THE UNIVERSITY OF MANITOBA STUDIES ON OVINE PLACINTAL ...

(iii) tsheep:
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Although there is no compelling evidence to
demonstrate the existence of ovine chorionic Aonadotropin,
indirect evldence has accumulated to suggest that there is
a luteotropln secreted by the conceptus, possìO.ly opL. For
the continuation of pregnancy in the ewe. the presence
of an act.ively secreting corpus Luteum is essential- during
the first 50-60 days. Ovariectomy, enucleation of the corpus
luteum, or hl¡pophysectomy, during this period will 1ead to
1¡6 tennination s¡ the pregnancy. After day 50-60 the
activity of the corpus luteum is maintained following
hypophysectomy, which suggests that the conceptus. may be
produci.ng a luteotropin that not onry maintains the funcÈional
activity of the corpus lutèum but may also stimulate
increased progesterone synthesis and secretÍon by the
conceptus. Hoh¡ever, it is not yet clear whether the
putative luteotropin is a dístinct hormone or ovine placental
Iact,ogen (opI.,).
One of the early píeces of evidence that suggested
ovine placentas might secrete a ÍElrnmotropin was a report by
Denamur and Mart,inet (1961). They found that hypophysectomy
of pregnant ewes aft.er 50 days of gestation did not affect
the normal course of pregnancy and the deveÌopment of the
ma¡nmary gland, aLthough the marnmary gland was not so
developed as in intact animaLs. Furthermore, trânsient
lactation also took place Ín these hypophysectomízed ewes.