Cervical collagen and biomechanical strength in non-pregnant women with a history of cervical insufficiency

Abstract

Background: It has been suggested that cervical insufficiency (CI) is characterized by a "muscular cervix" with low collagen and high smooth muscle concentrations also in the non-pregnant state. Therefore, the aim of this study was to investigate the biomechanical properties, collagen concentration, smooth muscle cell density, and collagen fiber orientation in cervical biopsies from non-pregnant women with a history of CI.

Methods: Cervical punch biopsies (2 x 15 mm) were obtained from 57 normal non-pregnant women and 22 women with a history of CI. Biomechanical tensile testing was performed, and collagen content was determined by hydroxyproline quantification. Histomorphometry was used to determine the volume densities of extracellular matrix and smooth muscle cells from the distal to the proximal part of each sample. Smooth muscle cells were identified using immunohistochemistry. Finally, collagen fiber orientation was investigated. Data are given as mean +/- SD.

Results: Collagen concentration was lower in the CI group (58.6 +/- 8.8%) compared with the control group (62.2 +/- 6.6%) (p = 0.033). However, when data were adjusted for age and parity, no difference in collagen concentration was found between the two groups. Maximum load of the specimens did not differ between the groups (p = 0.78). The tensile strength of cervical collagen, i.e. maximum load normalized per unit collagen (mg of collagen per mm of specimen length), was increased in the CI group compared with controls (p = 0.033). No differences in the volume density of extracellular matrix or smooth muscle cells were found between the two groups. Fibers not oriented in the plane of sectioning were increased in CI patients compared with controls.

Conclusions: Cervical insufficiency does not appear to be associated with a constitutionally low collagen concentration or collagen of inferior mechanical quality. Furthermore, the hypothesis that a "muscular cervix" with an abundance of smooth muscle cells contributes to the development of cervical insufficiency is not supported by the present study.

Figure 1

Collagen concentration in cases and controls. Collagen concentration (mg collagen/mg dry defatted weight (DDW) ×100) of cervical samples from women with a history of CI and controls (p = 0.033). After adjustment for age and parity by multiple linear regression, no statistically significant difference in collagen concentration was found between groups (p = 0.2).

Figure 2

F_max versus collagen content. Maximum load (F_max) of cervical samples in relation (linear regression) to collagen content mg/mm. Filled circle: Control (r = 0.76, p < 0.001). Open circle: CI (r = 0.76, p < 0.001) (Result on the control group has been submitted for publication 2009).

Figure 3

Normalized maximum load (S_max) of cases and controls. S_max, normalized maximum load, of cervical samples was increased in the CI group compared with controls (p = 0.033). Data were adjusted for age and parity by multiple linear regression (p < 0.001).

Figure 4

Collagen fiber orientation. Picro-Sirius stained sections from the human cervix. (A) Longitudinal section of a biopsy including epithelium (Bar: 500 μm); (B) collagen fibers in the center of the grid were divided into three categories based on their orientation (longitudinal axis of sections horizontal): 1) longitudinal or "parallel" fibers (deviating less than ± 45° from the longitudinal axis), 2) perpendicular fibers, (deviating between 46° and 90° or -46° to -90° from the longitudinal axis) representing circular or radial fibers, and 3) fibers shorter than 27 μm (not oriented parallel with the sectioning plane) representing circular, radial or wavy longitudinal fibers (arrows point at a longitudinal collagen fiber. Bar: 25 μm).