Abstract
Summary
Sprague-Dawley-Rolfsmeyer rats were hypophysectomized on days 11, 12 or 15 of pregnancy and sacrificed on day 20 to determine the extent of mammary development, as assessed by determination of nucleic acid content. The DNA of six abdominalinguinal glands in the hypophysectomized groups was not significantly different from that in the sham-operated pregnant or intact pregnant control groups. All groups maintaining pregnancy had significantly higher DNA contents in mammary glands than virgin control or hypophysectomized-aborted groups. In order to determine the minimal numbers of placental-fetal units required to maintain pregnancy and mammary gland growth, fetuses and placentas were removed on day 12 of pregnancy in addition to the pituitary so that only one fetus and one placenta remained in the uterus of a group of 6 rats with other groups having 2, 3, 4 or 5 remaining. Pregnancy was maintained with only one placental-fetal unit, but mammary gland proliferation was significantly lower than the control group on day 20 of pregnancy. Three to five concep-tuses supported mammary proliferation during the latter half of pregnancy at a level not significantly different from intact or sham-operated control groups. Removal of pla-cental units on day 12 in rats having pitui-taries intact resulted in no mammary DNA change when 1-5 units remained. Removal of pituitaries on day 12 and placental-fetal units on day 14 also resulted in no change in mammary DNA with as little as two placentas (minus all fetuses), while only one placenta remaining resulted in a significantly lower mammary DNA than in groups with 2 or more placentas.
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