THE UNIVERSITY OF MANITOBA STUDIES ON OVINE PLACINTAL ...

(Íi) coat:
-71
The goat is a spontaneous ovulator with an inter
estrus interval of 20 to 2l days. The lntegrity of the
pituitary is essential for the mAintenance of pregnancy in
thls species. In 1963, Co!"¿e et al reported that hypophysectomy
in pregnant goats causes abortion at any stage of pregnancy.
Ovariectomy at any stage of pregnancy also causes abortion
in the goat (Drumond et aL, L962¡ Meit,es et. al , l96f ) .
Since hypophysectomy in the goat will_ lead to abortion it
has been concluded that the conceptus does not, produce
a luteotropic factor, but, this may be a matter of degree.
It. has been noted by Van Rensburg (192I) that after an initiat
d.ecrease during the first 40 days of pregnancy the luteal
cells of the corpus luteum of the pregnant goat decrease
in size and then increase untiL day 110 before a slow
decLine to parturition sets in. The secretory actívity
of the corpus luteum tends to follor.r the change in the size
of the luteal cells with peripheral plasma progest,erone
concentrations reaching Èheir maximum about day tlO.
Therefore, it is possible that the increase in size
of the luteal cells over the períod 40-lIO days may arise
through stimulation by some placental luteotropins.
The grott¡th and differentiatÍon of the manmary
gland during pregnancy is a response to the acÈion of many
hotrmones (cowie and Tindal, I97L), one of which is prolactin.
This hormone has been consídered to exert a major influence
as a manìmotro¡rin. However, Buttle et al (19?2) ând Forsyth