Abstract
Introduction
Tendon cells originate from yet poorly described precursor cells and develop in a particular “niche” close to vascular walls. Several factors have been described to determine this niche such as mechanical stimuli, oxygen tension, composition and structure of the extracellular matrix (ECM). Also, the vasculature is considered to play a crucial role for tendon cell development, yet evidence of how this is accomplished is lacking. In this study we therefore focussed on the endothelium of tendon vessels postulating the existence of a paracellular barrier.
Materials and Methods
By electron microscopy, immunohistochemistry, and RT-PCR we investigated the presence of constituents making up such an endothelial barrier which we subsequently tested for its functionality by tracer injection. Moreover, we performed differentiation experiments into the adipogenic, chondrogenic and osteogenic lineage on tendon derived cells in the presence and absence of serum. Expression levels and activity of matrixmetalloproteinases (MMPs) were assessed by western blot and zymography.
Results
Perfusion with defined tracer substances revealed that the blood-tendon-barrier impedes the passive transport of macromolecules (>10 kDa) from the blood stream to the surrounding tendon tissue, but is permeable to molecules <287D. The expression of barrier-related proteins, such as zonula occludens protein-1 (ZO-1), occludin, claudin-3, and claudin-5, in human and murine tendon vascular cells further corroborates the assumption that a restrictive tissue barrier acts at the blood-tendon interface. In vitro experiments indicate that serum enhances the number of adipocytes and increases the amount of calcium deposits as demonstrated by Oil Red O and Alizarin S staining, respectively. Also, serum appears to significantly raise the expression level of MMP2 and MMP9 known to remodel the ECM.
Discussion
Here, we postulate the existence of a blood-tendon barrier, which separates tendon tissue from the systemic circulation in intact tendons and, thus, impedes the paracellular passage of blood-borne molecules >10kD. In vitro studies investigating the influence of serum on tendon derived cells show that serum considerably modifies the differentiation potential and increases the expression of matrix remodelling enzymes.
The role of this structure in tendinopathy, tendon regeneration and tendon development remains to be elucidated.
