Total protein content and protein synthesis within pre-elongation stage bovine embryos

Protein content was measured in zona-free bovine oocytes and pre-elongation stage embryos, following in vitro maturation, fertilisation, and then culture in Synthetic Oviduct Fluid medium supplemented with amino acids and 8 mg ml^-1 bovine serum albumin (BSA). Values (ng embryo^-1) of 122 ± 7.8, 137 ± 8.6, 111 ± 8.8, 115 ± 10.4, 139 ± 9.0 and 152 ± 10.1 were obtained for zona-free mature oocytes, 2-cell (day 2), 8-cell (day 3), compact morula (day 6), blastocyst (day 7), and expanded blastocyst (day 8) stage embryos, respectively. The protein content of day 7 zona-enclosed blastocysts was 337 ± 58.0 ng embryo^-1. These values suggest that prior to compaction and blastulation, the early cleavage stage bovine embryo has a higher rate of protein degradation than that of synthesis. Net growth is observed only after initiation of compaction. The protein content of day 7 blastocysts was measured in embryos following in vitro production and culture in the same media supplemented with either 0.5% w/v polyvinyl alcohol (PVA), 8 mg ml^-1 BSA, 8 mg ml^-1 BSA and further supplemented with 10% fetal calf serum (FCS) from the beginning of culture (FCS-D1), 8 mg ml^-1 BSA and 10% FCS from the fourth day of culture (day 5 of development) or from in vivo-derived day 7 blastocysts. Protein content was significantly (P< 0.05) lower in PVA-cultured embryos than other treatments. To determine if this difference in PVA-cultured embryos was due to a difference in the rate of protein synthesis, comparisons were made between day 7 embryos derived from BSA-culture and either PVA-culture, FCS-D1 culture or in vivo-derived embryos. Despite differences in diameter, no significant difference was observed in the incorporation of L-2,3,4,5,6-³H]-phenylalanine into the TCA-precipitable fraction in any of the three comparisons made. However, incubation in the presence of FITC-labelled BSA or β-casein and examination under either fluorescence or confocal microscopy revealed that protein in the extra-embryonic environment was actively taken up by the trophectoderm of day 7 blastocysts, most likely by endocytosis. These results suggest that exogenous protein is an important nutritive source, probably maintaining intracellular amino acid pools. Results obtained from the production of embryos in protein-free medium should be viewed with the knowledge that such embryos differ metabolically from those embryos grown in the presence of protein, including in vivo-derived embryos. Mol. Reprod. Dev. 50:139–145, 1998. © 1998 Wiley-Liss, Inc.