Normal and blocked pregnancy in the collared lemming: morphometrical and histological changes

Abstract

Normal and blocked pregnancy were studied in the collared lemming, Dicrostonyx groenlandicus, in the laboratory with individuals bred from wild stock caught at Eskimo Point, N.W.T. Morphometrical changes (corpora luteal area, ovarian weight, uterine weight, fetal crown–rump lengths, spleen and adrenal weights) during normal and blocked pregnancy were documented and found to be similar to those of other microtines. Mean maximum corpora luteal area, and ovarian and uterine weights increased continuously throughout gestation, as did the fetal crown–rump lengths. Spleen and paired adrenal weights increased similarly; however, they were not significantly different when corrected for body weight. Ovulation occurred between 9 and 12 h post coitum, as evidenced by the presence of ovulated follicles with stigmata, and luteinization was observed within 24 h. No identifiable differences were observed in the histology of corpora lutea and corpora albicantia of normal and blocked females although significant differences in size occurred. Accessory luteal structures (accessory corpora lutea and luteinized follicles) were found in very high numbers in this species compared with other microtines, peaking by day 4 post coitum and declining in number and size towards the end of gestation. It is concluded that the accessory luteal structures function in a supportive role to the corpora lutea of pregnancy during the early stages of gestation but undergo atrophy towards parturition, when the corpora lutea of pregnancy reach their full size and functional capacity. The presence of large numbers of accessory luteal structures and the similarity of histology between ovarian structures in normal and blocked pregnancy makes the identification of these conditions in the field impractical at present.