GESTATION 



1011 



hihition of pelvic muscle tension inhibited 

 relaxation in the guinea pig. In a similar 

 type of experiment Crelin (1954) tied to- 

 gether the innominate bones of a mouse be- 

 fore pregnancy and obtained some dorso- 

 ventral displacement of the pubic symphysis 

 but normal relaxation was inhibited. 



H. DILATION OF THE UTERINE CERVIX 



Dilation or softening of the uterine cervix 

 in the pregnant woman at the time of labor 

 has been known for a long time. This reac- 

 tion has been used to determine whether 

 delivery can be anticipated. Within recent 

 years this phenomenon has been described 

 in a number of animals and some analysis 

 of the hormonal control of the reaction has 

 been attempted. 



Relaxation of the uterine cervix of the 

 rat during pregnancy was first reported by 

 de Vaal in 1946 and confirmed by Uyldert 

 and de Vaal in 1947. Relaxation was meas- 

 ured by the insertion of a gauging pin into 

 a cervix that had been removed and the 

 diameter determined at the point where re- 

 sistance is first felt. The measurements re- 

 vealed a marked rise from approximately 

 3.5 mm. on the 17th day of pregnancy to 10 

 mm. at parturition. Recently, both Hark- 

 ness and Harkness (1956) and Yochim and 

 ZaiTow (1959) have taken in vitro measure- 

 ments of the relaxation of the uterine cervix 

 of the rat and observed marked relaxation 

 during the latter part of gestation and at 

 parturition. Yochim and Zarrow (1959) re- 

 moved the cervix, suspended it from a rod 

 and measured the stretch due to weights 

 added at fixed intervals until the cervix 

 broke. The amount of relaxation of the 

 cervix was determined by the amount of 

 stretch obtained with a weight of 50 gm. 

 Under these conditions, the curve for re- 

 laxation of the cervix showed two sloi^es as 

 pregnancy progressed (Fig. 16.35). The ini- 

 tial slope between day 12 and day 20 showed 

 a rise of approximately 4 mm., with an ex- 

 tremely abrupt rise of 14 mm. on day 21. 

 By 24 hours after parturition the degree of 

 dilation had fallen to 3 mm. It is of interest 

 that the curve for the tensile strength of the 

 cervix (expressed in grams force necessary 

 to tear 1 mg. cervical tissue in a rat weigh- 

 ing 100 gm.) was the opposite to that seen 



for cervical dilation. The tensile strength 

 fell from approximately 50 gm. force to a 

 low of 3 gm. at parturition and then rose 

 during the postpartum period. The drop in 

 tensile strength preceded the changes in the 

 dilation of the cervix and was essentially 

 completed 5 to 6 days before parturition or 

 when the abrupt increase in dilatability of 

 the cervix occurred. 



Similar changes have been described in 

 the dilatability of the cervix of the mouse 

 (Steinetz, Beach and Kroc, 1957) with in- 

 creased dilatability progressed beyond the 

 15th day (Fig. 16.36). The diameter of the 

 cervix increased from a])proximately 2 mm. 

 to about 5 mm. at delivery. It is apparent 

 that the rate of the reaction, i.e., dilation, is 

 much more rapid in the rat, although it is 

 possible that the method of measurement is 

 responsible for the differences. 



The induction of cervical dilation by re- 

 laxin was reported by Graham and Dracy 

 (1953) in the cow, and by Zarrow, Sikes 

 and Neher (1954) in the sow and the heifer. 

 Treatment with stilbestrol followed by re- 

 laxin caused a dilation of the uterine cervix 

 of the gilt from % or % inch to 1 inch 

 (Zarrow, Neher, Sikes, Brennan and Bul- 

 lard, 1956). Measurements were made by 

 the passage of aluminum rods, and, al- 

 though the technique is not too exact, the 

 differences are significant. Stilbestrol given 

 alone or in combination with progesterone 

 had no effect on the cervical dilation. On 

 the other hand. Smith and Nalbandov 

 (1958) have recently reported that estrogen 

 treatment constricted the uterine cervix of 

 the sow and that relaxin was without effect. 

 A cue with respect to the mechanism of ac- 

 tion of relaxin is given by the similarity of 

 the action of relaxin on the pubic sym- 

 physeal ligament and the uterine cervix. In 

 both instances, an increase in water content 

 and a marked dei)olymerizatioii occurs. 



Cullen and Harkness ( 1958) observed re- 

 laxation of the uterine cervix of the rat with 

 estradiol alone, or with estradiol and pro- 

 gesterone, or with estradiol and relaxin, but 

 maximal dilation was obtained only with 

 a combination of estradiol, progesterone, 

 and relaxin. In general Kroc, Steinetz and 

 Beach (1959b) obtained comparable results 

 in the rat. Estrogen alone caused some in- 



