Artificial insemination of all ejaculated sperm fractions accelerates embryo development and increases the uterine vascularity in the pig

The semen of boar is characterized by ejaculation in well-differentiated fractions with specific concentration, composition, and volume. The 'sperm-rich fraction' (SRF), the most concentrated seminal fraction, is habitually collected in insemination centers to make artificial insemination (AI) doses. The absence of the other fractions in AI doses could alter the uterine reaction to AI and not trigger essential responses that could maximize fertility. Thus, there is an urge to ascertain the impact of different ejaculate fractions on the uterus after AI to optimize the semen doses. This work analyzed specific parameters related to fertility in pregnant artificially inseminated sows (n = 15) with ac-cumulative fractions of the semen of boars (n = 6): F1, composed of the sperm-rich fraction (SRF); F2, composed of F1 plus the intermediate fraction; F3, composed of F2 plus the post-SRF. Non-inseminated sows (n = 5) were included as control (C). The different types of seminal dose did not affect the number of ovulated follicles (CL; corpora lutea, p > 0.05) but did affect the embryo development (p < 0.05). The proportion of embryos in morula stages was significantly higher in AI-F1 sows (84.4%, p < 0.05). Morulas and blastocysts were balanced in AI-F2 or AI-F3 (p > 0.05). Independently of the type of seminal dose (F1, F2, or F3), we observed by immunohistochemistry that AI significantly increased uterine vascularization, although with some anatomical differences. The cranial region of the uterine horns was significantly more vascularized in AI-F1 or AI-F2 sows (26.7 ± 2.3 and 28.6 ± 2.0%, respectively), and AI-F3 showed significantly less vascularization at that point (17.8 ± 1.6%, p < 0.05). To summarize, the synergistic effect of all ejaculate fractions accelerates embryo development, at least during the preimplantation period, and increases the uterine reaction to AI in certain parts of the uterus.