Towards the isolation of embryonal stem cell lines from the sheep.

Immunosurgical isolation of inner cell masses (ICMs) from sheep embryos was most efficient at the expanded, zona-intact blastocyst stage (day 7 to 8 post oestrus) before migration of endoderm cells beyond the boundary of the ICM across the blastocoelic surface of the trophectoderm. When cultured under conditions which allow the isolation of embryonal stem (ES) cell lines from mouse ICMs, sheep ICMs attached, spread and developed areas of both ES cell-like and endoderm-like cells. After prolonged culture only endoderm-like cells were evident. The implications for the isolation of ES cell lines from sheep embryos and possible species-specific requirements are discussed.