CONNECTIVE TISSUE CHANGES IN PROLAPSED TISSUE

1.5.6.1 OVERVIEW

There is a paucity of information detailing exact connective tissue that may lead to prolapse. In many aspects where knowledge exists some contradictory evidence is also present. No consensus has been reached regarding the exact role of all connective tissue components and sex steroid hormones in the aetiology of the disorder. Unraveling this may be the key to understanding the complex interplay between known risk factors and more certain triggers of the disease. Despite this observed lack of consistency in findings regarding pelvic connective tissue cells, certain aspects such as ageing of connective tissue structural proteins, repair after damage, ECM remodeling and hormonal effects show remarkable trends. This lights the path for new research linking connective tissue ageing, remodeling and development of prolapse.

Conditions affecting the integrity and elasticity of the connective tissue also influence the development of the disease. The increased occurrence of prolapse in patients with connective tissue disorders such as Ehlers Danlos syndrome and cardiomyopathy has been observed (6). In cases where prolapse has been noted in nulliparous individuals, the presence of a genetic connective tissue defect has been suggested (8).

1.5.6.2 COLLAGEN

Pelvic supporting ligaments from women with prolapse have shown decreased amounts of total collagen (23) but collagen Type III, which is associated with reduced fiber size and low mechanical strength of the connective tissue is generally increased in prolapsed vaginal tissues (6). Type III collagen is present in the skin of babies at birth whereas mature skin has collage

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type I (11,25). Its presence at early stages of development suggests that it is relevant in enabling connective tissues to expand to accommodate growth. Its increase in prolapsed tissues is a pointer to prolapse being a response to increased vaginal tissue loading. Findings on the levels of collagen I and collagen I/III ratios in prolapse tissues have been inconclusive (8,23,107).

Studies on collagen I mRNA in vaginal fascia and immunofluorescence of collagen in full thickness vagina apex tissue revealed no significant change between controls and test subjects (108,109). Another study comparing the total collagen content of the vaginal apex and parametrium of women with and without prolapse revealed a reduction of collagen in the parametrium where prolapse was present but no significant differences in the vagina apex (110). Others carried out on the arcus tendinous fascia and periurethral fascias showed decreased collagen type I (111,112). Differences observed could be the result of use of varied tissue types and different analysis techniques. It is however noteworthy that similar observations of low levels of collagen I have been noted in vagina tissue where disease is present (8).

1.5.6.3 MATRIX REMODELING ENZYMES

An increase in matrix metalloproteinase (MMP)-2 and MMP-9 has been consistently noted in prolapse tissues (23,108,113). Despite the use of different analytical methods, research suggests higher activity of these fibrillar collagens‘ degrading enzymes in various regions of the vaginal tissue and uterosacral ligaments. A congruent decrease in associated TIMPs activity has also been noted (45). Increase in tenascin, a marker of wound healing (113) and the aforementioned markers of collagen degradation suggests that prolapse reflects an on-

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going remodeling or tissue healing process with loss of homeostasis of this remodeling process. Overall, studies on amounts of collagen subtypes and regulatory enzymes suggest a dysfunctional regulation of collagen that results in its qualitative decline and quantitative imbalance in prolapsed tissues.

1.5.6.4 FIBROBLASTS

Fibroblasts are the predominant cells of the vagina connective tissue (3). These slow growing cells have been shown to change morphology and biochemistry in response to applied force. They are mechanosensitive (114). They undergo mechanotransduction like other mechanosensitive cells in load bearing tissues of the body (96). They are also responsible for the production of matrix remodeling enzymes (115). The mechanical function of fibroblasts can be hindered by abnormal underlying matrix architecture and in disease states they may produce abnormal ECM. For example, fibroblasts in prolapsed tissues were also noted to produce stiffer connective tissue matrix (116). Ageing also has an impact on vaginal tissue fibroblasts. It has been revealed that the fibroblasts show a numerical decline and reduced contractile ability (109) and reduced MMP-2 production (115) with age. The ageing sfibroblasts showed decreased wound healing abilities as well as a lack of ability to maintain tissue architecture.

Oestrogen can also influence vaginal fibroblasts behaviour and modulate its biochemical and mechanical activities. Repeated stretching of the fibroblasts correlates with production of matrix degrading enzymes (96). This effect can be modulated by the reproductive hormones, such as oestrogens via pro-trophic mechanisms (117). Oestrogen may increase cellularity but has no overall effect on maintaining tissue structure (8). Studies reveal the role of oestrogen in

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stimulating collagen turnover stress urinary incontinence (38,41), and this process can influence pelvic tissue ageing and remodeling. ECM genes associated with breakdown and resynthesis are altered during the female reproductive cycle and menopause (46). The pelvic tissues also change mechanically in response to cyclical changes in the hormones as well as during menopause (118). Further suggestive of the influence of ovarian hormones such as oestrogen on vaginal fibroblasts is the observation that the vaginal wall, like other pelvic tissues, undergoes remodeling during pregnancy (119). Although, there are few studies on fibroblasts in pregnancy, it is known that there is a control of the vaginal wall cell numbers by a poorly understood homeostatic balance between apoptosis and cellular proliferation. The apoptotic process is noted to decline in the second trimester (120). This is suggestive of increasing cell numbers and potentially increased vaginal tissue remodeling in pregnancy (120).

1.5.6.5 NON ENZYMATIC CROSSLINKING OF COLLAGEN

The most consistent connective tissue findings are the development of new crosslinks in prolapsed tissues (8). This is in keeping with the early hypothesis by Jackson suggesting prolapse to be the result of increasing trivalent non-enzymatic crosslinks gradually dominating slow turn-over proteins ultimately leading to an older, stiffer connective tissue (23). Notably, he found higher amounts of an AGE, pentosidine, in the tissues. Since then a few other studies have reiterated this finding, showing no change in divalent, enzymatic pyridinoline crosslinks but an increase in advanced glycation of pelvic connective tissue collagen (8,45). This, as well as the presence of disorganized collagen structure in prolapsed tissues correlates with the increasing incidence of the disease observed with increasing age.

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