Introduction

The first visible differentiation of the mammalian embryo occurs with formation of the blastocyst. At this stage the embryo can be seen to have two distinct cell types: the trophectoderm cells, which have limited developmental capacity that restricts them to the extraembryonic lineages, and the pluripotent inner cell mass (ICM) cells, which give rise to all of the differentiated cell types of the adult, including the germ line (see Chapters 4 and 5). As the ICM cells proliferate, groups of cells become committed to specific developmental pathways. By 7.5 days of development, proliferating pluripotent cells are restricted to the embryonic ectoderm of the egg cylinder, and it is these cells that give rise to the tissues of the adult mouse. The methods for rescuing the undetermined pluripotent cells present during the first 8 days of embryonic development and the establishment of permanent tissue culture lines from them are the subjects of this chapter.

Isolating cell lines from embryos into tissue culture provides an opportunity to perpetuate a given cell phenotype indefinitely. Permanent cell lines established under in vitro conditions provide large quantities of homogeneous material that can be used for biochemical analysis of the patterns of gene expression. Embryo-derived cell lines are becoming increasingly important as sources of material to facilitate molecular investigation of the differentiation processes occurring in the embryo.

Cell lines from embryos can be classified as being of either a differentiated or an undifferentiated stem cell phenotype. Differentiated cell lines are relatively easy to define, both phenotypically and by expression of particular biochemical markers (Sherman 1975; Evans 1981).