Early embryo development, number, quality, and location and the relationship with plasma progesterone in dogs

This research assessed the developmental stages and morphological quality of dog embryos collected during different stages of pregnancy as well as the relationship with serum progesterone recorded at insemination and embryo collection. Embryos were collected from 23 young mature bitches, that had been inseminated with fresh semen 3–6 days after the LH surge (day 0). Embryo flushing was performed on pregnancy days 8–11 (Group 1), 12–15 (Group 2), or 16–20 (Group 3). The location, number and morphological characteristics of the embryos were evaluated. A total of 120 embryos and 25 unfertilized oocytes were collected from bitches with a total of 156 corpora lutea (CL). The mean total embryo yield (total of embryos/CL) was 76.7 ± 5.9%, and the mean embryo recovery rate (number of flushed embryos/number of CL) was 70.6 ± 6.6%. The mean ovulation rate was 6.8 ± 0.5 and the mean number of embryos per bitch was 5.2 ± 0.6. Oocyte fertilization occurred following oocyte maturation. Most embryos in Group 1 (70.0 ± 18.6%) were collected at the 2 to 16 cell stage. The morula stage was first observed on day 11. Expanded blastocysts (EBLs) and hatched blastocysts were first flushed from the uterus on days 13 and 14, respectively. The EBL was the most abundant stage in Groups 2 and 3. After day 19, some embryos (n = 8) had already adhered to the endometrium. Although most recovered embryos were classified as very good, a greater number of low quality embryos was collected in the later gestational periods. A significant variation in the embryonic stages and location of embryos in early canine pregnancy was observed, as embryos entered the uterus independently of their developmental stage. Embryo yield and quality were independent of the serum progesterone concentration at insemination and recovery.