Luteal Function as Related to Pituitary and Ovarian Cytology and Embryo Development in the Bovine

Embryos recovered 15 days after insemination from 20 of 27 untreated Holstein heifers and those recovered from 18 of 43 heifer treated daily with 7 U.S.P. units of oxytocin per hundredweight per day did not differ in total length, embryonic disc size, or mesodermal development attained.

Abstract

Embryos recovered 15 days after insemination from 20 of 27 untreated Holstein heifers and those recovered from 18 of 43 heifers treated daily with 7 U.S.P. units of oxytocin per hundredweight per day did not differ in total length, embryonic disc size, or mesodermal development attained.No embryos were recovered from animals with less than 100 ~g. of total progesterone in their corpora lutea; normal embryo development appeared possible when the level of luteal progesterone was above this figure.The weights of the corpora lutea of the 15 oxytoein-treated heifers which had precocious ovulations were significantly (P < 0.01) reduced, and these corpora contained few normal luteal cells.Ten cystic corpora lutea were found in treated heifers and one in an untreated heifer.These cystic corpora lutea were all formed after ovulation; the cysts were 7 ram.or more in diameter and were separated from the surrounding luteal cells by fibrous tissue.Only two heifers with cystic corpora lutea had embryos.Low levels of progesterone were found in cystic corpora lutea, except in one heifer that had a normal embryo.Although an occasional cystic corpus luteum can produce enough progesterone to maintain an embryo until the 15th day, most of them appear to produce little progesterone and they are not likely to be associated with live embryos.The gonadotrophin-producing delta cells in the anterior pituitary were degranulated in some of the heifers which ovulated precociously and in the control heifers that did not have embryos.In a previous paper, Staples and Hansel (10) studied the effects of altering the level of progesterone in the corpus luteum by oxytocin injections on embryo survival at 15 days in 70 Holstein heifers.In the present paper the effect of varying luteal progesterone levels on the rate of embryo development and on pituitary, ovarian, and uterine cytology in these same heifers is considered.The heifers in this experiment were slaughtered at the 15th day after breeding, because it is at this time that the trophoblast normally elongates at a very rapid rate, mesoderm formation begins, and the embryonic disc becomes oval in shape.It was hoped that in cases where embryonic development was inhibited, spherical blastocysts having no mesoderm and round discs would be found.The relative simplicity of the embryo at this stage, as well as the fact that luteal progesterone levels would still be maximal in those heifers which failed to conceive, were additional factors which led to choice of the 15th day.