Immunohistochemical Studies on Oestrogen Receptor Alpha (ERα) and the Proliferative Marker Ki-67 in the Sow Uterus at Oestrus and Early Pregnancy

Oestrogen receptor alpha (ERalpha), the main subtype in the uterus, is involved in the regulation of uterine growth/proliferation. A relationship between ERalpha and proliferative activity has been shown in the cyclic sow uterus, but to our knowledge, no study has been carried out on early pregnant sows. Therefore, by means of immunohistochemistry and use of mouse monoclonal antibodies to ERalpha and a proliferative marker, Ki-67, the localization of these proteins was investigated in the sow uterus during early pregnancy. Eighteen crossbred multiparous sows were artificially inseminated once at 20-15 h before expected ovulation. After artificial insemination (AI), they were slaughtered at five different times: at oestrus, 5-6 h after AI (n = 4), 20-25 h after ovulation (n =4), 70 h after ovulation (n = 4), on day 11 (the first day of standing oestrus = day 1, n = 3) and on day 19 (n = 3). Immediately after slaughter, uterine samples were collected at the mesometrial side of the uteri, fixed in 10% formaldehyde and embedded in paraffin. Immunohistochemistry was performed by using mouse monoclonal antibodies to ERalpha (C-311) and Ki-67 (MM1). All sows slaughtered after ovulation were pregnant. In general, positive immunostaining for ERalpha and Ki-67 was found in the nuclei. Variations in staining intensity and proportion of positive nuclei were observed in different uterine compartments and stages of early pregnancy. The highest level of ERalpha presence in the surface epithelium and myometrium was found at oestrus (5-6 h after AI), and low levels of ERalpha in these compartments were observed as early as 20-25 h after ovulation. In the glandular epithelia, presence of ERalpha was highest at 70 h after ovulation. The largest number of ERalpha-positive cells in the stroma was observed at oestrus and early after ovulation. Low proliferation was observed, and with no significant difference in tissue compartments except in the glandular epithelium. High proliferative activity in the glandular epithelium at 70 h after ovulation indicated involvement in preparation for secretory activity and growth during pregnancy establishment. Significant positive correlation was found between the number of ERalpha-positive cells in the stroma and Ki-67-positive cells in the surface epithelium. In conclusion, the present study showed differences in immunolocalization of ERalpha and the proliferative marker Ki-67 in different tissue compartments of the sow uterus at oestrus and early pregnancy. In some uterine compartments, the patterns of ERalpha and Ki-67 immunostaining seemed to be influenced by insemination and the presence of embryos, in addition to the effects of steroid hormones.