76 RELATIONSHIP BETWEEN BLASTOCYST CELL NUMBER AT EMBRYO TRANSFER AND CONCEPTUS ELONGATION ON DAY 14: CAN SUPPLEMENTARY PROGESTERONE RESCUE SMALL EMBRYOS?

Conceptus size on Day 14 following multiple embryo transfer of Day 7 in vitro-produced blastocysts varies greatly within animal. One explanation for this variation may be related to blastocyst cell number at the time of transfer. The aim of this study was to examine the effect of Day 7 blastocyst cell number on Day 14 conceptus size and to examine the effect of progesterone (P4) supplementation on embryo development following the transfer of Day 7 blastocysts containing a low total cell number. The oestrous cycles of crossbred beef heifers were synchronised using an 8-day CIDR (1.38 g of P4) treatment with administration of a prostaglandin F2α analogue (Estrumate™, Merck Animal Health, Summit, NJ, USA) on the day before CIDR removal. Only those heifers recorded in standing oestrus (Day 0) were used. Heifers were randomly assigned to 1 of 4 treatment groups: (1) Control: large blastocysts (high total cell number), (2) Control: small blastocysts (low total cell number), (3) small blastocysts plus a single intramuscular injection of 3000 IU of hCG (Chorulon®, Merck Animal Health) on Day 2 after oestrus, or (4) small blastocysts plus insertion of a vaginal progesterone insert (PRID, 1.55 g of P4) between Days 3 to 5 after oestrus. In vitro-produced blastocysts were transferred to each heifer on Day 7 (n = 10 blastocysts per heifer) and conceptuses were recovered at slaughter on Day 14. A sample of blastocysts was fixed and stained using Hoechst 33342 to determine the mean total cell number per group. Daily blood samples were collected from Day 0 to 14 to measure serum P4 concentrations. Data were analysed using the PROC MIXED procedure of SAS (SAS Institute Inc. Cary, NC, USA). Total cell number on Day 7 was significantly lower in small compared with large blastocysts (72.4 ± 3.93 v. 144.8 ± 3.90; P < 0.05). Conceptus recovery rate was 53.8% overall (140/260) and was highest in the large blastocyst group (68.3%, 41/60) compared with the other groups (45.7–55.0%). Concentrations of serum P4 were similar in the 2 unmanipulated recipient groups but were significantly elevated (P < 0.05) by Day 8 in hCG-treated heifers and on Days 4 and 5 in the PRID group (P < 0.003). In the absence of supplemental P4, Day 14 conceptuses resulting from the transfer of small blastocysts (2.48 ± 0.54 mm) were smaller than those from large blastocysts (3.32 ± 0.52 mm). Administration of hCG on Day 2 approximately doubled conceptus length on Day 14 (4.94 ± 1.15 mm), whereas insertion of a PRID from Day 3 to 5 increased conceptus length ~5-fold (13.09 ± 2.11 mm; P < 0.05) compared with controls. In conclusion, results indicate that supplemental P4 is capable of “rescuing” poor quality blastocysts, presumably via the now well-described actions on the endometrium and consequent effects on uterine lumen fluid composition. Supported by Science Foundation Ireland (07/SRC/B1156).