Influence of pre-ovulatory insemination and early pregnancy on the infiltration by cells of the immune system in the sow endometrium

The aim of this study was to investigate the distribution of leukocytes in the sow endometrium following insemination and during early pregnancy. Cross-bred multiparous sows (Swedish Landrace x Swedish Yorkshire) were artificially inseminated (AI) once at 20-15 h before ovulation. Blood samples were collected from the jugular vein 1 h before slaughter for analyses of oestradiol-17beta and progesterone levels. The sows were slaughtered at 5-6 h (group I, n = 4) after AI or at different times after ovulation: 20-25 h (group II, n = 4), 70 h (group III, n = 4), day 11 (group IV, n = 3; first day of standing oestrus = day 1) and day 19 (group V, n = 3). Uterine horns were flushed to control for the presence of spermatozoa and neutrophils (groups I-IV) and/or for recovery of oocytes and/or embryos (groups II-IV, control of pregnancy). Mesometrial uterine samples were fixed, embedded in plastic resin and stained with toluidine blue. The surface and glandular epithelia as well as subepithelial and glandular connective tissue layers were examined by light microscopy. A marked number of neutrophils and spermatozoa were observed in the flushings from the uterine horns of sows slaughtered at 5-6 h after insemination. All animals slaughtered after ovulation were pregnant but no morphological effect of pregnancy was observed until day 11. In the surface epithelium, the largest numbers of intraepithelial lymphocytes were found in groups II and III, the smallest number was found in group V. The largest number of lymphocytes within the glandular epithelium was found in group III. The largest number of macrophages within the surface and glandular epithelia were found in group I. Neutrophils were found within the surface epithelium only in groups I and II. In the subepithelial connective tissue layer, a high infiltration of neutrophils was found in groups I and II while the largest number of eosinophils was found in group IV. The largest number of lymphocytes was observed in group V. In conclusion, this study showed a variation in the infiltration and distribution of neutrophils, lymphocytes, macrophages, eosinophils and plasma cells in the endometrium following insemination and during different stages of early pregnancy. Particularly, the pattern of lymphocyte presence on day 19 of pregnancy, indicate that the lymphocyte function may play a role during embryonic attachment in the pig.