GESTATION 



1005 



in the castrated mouse. One nig. progester- 

 one per day maintained pregnancy in 83 

 per cent of the mice castrated on day 14 of 

 gestation, but 0.5 mg. maintained preg- 

 nancy in only 30 per cent of the animals. 

 The addition of 1.5 ^g. estradiol per day 

 was without effect. On the other hand, the 

 addition of relaxin to the estradiol and 0.5 

 mg. progesterone gave pregnancy mainte- 

 nance in over 80 per cent of the mice as 

 compared with 30 per cent when progester- 

 one alone was given. 



Smithberg and Runner (1956) induced 

 ovulation and mating in prepubertal mice 

 (age 30 to 35 days) and obtained 100 per 

 cent implantation with 0.5 to 1 mg. proges- 

 terone daily and approximately 90 per cent 

 successful pregnancies when 2 mg. proges- 

 terone were given. A comparison of the 

 amount of progesterone required for main- 

 tenance of pregnancy in the normal and 

 castrated prepubertal mouse is given in Fig- 

 ure 16.34. In an interesting application of 

 the information available on the induction 

 of ovulation and maintenance of pregnancy, 

 Smithberg and Runner (1957) were able to 

 obtain successful pregnancies in genetically 

 sterile, obese mice. 



Haterius (1936) observed that distortion 

 of the fetus occurred following ovariectomy 

 in the rat. This has been confirmed by 

 Zeiner (1943) in the rat and by Courrier 

 and Colonge (1950) in the rat and rabbit. 

 It was noted that castration greatly com- 

 pressed the fetuses and eventually caused 

 death. Courrier and Colonge (1950) in very 

 elegant experiments showed that removal of 

 the rabbit fetus into the peritoneal cavity 

 prevented the distortion and death which 

 ordinarily followed castration. Frazer 

 (1955) obtained similar results in the rat 

 and concluded that fetal death after cas- 

 tration of the mother follows a rise in intra- 

 uterine pressure which is associated with an 

 increased tone of the circular uterine mus- 

 cle fibers. Consequently the increased sur- 

 vival of the extra-uterine fetuses following 

 ovariectomy in the mother is the result of 

 the removal of this pressure by the circular 

 muscle of the uterus. 



Many investigators have demonstrated 

 that gestation can be prolonged by inhibit- 

 ing parturition. Both the injection of large 

 doses of progesterone or the formation of 



0.25 0.5 1.0 2.0 



PROGESTERONE (mg) 



Fig. 16.34. Daily dose of progesterone required 

 to maintain pregnancy in the normal and cas- 

 trated prepubertal mouse. (From M. Smithberg 

 and M. N. Runner. J. Exper. Zool., 133, 441, 

 1956.) 



a new set of functional corpora lutea during 

 pregnancy will prevent parturition. The in- 

 jection of an ovulating dose of HCG on the 

 25th day of pregnancy in the rabbit delayed 

 parturition for 15 days after the injection, 

 i.e., until the 40th day of gestation (Snyder, 

 1934). The fetuses survived in utero for 

 only 3 days and grew to greater than nor- 

 mal size during this period. The placentas 

 persisted until day 41 of gestation. Com- 

 parable results were obtained following 

 daily injections of progesterone into preg- 

 nant rabbits (Zarrow, 1947a). Haterius 

 (1936) obtained prolongation of pregnancy 

 in the castrated rat by removing all the 

 fetuses except one, leaving all placentas 

 intact. Recently a comparable experiment 

 was performed in tlie rabbit with intact 

 ovaries (Hafez, Zarrow and Pincus, 1959). 

 In 2 of 10 rabbits, live fetuses were obtained 

 l)y cesarean section on day 36. However, in 

 8 of the 10, delivery was delayed beyond 

 day 36, although some degree of fetal re- 

 sorption was present in all instances. Pro- 

 longation of pregnancy in the rat was ob- 

 tained by the injection of prolactin (Meites 

 and Shelesnyak, 1957), but only if the ova- 

 ries were present. 



E. PITUITARY GLAND 



In general, hypophysectomy before mid- 

 pregnancy leads to resorption. This is es- 

 pecially true of the rat and mouse. On the 



