Primo-vessels have been observed in the rat abdominal cavity as floating thread like structures on and not adhering to fascia-wrapped internal organs. To date their presence, locations, and lengths have been irregular and unpredictable, and their identification not regularly repeatable, thus they have remained a nagging enigma in primo-vascular system research for several years. In this work, locations were found where primo-vessels were regularly present and observed repeatedly. These vessels were not floating or freely movable but lay in a regular position in the mesentery in the abdominal cavity of the rat, being observed between the cecum and small intestine and between the colon and mesentery root. The difference between a lymph vessel and a primo-vessel is described in anatomical and histological aspects. In addition, trypan blue was found to enter primo-vessels through the surrounding membranes and filled spaces between fibers comprising the primo-vessels. It is conjectured that the previously observed floating primo-vessels had anomalously and irregularly emerged, for some unknown physiological reasons, from primo-vessels normally located in the fascia-like mesentery.
Novel threadlike structures (NTSs) on the surfaces of mammalian abdominal organs have recently attracted interests regarding their ability to transport fluid, enable cell migration, and possibly facilitate cancer metastasis. Nevertheless, histological studies of NTSs have been sporadic and often have inconsistent interpretations of the NTS internal structure. In this article, we provide a synthetic and consistent view of the NTS internal structure: the NTS is a loose bundle of fibrous stroma that forms interstitial channels and microsinusoids infiltrated with inflammatory cells. The fibroblasts are embedded in the stroma and mostly aligned along the major axis of the NTS. The sinusoids, which are in inconsecutive cross sections, have boundaries more or less delineated by extracellular fibers, partly surrounded by endothelial-like cells, or both. We compare these morphological features to other well-known connective tissues (i.e., trabecular meshwork and lymphatic capillary) and discuss the biomechanical and biological functions of NTSs based on their structural characteristics.
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