Production of sheep embryos in vitro and development of progeny following single and twin embryo transfers.

An in vitro culture system for producing ovine embryos is described, in which heat inactivated sheep serum was used as a protein source for maturation, fertilization and 7-d culture phases. Ovaries obtained from a commercial abattoir were used as the source of mature ewe (285) and lamb oocytes (356), which were subsequently cultured in this system to yield similar mean cleavage rates of 91 and 92%, respectively, but significantly different (P < 0.025) proportions for blastocysts/cleaved oocytes (46 and 18%). At Days 7 or 8 of culture, embryos from each source were transferred, either singly (ewe-derived) or in pairs (ewe- and lamb-derived), to hormonally synchronized recipient ewes, resulting in the birth of lambs ranging in weight from 2.5 to 8.8 kg for singletons and 2.6 to 8.0 kg for twins. Mean gestation length of 153.4 +/- 0.5 d (range 151 to 160) was slightly longer than the expected norm of about 150 d. The pregnancy rate was significantly higher after the transfer of embryo pairs (64%) than single (39%) embryos, while survival of lambs to weaning was greater for singleton (80%) than for twin lambs (43%). Some factor(s) in the culture media, such as growth factors in the sera, may have a mitogenic effect on embryonic cells, or it may alter the distribution of cells to the trophectoderm and inner cell mass, resulting in enhanced body growth rates.