Abstract
Collagen fibrils, a major component of mitral valve leaflets, play an important role in defining shape and providing mechanical strength and flexibility. Histopathological studies show that collagen fibrils undergo dramatic changes in the course of myxomatous mitral valve disease in both dogs and humans. However, little is known about the detailed organization of collagen in this disease. This study was designed to analyze and compare collagen fibril organization in healthy and lesional areas of myxomatous mitral valves of dogs, using synchrotron small-angle x-ray diffraction. The orientation, density, and alignment of collagen fibrils were mapped across six different valves. The findings reveal a preferred collagen alignment in the main body of the leaflets between two commissures. Qualitative and quantitative analysis of the data showed significant differences between affected and lesion-free areas in terms of collagen content, fibril alignment, and total tissue volume. Regression analysis of the amount of collagen compared to the total tissue content at each point revealed a significant relationship between these two parameters in lesion-free but not in affected areas. This is the first time this technique has been used to map collagen fibrils in cardiac tissue; the findings have important applications to human cardiology.
| Original language | English |
|---|---|
| Pages (from-to) | 2472-2476 |
| Number of pages | 5 |
| Journal | Biophysical Journal |
| Volume | 93 |
| Issue number | 7 |
| DOIs | |
| Publication status | Published - 1 Oct 2007 |
Funding
We have shown that small-angle x-ray diffraction may be used to study the different characteristics of the axially projected structure of collagen fibrils in myxomatous mitral valve. We have applied this technique to determine the various attributes of collagen fibrils in the diseased canine mitral valves and here present detailed maps of collagen fibril orientation, collagen density, the degree of alignment of the fibrils, and the relative amount of tissue density. This allows us to understand better the spatial arrangements of collagen within diseased valves. Any changes that affect the quality, quantity, or organization of collagen could lead to the mechanical failure and dysfunction of mitral valves. The identification of collagen fibril orientation is one of the most striking findings of this study and shows that the mass of preferentially aligned collagen runs differently in different regions. In the main lesion-free body of the leaflets, collagen has a transverse course between the commissures. Near the free edge, it is almost a continuation of the collagen fibrils in the chordae tendineae. Because fiber orientation is the most effective way to optimize strength without increasing weight, fiber direction tends to reflect the prevalent tensile forces acting on a tissue (7) . Therefore, knowledge of pattern of fibril alignment would have important implications for mechanical, bioprosthetic, and tissue-engineered valves and may inform the design of mitral valve substitutes. In contrast, diseased areas showed no distinct preferred orientation, and the collagen fibrils were disorganized with a haphazard and irregular arrangement. This confirmed what was previously suspected from histopathological and transmission electron microscopy studies (8) . In other tissue types dynamic changes in collagen alignment can occur in response to continuous mechanical force, but the short time frame of valve leaflet movement is unlikely to allow collagen fibrils to rearrange in response to changes in force alone. In MMVD, which is an age-related disease, the alignment of fibrils declines, whereas in aged specimens of other tissues, collagen tends to have the best-defined patterns, which indicates a greater degree of regularity from increased cross-linking of the collagen molecules (9) . This suggests that the collagen changes that occur with MMVD are not simply an aging process. The quantity of collagen was also greatly reduced in the affected areas compared to nonaffected areas. Because the tensile strength of collagen fibrils is directly proportional to the mass of the fibrils, it can be assumed that the affected areas will be less capable of resisting the load and strain imposed by ventricular systole. The free edge of normal leaflets is the thickest part of the leaflet ≤ 0.01, = 120 points) revealed that there was significant relationship between these two parameters in lesion-free areas. However, this relationship did not hold true in affected areas, where depletion of collagen is not reflected in a parallel decline in total tissue content. One possible explanation is that MMVD involves increased production and deposition of glycosaminoglycans (GAGs). The different types of GAGs have diverse properties for absorbance and for destructive and constructive interference (10) , and in healthy valves, collagen content is related to tissue thickness. The plots of baseline scattering in this study showed notable variation across the leaflets, which did not correspond to diseased areas. Regression analysis of the amount of collagen compared to the total tissue content at each pixel ( p n (1) , and although some of them could produce a clear and ordered diffraction pattern, others would only contribute to tissue density. It is also believed that GAGs in the right proportions do play a role to maintain collagen fibrils in specific spatial order. Thus, the increasing amount of the GAGs would not only disturb that spatial order but, by reducing the physical space, might also encourage the process of collagen depletion. Furthermore, determining the exact types of concerned GAGs as another step forward in understating of the disease is suggested. An additional confounding factor is that the type of collagen present in connective tissues is linked to the type and quantity of related GAGs. Type III collagen tends to be associated with more GAGs, although there are conflicting reports on the changes in Type III collagen content in human myxomatous mitral valve disease (9,11) , and the exact makeup of collagen types in canine MMVD still needs to be determined. This work was supported by the CCLRC, the Cavalier King Charles Spaniel Club, and the Kennel Club. Mojtaba Hadian is sponsored by Iran Ministry of Science. The authors would like to thank Kalotina (Tina) Geraki for her technical support.
ASJC Scopus subject areas
- Biophysics
