Luteal versus placental progesterone: the situation in the cow, pig and bitch.

Placenta and corpus luteum (CL) have acquired different functions for progesterone synthesis and maintenance of pregnancy in the course of placentalia evolution. In the cow and pig, progesterone is produced by the same CL during the entire gestation period with minor or no contribution from the placenta. In the case of an infertile cycle, CL survival is terminated by PGF2 alpha produced by the endometrium. PGF2 alpha secretion is inhibited by the trophoblast in order to guarantee continuous progesterone production. Also in the dog, only the CL synthesizes progesterone and the secretion pattern is similar in pseudopregnant and pregnant animals. The dog CL seems to have an internally programmed life span.