GESTATION 



1009 



a sex dimorphism exists in this species. The 

 pubic cartilages ossify in both sexes and 

 unite to form a complete pelvis with a rigid 

 symphysis pubis. At this stage, the pelvis is 

 too small for the passage of the young, but 

 with the first estrus in the female, the pubic 

 bones are gradually resorbed, leaving the 

 pelvis open ventrally. The pelvis in the male 

 remains intact (Hisaw, 1925). Treatment 

 with estrogen alone can readily bring about 

 the resorption of the pubic bones. 



A third type of adaptive mechanism has 

 been described in great detail in the guinea 

 pig and led to the discovery of the hormone, 

 relaxin. A sex dimorphism of the pelvis ex- 

 ists in the guinea pig, as in the pocket go- 

 pher, but in addition parturition is further 

 facilitated by marked relaxation of the 

 pubic ligaments and of the sacroiliac joint. 

 Thus far extensive pelvic relaxation has 

 been described in the guinea pig (Hisaw, 

 1926, 1929 », mouse (Gardner, 1936; Newton 

 and Lits, 1938; Hall and Newton, 1946a), 

 women (see review by Hisaw and Zarrow, 

 1951), and rhesus monkey (Straus, 1932; 

 Hartman and Straus, 1939). No relaxation 

 of the pubic symphysis has been reported in 

 the ewe but a relaxation of the sacroiliac 

 joint and an elongation of the sacrosciatic 

 ligament was noted the 2nd to 3rd month of 



gestation. These changes increased as preg- 

 nancy progressed (Bassett and Phillips, 

 1955). Treatment with stilbestrol alone 

 caused a marked loosening of the sacroiliac 

 joint and the sacrosciatic ligament. The ad- 

 dition of relaxin to the treatment was with- 

 out effect (Bassett and Phillips, 1954). 



The role of relaxin in the relaxation of 

 the pubic symphysis has been studied most 

 extensively in the guinea pig and mouse. 

 The work before 1950 was reviewed by Hi- 

 saw and Zarrow in 1951. The controversies 

 (de Fremery, Kober and Tausk, 1931 ; Ha- 

 terius and Fugo, 1939) as to whether such 

 a hormone exists need not be discussed here, 

 in detail, except to point out that the evi- 

 dence supporting this opinion is more than 

 adeciuate. Zarrow ( 1946, 1948) showed that 

 pubic relaxation could be induced by es- 

 tradiol alone, by a combination of estra- 

 diol and progesterone, or by relaxin in 

 an estrogen primed animal (Table 16.13). 

 The difference in the time required to in- 

 duce relaxation, i.e., 23 days for estrogen 

 alone, 13 days for estrogen and progesterone, 

 and 6 hours for relaxin, and data indicating 

 that progesterone caused the presence of 

 relaxin in the blood of guinea pig only if a 

 uterus was present led to the concept that 

 pubic relaxation may be produced independ- 



TABLE 16.13 



Relaxation of the symphysis pubis and relaxin content of blood, urine, and uteri of castrated and castrated, 



hysterectomized guinea pigs after treatment with moderate doses of estradiol and progesterone 



(From M. X. Zarrow, Endocrinology, 42, 129, 1948.) 



One guinea pig not included in the table refiuired 22 days of treatment for pubic relaxation. 



