The internal milieu of the pregnant rat is not growth promoting for maternal tissues and it reduces growth of fetal paw transplants.

We have investigated whether the internal milieu of the pregnant female rat is growth promoting to fetal or adult tissues. Adult female littermates were divided into two groups, one of which remained virginal while the other was bred repeatedly. Four successive pregnancies did not affect skeletal growth (i.e. tail length increase) or BW gain. Three-month-old females were hypophysectomized (HX) or sham HX on day 13 of pregnancy. Age-matched virgins were treated similarly. Eight days after surgery, the width of the tibial epiphysial plate of the HX virgins had decreased from 146.2 +/- 5.6 mu to 98.4 +/- 2.5 mu. In the pregnant rats, the width of the plate in the shams was reduced to 120.8 +/- 7.6 mu, and hypophysectomy tended to further reduce its width to 103.2 +/- 8.4 mu. Paws from 14- to 15-day fetal rats were transplanted under the kidney capsule of adult nonpregnant or pregnant hosts and grown there for 6 or 11 days. In the nonpregnant hosts the paws grew substantially during both periods, but in gravid females their growth was inhibited by about 35%. Paws were also transplanted to the kidney of HX pregnant and HX nonpregnant females to determine whether the internal milieu of the gravid female retains more growth-promoting/growth-supporting activity after pituitary removal than does that of HX virgin hosts. In both types of HX hosts, transplant growth was inhibited to the same degree (65-70%) relative to growth in intact hosts. The growth-inhibiting properties of the internal environment of the gravid hosts were not due to ovarian steroids because injections of estradiol plus progesterone had no effect on transplant growth. We also tested the effects of placental transplants and placental extracts on the growth of transplanted paws in juvenile female hosts. Both placental treatments caused inhibition of transplant growth by 25-30%. Overall, our results indicate that the internal milieu of the pregnant rat is not somatotrophic for her tissues, and it actually suppresses growth of fetal tissue transplants. The placenta may be responsible for this growth-inhibitory effect.