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Fas is a membrane-bound protein which upon activation causes programmed cell death. Fas ligand (FasL) binds Fas on target cells. Both these factors are known to regulate apoptosis at implantation in different species and thus might be involved in the regulation of implantation in dogs. The aim of the study was to assess the expression of Fas and FasL in canine uterine tissue throughout pregnancy as well as in pre-implantation embryos using RT-PCR and RT-qPCR. Uterine tissues was collected from of 21 healthy pregnant bitches (group I: days 10-12, n = 5; group II: days 18-25, n = 6; group III: days 28-45, n = 6) and from 4 non-pregnant bitches (controls: days 10-12). Pregnancy stage was determined by days after mating, that is, 2-3 days after ovulation as determined by vaginal cytology and progesterone measurement. After ovariohysterectomy, uteri from group I bitches were flushed with PBS and the embryos washed and stored frozen at -80°. Tissues from the other groups were taken from the implantation and placentation sites, respectively, covered with Tissue Tek(®) and frozen at -80°. Extraction of RNA was performed with Trizol Reagent and RT-qPCR using SYBR green probes. In pre-implantation embryos, only FasL but not Fas could be detected. In all tissues from pregnant and non-pregnant bitches, both parameters were detectable. Before implantation (group I) expression of FasL resembled that of non-pregnant bitches in early dioestrus and decreased significantly during implantation and thereafter (p < 0.05). Expression of Fas did not change significantly until day 45. The relative expression of Fas exceeded that of FasL at each stage investigated, which is comparable to observations of other species; however, high standard deviations indicate high individual differences. These preliminary results point towards a regulatory function of the Fas/FasL system during early canine pregnancy., (© 2012 Blackwell Verlag GmbH.)