The inhibition of implantation of ova in rodents with active mammary glands has been ascribed to various factors. Early literature favored inanition. The ability of certain animals to maintain the mammary gland and the activity in the uterus, as well as other variations in the effects of the suckled litter, have been found however (Kirkham,(16)(17)(18) ; Hain, '34; Krehbiel, '41 a). To this may be added the fact that irrespective of the number of suckled young it is possible to produce deciduomata during post-partum lactation (Krehbiel, '39 ; Weichert, '40). Thus, it is appafent that if other factors are present in the proper amounts implantation will occur and continue to term despite the demands of the suckled litter on the maternal system. Mirskaia and Crew ( '30) attributed the delay to an inability of the corpus luteum to maintain both lactation and the proper sensitization in the uterus. Hain ('34) refuted this assumption and Selye and RkKeown ( '34) have demonstrated the negative role played by the corpus luteum in the maintenance of lactation. Weichert ('40)' however, noted that the corpora lutea of animals suckling large litters were smaller, just before parturition, than those of animals providing for smaller litters. Assuming the same condition existed during the pre-implantation period, he attempted to obviate the effects of lactation by the injection of progesterone during the first days of pregnancy. Questionable results were ob-381
Reproductive decline in older female rodents has been interpreted as being due to deficiencies in various segments of the reproductive tract. Previous observations, such as those of Ingramamong others, have attributed deficiencies to one or all of the reproductive organs. In the present observations of a colony of aged females, examples of deficiencies at all levels of the reproductive tract were found. There were, however, a number of these animals which did produce a normal complement of ova and which fostered their continued development.Sixty-six virgin or multiparous rats, aged 18 months or older, were caged with males and matings were noted as having taken place by the presence of spermatozoa in the vaginal smear. The females were subjected to laparotomy or were killed at intervals from 1 hr following mating until term (Table 1). An additional fifteen rats, 18 months or older, were bilaterally ovariectomized. Three weeks after spaying, these females were started on hormone replacement therapy in the form of 2 mg progesterone daily.On Days 5 and 6 of progesterone treatment, each animal received 0-5 µg oestradiol propionate. On Day 6, needle stabs (six) were made in each uterine horn. On Days 9 and 10, the animals were killed. Most of the animals were injected with either Evan's blue or pontamine blue just before being killed to facilitate the observation of any decidual reactions.In thirty-five of sixty-six animals, evidence of pregnancy was observed (Table 1 ). Ova were found in the normal stages of cleavage and in the appropriate segment of the oviducts, with the following exceptions. Five of the twenty animals examined during the tubai period had a purulent salpingitis. Corpora lutea were absent in three rats. In two females, mating occurred after 09.00 hours. Ovulation was taking place in these two animals when they were killed 1 hr after mating.Thirty-six animals were examined on Days 4 to 9 of pregnancy. The uteri obtained during Days 4 and 5 (preimplantation stages) were serially sectioned. In the six negative cases, various degrees of leucocytic infiltration were found in the mesometrial segment of the lumen. In some, the migration of lympho¬ cytes was massive. Abnormal ova were surrounded by polymorphonuclear leucocytes. When implantation had occurred in the animals killed on Days 6 to 9 of pregnancy, the average number of decidual reactions per uterine horn 121
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