessel had a sinus (3–4 mm in diameter) surrounded by a layer of cells, and had a bundle-like collagen architecture (covered by a cellular layer) in which collagen fibers were regularly aligned along the longitudinal direction of the primo-vessel. Nanoparticles were also found in the cellular or extracellular layer. Since it seems evident that the primo-vascular system mainly exists in the fascia, a new perspective is needed to investigate the whole mechanism of the propagation of acupoint stimulation through the primo-vascular system in relation to fascia physiology. 1 Introduction Anatomical and histological studies on acupuncture points (acupoints) and meridi- ans have been one of the most important approaches to revealing the mechanism of acupuncture analgesia and therapeutics since the early era of scientific investigation B. Sung (*) Department of Physics and Astronomy, Biomedical Physics Laboratory, Seoul National University, Seoul 151-747, South Korea e-mail: baeckkyoung@gmail.com Chapter 20 Network of the Primo Vascular System in the Rat Hypodermis Byung-Cheon Lee, Zhendong Su, Baeckkyoung Sung, Ki Woo Kim, Jin-Myung Cha, Jin-Kyu Lee, Byung-Joon Chang, and Kwang-Sup Soh