The effects of oestradiol benzoate, progesterone, relaxin and ovariectomy on cervical extensibility in the late pregnant rat

Changes in the tensile properties and fine structure of the rat cervix in late pregnancy and during parturition

“…However, it is possible to produce only very small dilatations of the cervix of the non-pregnant animal before fracture occurs (De Vaal, 1946;Harkness, 1964;Hollingsworth, Isherwood & Foster, 1979). The cervical wet weight increases 3-fold during pregnancy (Harkness, 1964;Hollingsworth et al, 1979) but this increase in size alone would appear to be insufficient to account for the marked change in ability to be dilated at term and implies an alteration in the tensile properties or softening.…”

“…However, it is possible to produce only very small dilatations of the cervix of the non-pregnant animal before fracture occurs (De Vaal, 1946;Harkness, 1964;Hollingsworth, Isherwood & Foster, 1979). The cervical wet weight increases 3-fold during pregnancy (Harkness, 1964;Hollingsworth et al, 1979) but this increase in size alone would appear to be insufficient to account for the marked change in ability to be dilated at term and implies an alteration in the tensile properties or softening. Harkness (1964) and Hollingsworth et al (1979) observed that the rat cervix in vitro exhibits significant creep when a load is applied and that the creep rate (previously called extensibility) was small for the cervix of the non-pregnant rat and increased progressively between Day 16 and 22 of pregnancy.…”

“…The cervical wet weight increases 3-fold during pregnancy (Harkness, 1964;Hollingsworth et al, 1979) but this increase in size alone would appear to be insufficient to account for the marked change in ability to be dilated at term and implies an alteration in the tensile properties or softening. Harkness (1964) and Hollingsworth et al (1979) observed that the rat cervix in vitro exhibits significant creep when a load is applied and that the creep rate (previously called extensibility) was small for the cervix of the non-pregnant rat and increased progressively between Day 16 and 22 of pregnancy. We have extended our previous comparisons of cervices from non-pregnant and early Day-22 pregnant rats by using further tensile tests, determination of collagen content and electron microscopy.…”

“…Any progressive increase in creep during the short parturient period would have contributed to this variation. Parturition in this strain of rats starts between 12:00 h on Day 22 and 16:00 h on Day 23 (Whalley & Riley, 1977 (Hollingsworth, 1981;Hollingsworth, Gallimore & Williams, 1980 (Hollingsworth et al, 1979). The time (Tc) for the inner circumference of the cervix after load application to reach the mean fetal head circumference at term (43-6 mm) was calculated from a computer-fitted curve (Hollingsworth et al, 1979 (after Hollingsworth, 1981).…”

“…Parturition in this strain of rats starts between 12:00 h on Day 22 and 16:00 h on Day 23 (Whalley & Riley, 1977 (Hollingsworth, 1981;Hollingsworth, Gallimore & Williams, 1980 (Hollingsworth et al, 1979). The time (Tc) for the inner circumference of the cervix after load application to reach the mean fetal head circumference at term (43-6 mm) was calculated from a computer-fitted curve (Hollingsworth et al, 1979 (after Hollingsworth, 1981). A modified DC/25 mm linear variable differential transducer (Sangamo transducers, Bognor Regis, Sussex; t) was incorporated with one end of the transducer armature connected to the stirrup (s) on which the cervix (c) was placed and to the other end of the armature a thread was tied which went over the wheel (w) to the balance pan (b).…”