GESTATION 



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o o CERVICAL DILATABILITY 



(5-12 MICe/POINT) 



^^ - INTERPUBIC LIGAyCNT 



• • (5-12 MICE/POINT) 



□ D RESPONSE TO OXYTOCIN 



( 3-25 MICE/POIMT) 

 STANDARD ERROR OF 

 THE MEAN 



DIESTRUS ESTRUS IS 16 17 18 19 20 

 CYCLE DAYS PREGNANT 



1 

 DAYSP 



4 

 OSTPA 



RTUM 



PREGNANCY 



PARTURITION 



Fig. 16.36. Increased length of the pubic ligament, inciea.sed cervical dilatability, and 



increased responsiveness to oxytocin with the length of pregnanc.y in the mouse. L = 



lactating; NL = not lactating. (From B. G. Steinetz, V. L. Beach and R. L. Kroc, En- 

 docrinology, 61, 271, 1957.) 



XII. Uterine Myometrial Activity 



The classical and well known description 

 of uterine muscular activity has been more 

 than adequately reviewed by Reynolds 

 (1949). Since then Csapo and his colleagues 

 have reported a series of elegant experi- 

 ments involving the action of estrogen and 

 progesterone on the uterine myometrium 

 and have evolved the concept of "i)rogester- 

 one block" in the control of uterine activity 

 (1956a, 1956b). It has been shown that the 

 ovarian steroid hormones regulate myo- 

 metrial activity and that the uterine con- 

 tractions are dependent on the relative 

 amounts of the two hormones. Contractility 

 is dependent basically on the concentration 

 of the high energy phosphates which are 

 maintained by estrogen w^iich in turn is 

 involved in the synthesis of these substances 

 (Csapo, 1950; Menkes and Csapo, 1952). 

 Discovery of the staircase phenomenon in 

 the uterine myometrium similar to that ex- 

 hibited by cardiac muscle led to a marked 

 difference between the action of estrogen 

 and progesterone (Csapo and Corner, 1952 ) . 



With decreasing freciuency of electrical 

 stimidation in an isometric arrangement, 

 tension decreased if the uterus was domi- 

 nated by estrogen and increased if it was 

 dominated by progesterone. Uteri from cas- 

 trated rabbits were insensitive to the fre- 

 quency of electrical stimulation. Thus estro- 

 gen induced a "positive staircase" response 

 and progesterone a "negative staircase" re- 

 sponse, although in the latter instance some 

 estrogen is also present. These staircase re- 

 sponses have been used successfully as a 

 measure of hormone domination and have 

 been shown to be a function of the Na+ and 

 K+ gradients across the myometrial cell 

 membrane. 



Uterine motility during estrus, the di- 

 estrum, and pregnancy has been described 

 by many investigators in great detail (for 

 a review see Reynolds, 1949). The diestrous 

 uterus shows extremely slow, feeble, unco- 

 ordinated movements. The contractions may 

 arise in any part of the uterus and extend 

 in any direction. At estrus, the uterine con- 

 tractions become rhythmic and sweep over 



