49 The dynamic changes in uterine morphology and function during the proestrus-diestrus transition in beef cattle

Abstract

In cattle, early development of the non-attaching embryo depends exclusively on the biochemical properties of the intrauterine milieu. As the embryo transitions from the oviduct to the tip of the uterine horn around 4 days after oestrus, little is known about uterine functions that modulate the milieu composition and potentially determine pregnancy success. The objective was to characterise changes in uterine luminal fluid (ULF) accumulation and absorption, endometrial vascularity, and endometrial thickness in response to progesterone fluctuations during the peri-ovulatory period in beef heifers. Fourteen Bos indicus heifers that presented a prostaglandin F_2α (PGF_2α) responsive CL received 500 µg of PGF_2α analogue i.m. (PGF) and were examined daily by rectal B-mode and pulse-wave colour-Doppler ultrasonography until the fifth day after oestrus (oestrus = Day 0). Plasma progesterone (P4) concentrations were measured daily. Uterine luminal fluid score was given based on the accumulation of fluid throughout the entire organ (0, no fluid, to 4). Measurement of the main ovarian structures, endometrial thickness, uterine luminal fluid score, and luminal fluid area of the cranial portion of the uterine horn ipsilateral to the preovulatory follicle and subsequent CL were executed by B-mode ultrasonography and endometrial vascularity by colour Doppler. The fixed effect of day was analysed by ANOVA. There was an effect of day on endometrial vascularity, endometrial thickness, and uterine luminal fluid score (P ≤ 0.01) as well as uterine luminal fluid area (P ≤ 0.05). Endometrial vascularity and endometrial thickness increased after PGF injection to peak on Day −1, whereas uterine fluid score peaked on Day 0. Interestingly, cranial uterine luminal fluid area peaked twice, on Day −1 and Day 5. Vascularity, thickness, and fluid accumulation decreased gradually from Day 0 to Day 5. There was a negative correlation (r = −0.55) between the sum of daily P4 concentrations during proestrus (i.e. from PGF to oestrus) and the greatest ULF score measured (P = 0.05). Length of proestrus was correlated negatively (r = −0.69) with the increase in uterine body thickness during proestrus (P = 0.03). The sum of daily luminal fluid scores, from PGF injection to Day 5, was double for heifers grouped within the upper third for that variable compared with those grouped within the lower third. In conclusion, peri-ovulatory transformations in uterine morphology and function are dramatic and vary in duration and in intensity among animals. Temporal changes in intraluminal fluid accumulation and absorption at the site of embryo arrival may change its biochemical composition. Potential implications for embryo receptivity and reproductive outcomes deserve further investigation.