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Embryonic resorption is a common phenomenon in polytocous mammals. The process of resorption is not associated with any obvious clinical symptoms. Therefore, studies are usually based on post mortem examinations or counts after ovariohysterectomy (OHE). The present study is the first longitudinal in vivo investigation describing the incidence and morphology of embryos undergoing resorption in a small polytocous species, the European brown hare (Ebh) (Lepus europaeus). High frequency ultrasonographic examinations enabled early detection of resorption sites from day 8 of pregnancy onwards. Resorptions were monitored in context of embryonic and fetal development and classified into (i) preimplantation resorptions, (ii) peri-implatation resorptions and (iii) post-implantation resorptions. In total, 42% of embryos underwent resorption. Parallel monitoring of the ovaries revealed in 91% of the cases the regression of a CL while an embryo underwent resorption. The number of resorptions did not significantly differ from the number of CL in regression suggesting a one-resorption-to-one-regression-relationship. Two minimally invasive ultrasound guided biopsy techniques were established for the genetic analysis of resorption sites.
Resorption material was gained in vivo by aspiration of resorption fluid or by biopsy of extraembryonic tissue from the placenta (CVS). Further analysis was performed by microsatellite analysis (paternity testing) and SRY testing.
Paternity was determined in 11 of the fluid aspirates (n = 28) and six of the placental biopsies (n = 8). The lower success rate in the fluid samples is attributed to the high abundance of maternal cells which was confirmed by analysis of fluid sample smears.
By SRY testing male sex of the resorbing embryo was identified in ten of the fluid samples (n =28) and one of the placental samples (n = 8).
A negative influence of the biopsy techniques on the viability of the remaining embryos or the reproductive performance of the mother was not observed.