Development of sheep embryos in vitro in a medium supplemented with different serum fractions

Sheep embryos will generally develop into expanded blastocysts in vitro only in culture media supplemented with serum or serum components. In order to better understand how serum supports embryo development, a batch of ovine serum was fractionated by (a) ultrafiltration into two components containing substances with molecular weights greater and less than 10 Kd (kilodaltons), and (b) gel filtration into protein fractions 1, 2 and 3, containing groups of proteins with mean molecular weights of about 500, 150 and 65 Kd, respectively. The principal protein in fraction 3 was albumin. Day 6 sheep morulae were cultured in vitro for 48 hours in a bicarbonate-buffered salt solution supplemented with various concentrations of ovine serum or of these components or protein fractions of serum. Morulae could develop to fully expanded blastocysts in medium supplemented with whole serum or with the greater than 10 Kd component or protein fraction 3 only, but could not develop in medium supplemented with the less than 10 Kd component only or with the less than 10 Kd component and protein fractions 1 or 2. However, the proportion of embryos that developed fully in medium supplemented with the greater than 10 Kd component or protein fraction 3 was increased by adding the less than 10 Kd component of serum to the medium. The addition of protein fraction 2 decreased the proportion of embryos that developed to expanded blastocysts in medium containing protein fraction 3 and the less than 10 Kd component, but not in medium containing whole serum. Since the compositions of different sera may vary markedly, these results suggest (a) reasons why different sera vary in their ability to support embryo development in vitro, and (b) factors which may influence development of the sheep embryo in the uterus, where plasma proteins comprise nearly all the protein in the fluid bathing the embryo.