Mammalian chimaeras / Anne McLaren.

  • Anne McLaren
Date:
[1976]
Catalogue details
    16/168 (page 6)
    6 What is a chimaera? when Tarkowski reported the successful aggregation of cleaving mouse embryos. Embryos aggregated in pairs formed single large blastocysts in culture. After transplantation to the uterus of a foster-mother, these blasto¬ cysts developed into foetuses of apparently normal size and morphology, though the sex ratio was abnormal and one hermaphrodite was observed (see Chapter 4). Some of the embryos were from an albino and others from a coloured strain; subsequent histological examination of the pigmented retinas of the foetuses and newborn animals yielded morphological evidence of chimaerism (Tarkowski, 1963, 1964¿?). A similar method devised indepen¬ dently by Mintz was mentioned briefly in 1962 (Mintz, 1962«, b) and described more fully two years later (Mintz, 1964ö). The techniques used were in some ways more convenient than those of Tarkowski (see Chapter 2), and embryos were aggregated not merely in pairs, but in groups of up to ten, forming giant blastocysts. Adult chimaeric mice were described first by Mintz (1965«, b) and later by Mystkowska & Tarkowski (1968) and others. The term 'chimaera' was used by Tarkowski (1961). Mintz (1967ö) coined an alternative term, 'allophenic', to refer to animals derived from embryo aggregation; however, Hadorn (1945, 1961) had previously used the terms 'allophene' and 'allophenic' in a difi'erent sense, referring to any characteris¬ tic of a cellular system which derives not from its own genetic constitution, but from the genetic constitution of some other cellular system (see dis¬ cussion after McLaren, 1972fl). Chimaeras made between embryos from dif¬ ferent parents are often referred to as 'tetraparental' animals. In earlier papers the embryos were described as 'fused' rather than 'aggregated', and the resulting animals were often referred to as 'fusion chimaeras'. This terminology has now been largely abandoned, in view of the potential confusion with virus-mediated cell fusion, and the expression 'aggregation chimaera' has been substituted. A В denotes a chimaera formed by aggregating a strain A with a strain В embryo. An alternative technique for making mammalian chimaeras has been de¬ vised by Gardner (1968). Cells from a dissociated blastocyst are injected singly or in groups into the blastocoele of an intact blastocyst, and become incorporated into the inner cell mass of the recipient embryo, giving rise to an animal that may be termed an 'injection chimaera'. Details of the tech¬ nique are given in the next chapter. A -» В denotes a chimaera formed by injecting strain A cells into a strain В blastocyst. Methods for producing mammalian chimaeras have all been worked out on mouse embryos, for which much is known of the requirements for growth in vitro during the pre-implantation period, and have been slow to spread to other species. An attempt was made to produce aggregation chimaeras in sheep (Pighills, Hancock & Hall, 1968), but the only lamb thought from its transferrin type to be tetraparental was stillborn. More recently, injection chimaeras have been successfully produced in sheep (Tucker, Moor & Row-