A transplant system for studying hormonal control of growth of fetal rat tissues: effects of hypophysectomy growth hormone, prolactin, and thyroxine.

Much uncertainty still exists about whether mammalian fetuses are dependent on and/or responsive to hormones that are growth promoting postnatally. To allow this problem to be investigated more meaningfully, a transplant system was developed. Paws from 14- to 15-day-old fetal rats were incubated under the kidney capsules of 28- to 30-day-old syngeneic female hosts for up to 11 days. After an initial lag period, the growth rate of the transplants was similar to that of in situ paws. The implants differentiated normally, and their histological structure as well as various biochemical indices were similar to those of in situ paws. Growth of paws in hypophysectomized (HX) hosts was reduced to 35% of that of transplants in intact hosts, but the HX hosts themselves showed total cessation of growth. Injections of T4 or PRL into HX hosts did not improve transplant growth, but GH restored paw growth almost completely. The combination of GH plus T4 was no more effective than GH alone. Doses of GH (in HX hosts) that restored growth of the fetal tissue to 80-95% of that of paws in intact hosts caused only a 20-60% restoration of growth of the HX hosts themselves. Our results indicate that the internal milieu of juvenile female rats is as suitable for the growth of fetal paws as the normal intrauterine environment. Furthermore, GH is needed to maintain that internal milieu for optimal growth of fetal tissue. Although the tissues of the fetal paw are less dependent on GH than are those of the juvenile hosts, they appear to be more responsive to the hormone.