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Agricultural Research Council Unit of Reproductive Physiology and Biochemistry, University of Cambridge, England
Abstract
The relationship that exists between the embryo and the corpus luteum has been analysed particularly in relation to the effects of the embryo on luteal function in early pregnancy. The animals that have been discussed, namely the sheep, pig, guinea-pig, rat, mouse and some Mustelids and Marsupials, serve to illustrate the marked differences in the role of the embryo during pregnancy.
In some animals of which the sheep and the pig are good examples, the presence of embryos in the uterus prolongs the life span of the corpora lutea but does not increase their functional capacity. In these species the level of progesterone secreted by the corpus luteum of pregnancy is similar to the maximum level secreted in the oestrous cycle.
The first effect of the embryo on luteal function in the sheep is seen on the 12the to 13th day after oestrus. In the pig the embryo first assumes its function of extending the life span of the corpus luteum between day 7 and 14. In these animals the corpora lutea of pregnancy therefore become dependent for their maintenance upon the presence of embryos in the uterus at least 1 to 3 days before the lytic effect of the non-pregnant uterus becomes apparent. The embryos exert their effect on the corpora lutea during early pregnancy in a local manner. This is demonstrated by the fact that unilateral luteal regression can be induced in pregnant sheep and pigs by retaining a segment of one uterine horn in a non-gravid condition. The corpora lutea in the ovary adjacent to the non-gravid uterine segment regress while those corpora lutea in the ovary adjacent to the pregnant horn are maintained by this procedure. The active principle in the sheep embryo is probably both age and species specific; its chemical nature is not known but embryonic homogenates stored at –20° C. retain their ability to prevent luteal regression if infused into the uteri of non-pregnant animals. It is suggested that the embryo during early pregnancy in sheep and pigs acts either by ‘protecting’ the corpora lutea from the lytic effect of the uterus or else by preventing the uterus from acquiring its lytic properties.
In the sheep, the unilateral relationship between the conceptus and the corpus luteum persists for at least the first 50 days of pregnancy and possibly longer. By contrast, conceptuses in the pig cease to affect luteal function in a local manner after the 12th to 16th day of pregnancy. The placenta in the sheep secretes sufficient luteotrophic and steroid hormones after day 50 to maintain the pregnancy independently of the pituitary gland or ovaries. On the other hand removal of either the ovaries or the pituitary at any time during gestation in pigs terminates pregnancy unless exogenous hormones are given.
The guinea-pig, rat and mouse have been selected as examples of animals in which both the life span and the functional activity of the corpora lutea are increased during pregnancy. The presence of embryos in the guinea-pig first affects luteal function within 10 to 14 days of mating. Histological and functional changes in the corpora lutea of pregnant rats and mice indicate that luteal function in these species is first affected by the presence of embryos at about the time of implantation. This early effect of pregnancy is not understood, but the embryos might initially exert both an "anti-luteolytic" and a directly "luteotrophic" effect on the corpora lutea. It has been suggested that the secretion of placental luteotrophins in these species is dependent upon an interaction between the maternal and foetal components of the placenta. A clearly defined placental luteotrophin is present in these rodents by about day 7 post coitum. After the 12 th day of pregnancy luteotrophins of placental origin can entirely replace the pituitary luteotrophic complex in rats and mice. In the guinea-pig the embryos maintain and stimulate the corpora lutea of pregnancy independently of hypophyseal control from the second or third day after mating.
In a few species such as the Mustelids and Marsupials the embryo appears to have little or no effect on luteal function. This is evident since there is no difference between the function or life span of corpora lutea in pregnant and pseudo-pregnant females in these species.
1 I am particularly grateful to Mr. L. E. A. Rowson Dr R. Deanesly, Dr. D. Kirby, Dr. R. M. Melampy and Dr. I. Hashimoto for their generosity in allowing me to refer to some of their unpublished observations. Dr. Mary Hay deserves special thanks for her invaluable editorial assistance in the preparation of the manuscript. Finally, I would like to thank Dr. T. R. R. Mann, F.R.S., for his interest and advice throughout the period of this study.
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