Discovery of a novel fibrous tissue in the spinal pia mater by polarized light microscopy

Abstract It is very well known that spinal meninges are composed of three layers, dura, arachnoid and pia mater, and that the main components of pia mater are collagen and reticular fibers. However, the distribution of those fibers has not been extensively investigated but just described as a mesh of fibers. In this study, we detected novel structures, which are composed of unidirectionally arranged fibers, in a rat spinal pia mater by using a polarized light microscope. They were seen as three parallel lines, one of which ran along a posterior spinal vein and the rest two of which ran along a pair of posterior spinal arteries. Histological analysis including Masson's trichrome, picrosirius-red staining, Gordon & Sweet's staining and immunohistochemistry with anti-collagen type 1 and 3 antibodies uncovered that they are mainly composed of collagen fibers and some reticular fibers. In addition, a putative primo vessel was detected in the novel fibrous tissue, which was proven out to be different from a blood vessel. In conclusion, we report a newly detected fibrous structure in the spinal pia mater, which may contribute to provide tensile force to the spinal meninges and to harbor the primo vascular system inside.