LAMB BIRTH-WEIGHT IS AFFECTED BY CULTURE SYSTEM UTILIZED DURING IN-VITRO PRE-ELONGATION DEVELOPMENT OF OVINE EMBRYOS

It has previously been reported that ovine embryos cultured in Synthet ic Oviduct Fluid medium supplemented with 20% human serum (SOF + HS) d evelop into lambs with a high birth weight. We have investigated this phenomenon by culturing ovine zygotes in SOF + HS or a serum-free vers ion of Synthetic Oviduct Fluid with BSA and amino acids (SOFaaBSA) in place of serum. Zygotes were either obtained from superovulated and na turally mated ewes or produced in vitro. Embryos were subsequently tra nsferred to synchronized recipient ewes (n = 63). An additional group of ewes (n = 16) served as flock fertility and lambing controls. Devel opment of zygotes to stages suitable for transfer (i.e., good to excel lent compact morulae or blastocysts) was not affected by medium (SOFaa BSA = 53 +/- 5% vs. SOF + HS = 59 +/- 5%) but was affected by source ( in vivo-derived = 74 +/- 5% vs. in vitro-derived = 35 +/- 5%, p < 0.00 1). Embryos incubated in SOF + HS were morphologically different from those incubated in SOFaaBSA, having abundant lipid droplets. Pregnancy rate (65%) and embryo survival (48%) of recipients determined by ultr asonography on approximately Day 60 of pregnancy did not differ betwee n medium treatments or source of embryo. Mean weight of lambs from emb ryos cultured in SOF + HS (4.2 +/- 0.2 kg) was significantly heavier t han that of controls (3.4 +/- 0.2 kg, p < 0.01) or of lambs from embry os cultured in SOFaaBSA (3.5 +/- 0.2 kg, p < 0.05). Furthermore, mean gestation length was longer in recipients receiving embryos incubated in SOF + HS (147 +/- 1 days) than in SOFaaBSA (145 +/- 1 day, p < 0.05 ). Reasons for this birth weight and gestation length difference are u nclear, but our data suggest that different culture conditions can pro duce embryos with differing morphology, apparent chemical composition, and rate of development, resulting in lambs with differing gestation length and birth weight.