ENDOCRINE CHANGES AND LUTEAL MORPHOLOGY DURING PREGNANCY IN MUSKOXEN (OVIBOS-MOSCHATUS)

Progesterone, oestradiol and oestrone were measured in plasma from fou r captive muskoxen during three consecutive pregnancies (1983-1984, 19 84-1985 and 1985-1986). Jugular blood samples were collected weekly (1 983) or on an alternating 3:4 day schedule (1984-1986) during the firs t 12-15 weeks and last 6-10 weeks of pregnancy. Sampling during mid-pr egnancy was at intervals of 2 weeks (1983 and 1985) or 1 week (1986). Duration of gestation was about 34 weeks (235 +/- 4 (SD) days (n = 10) , range 230-242 days). Progesterone remained at concentrations similar to those found during the luteal phase of the oestrous cycle for the first 10-12 weeks (mean +/- SEM 1.6 +/- 0.1 ng ml-1) after which it ro se to a peak (mean 5.5 +/- 0.65 ng ml-1) between weeks 12 and 20. In a ll ten pregnancies progesterone concentrations declined dramatically b etween weeks 20 and 22 to luteal phase values where they remained unti l parturition. The decline was accompanied by an increase in oestradio l and oestrone concentrations which reached mean peak values of 199.23 +/- 87.23 pg ml-1 and 980.48 +/- 203.91 pg ml-1, respectively. Corpor a lutea collected from wild muskoxen between 45 and 80 days gestation all showed histological evidence of regression, while corpora lutea fr om mid-gestation (112-125 days) were in advanced stages of involution. Repeated ovarian ultrasonography of captive muskoxen during the first 100 days of pregnancy confirmed these findings. The unusual, early re gression of the corpus luteum of pregnancy indicates that progesterone and oestrogen of mid- and late pregnancy are probably of placental or igin. The temporal relationship between the concentrations of these ho rmones during late pregnancy differs from those of domestic species an d is apparently unique among ruminants.