Development of isolated sheep inner cell masses/embryonic discs in vitro.

Ovine inner cell masses (ICMs)/embryonic discs cultured in vitro, in conditions copying those in which mouse embryonic stem cells (ESCs) arise from mouse blastocysts, give rise to ectodermal colonies. Day 10-11 ICMs/epiblasts produce ectodermal colonies sufficiently often (55-60%) for it to be considered worthwhile trying to generate presumed ESCs from them. Younger ICMs can only be taken into account if culture conditions can be improved so that ICM/ectodermal cells are more numerous. Older embryonic discs (12-13 day) are inconvenient because of the problem of endoderm overgrowing ectoderm. Secondary cultures of ectodermal colonies form epithelial or mesenchymal cells, which can be passaged at least seven times (50 days).