Innervation patterns and locally produced signal substances in the human patellar tendon : of importance when understanding the processes of tendinosis

Abstract: Tendinosis is a condition of chronic pain that afflicts several human tendons, not least the patellar tendon, in which case it is often clinically referred to as ‘jumper’s knee’. The exact mechanisms behind tendinosis are yet not fully understood. One draw-back in the case of patellar tendinosis has been the lack of knowledge of the innervation patterns of the human patellar tendon. It cannot be excluded that the processes of tendinosis are influenced by nerve mediators, released from nerve endings or from stimulated cells inside the tendon.Thus, the studies of the present thesis aimed to 1) map the general, sensory, cholinergic and sympathetic innervation patterns of the human patellar tendon, in both the tendon tissue proper and the loose paratendinous connective tissue surrounding the tendon, and 2) investigate the possible existence of a production of signal substances, traditionally associated with neurons, in non-neuronal tendon cells, and to see if there are signs of local cholinergic and catecholaminergic signaling pathways. Biopsies of both normal pain-free patellar tendons and patellar tendons from patients with chronic painful tendinosis were collected and investigated. The main method utilized was immunohistochemistry, using antibodies directed against synthesizing enzymes for acetylcholine and catecholamines, against muscarinic and adrenergic receptors, and against markers of general and sensory innervation. In situ hybridization (ISH) to detect mRNA for the cholinergic/catecholaminergic synthesizing enzymes was also used.It was found that the loose paratendinous connective tissue of the patellar tendon was rather richly innervated by nerve structures. These consisted of large nerve fascicles, as well as perivascular innervation in the walls of some of the larger arteries and smaller blood vessels. It was found that part of the nerve structures corresponded to sensory afferents, and that some conformed to cholinergic and, especially, sympathetic nerve fibers. The tendon tissue proper was strikingly less innervated than the paratendinous tissue. The sparse innervation that was found in the tendon tissue proper was seen in narrow zones of loose connective tissue and blood vessels, interspersed between the collagen bundles. The overall impression was that the patterns of distribution of the general, sensory, and autonomic innervations of tendinosis tendon tissue were similar to those of normal tendon tissue proper.The most pioneering findings were the immunohistochemical observations of an expression of enzymes related to production of both acetylcholine and catecholamines within the tendon cells (tenocytes) themselves, as well as of a presence of the receptors for these substances on the same cells; features that were predominantly seen in tendinosis tendons. The observations of the synthesizing enzymes for acetylcholine and catecholamines in tenocytes were confirmed by ISH findings of mRNA for these enzymes in the tenocytes. Immunoreactions for muscarinic and adrenergic receptors were also found in blood vessel walls and in some of the nerve fascicles.In summary, this thesis presents novel information on the innervation patterns of the human patellar tendon, in healthy individuals with pain-free tendons as well as in patients with chronic painful tendinosis. Furthermore, it gives the first evidence of the presence of a local, non-neuronal production in the tendon tissue of signal substances normally seen in neurons, and a basis for these substances to affect the tenocytes as these cells also display muscarinic and adrenergic receptors. Thus, the results indicate an existence of autocrine and/or paracrine cholinergic/catecholaminergic systems in the tendon tissue; systems that seem to be up-regulated in tendinosis. This is of great interest as it is known that stimulation of receptors for both catecholamines and acetylcholine can lead to cell proliferation, interfere with pain sensation, influence collagen production, and take part in vasoregulation, as well as, in the case of adrenergic receptors, promote cell degeneration and apotosis. All these processes represent biological functions/events that are reported to be affected in tendinosis.In conclusion, despite the fact that there is very limited innervation within the patellar tendon tissue proper, it is here shown that effects of signal substances traditionally associated with neurons seem to occur in the tissue, via a local production of these substances in tenocytes.