As a D.Phil. student at the University of Oxford in the late 1970s, Beddington took on the challenge of understanding events in the early-stage mouse embryo. Mouse embryos grow outside the protective environment of the uterus only for short periods of time. Moreover, early axis formation is accompanied by extraordinary growth and cell movements. Undaunted by these complexities, Beddington painstakingly grafted pieces of tritium-labelled tissue from different regions of donor embryos to various sites in a host embryo. She then scrutinized histological sections to see whether donor tissues could adopt new fates or whether the graft's developmental potential was restricted by its site of origin. Together with work by Kirstie Lawson and Patrick Tam, her papers provided the all-important fate map of post-implantation lineages in the mammalian embryo. Without this basic information, all the subsequent work on how, when and what a cell is instructed to become in the embryo would have been impossible.
At the Imperial Cancer Research Fund laboratories in Oxford, Beddington harnessed the new transgenic technology to generate strains of mice expressing β-galactosidase. She fondly dubbed her most famous animal Levi — a mouse with 'blue genes' that simplified analysis of tissue grafts and embryo manipulation. She also started working on embryonic stem cells, made from mutant mice, that provided insights into the cellular defects underlying the truncated body axis of the classical T (brachyury) mutation.
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