Introduction

In Greek mythology, a chimaera was a monster bearing a lion's head, goat's body and serpent's tail (Fig. 8.1). In the present chapter, however, chimaeras will be regarded as composite animals containing genetically different cell populations derived from more than one zygote, and primary chimaeras are those in which the genetically different cell populations have coexisted from a very early stage of embryogenesis or from fertilisation itself. In a mosaic, by contrast, the cells are derived from a single zygote lineage (Mintz, 1974). Artificially generated chimaeras have been used quite extensively in experimental embryology since the 1960s (e.g. Tarkowski, 1961, 1964; Mintz, 1962, 1964, 1965a, b), and especially during the 1970s and 1980s to examine somatic cell lineages (McLaren, 1976; Gardner, 1978, 1982; Gardner & Rossant, 1979; Beddington, 1982; Le Douarin & McLaren, 1984; Bradbury, 1987), not least with the focus on malignancy and on possible ways of controlling the growth of tumours (Brinster, 1974; Illmensee & Mintz, 1976; Hardy et al., 1990). The approach of bringing cell lineages together from two or more different individuals to produce a viable conceptus has permitted analysis of a number of steps in the process of differentiation such as the time of allocation of commitment, and has shed light on the origin and fate of individual cell lines (Gardner, 1985). The experimental generation of chimaeras has also enabled interactions between germ cells and the surrounding soma to be analysed under a variety of conditions, and has led to conclusions concerning differentiation and organogenesis, and survival of oogonia in testicular tissue or spermatogonia in ovarian tissue.