Evolution of disc degeneration in lumbar spine: a comparative histological study between herniated and postmortem retrieved disc specimens

Abstract

This is a prospective comparative histological study on blood supply between lumbar herniated discs and postmortem retrieved ones. The aim of this study is to observe the evolution of disc degeneration in relation to its blood supply changes. Disc vascularization is present early in life, but the nucleus pulposus becomes avascular after adolescence. Vascularization of the annulus fibrosus (AF) probably also occurs late in life in association with degenerative changes and in response to trauma. Capillary neoformation and hypervascularity in degenerated discs have also been mentioned, based on animal cases. In the present study, intervertebral lumbar disc specimens were surgically removed from 84 patients with an average age of 41 years (range 24-60 years) operated on for disc herniation. In addition, control autopsy specimens were selected from 24 cadavera with an age of 39 years (range 24th gestation week to 80 years). The material was fixed in neutral buffered formalin, and 4-microm-thick sections were stained with hematoxylin-eosin and reticulin. In addition, with use of an immunohistochemical avidin-biotin complex technique, paraffin sections were stained for Ulex europaeus agglutinin receptors (UEA-1) after binding UEA-1 to the tissue. In surgical specimens, small blood vessels were identified in 45% of the disc cases. They were of the capillary-type vessels and were intermingled with proliferating endothelial cells, fibroblasts, and few mononuclear cells. All of them were located along the edges of the surgically retrieved fibrocartilage fragments. Sometimes thin bands of fibrin were attached to them and extravasated erythrocytes were occasionally seen. In autopsy specimens, blood vessels were identified in 78% of the retrieved discs. In contrast to the edge neovascularity observed in surgical specimens, capillaries were observed at the outer layer of AF surrounded by dense hyalinized and inactive-appearing collagen. From these results it is concluded that the blood vessels in extruded tissue from every type of herniation are newly formed, possibly through metaplasia of undifferentiated mesenchymal cells. It is also possible that they are derived from blood vessels that have invaded the AF as a result of disc degeneration. The finding of detection of progressive disc degeneration in both groups after 20-25 years seems to be of special interest because disc degeneration is a process that may not be directly correlated to disc herniation in these age groups.