Cell Stem Cell Obituary Anne McLaren The July 7 automobile accident that claimed the lives of Anne McLaren and her former husband, Donald Michie, marked the end of a 5 decades long era that gave birth to the fields of mammalian reproductive genetics and stem cell biology. Anne McLaren made pioneering contributions to two major pillars of both fields: mammalian em- bryonic development and the ethical implications of embryo research. On April 26, 1927, Anne McLaren was born into a liberal British industri- alist family, daughter of Henry McLa- ren, second Baron Aberconway, and Christabel McNaughton. Anne spent her childhood between the family’s homes in London and Bondnant, North Wales, and was educated at Longstow Hall, Cambridgeshire. She attended Oxford University, Lady Margaret Hall, attaining a first in zoology, then continued at Oxford for her doctor of philosophy, awarded in 1952. She then took up her first post as research associate with Peter Medawar at Uni- versity College London, be- ginning a collaboration with Donald (a recruit to genetics from his codebreaking work at Bletchley Park), whom she had married earlier that year. Together, they studied the genetic and environmen- tal basis of skeletal variation in mice, a subject to which she would return in her sub- sequent studies of mamma- lian chimeras. Anne and Donald continued their stud- ies after moving to the Royal Veterinary College, London, in 1955, where they met John Biggers. Following the demonstration by Wesley Whitten that early mouse embryos could be cultivated in a simple solution of salts and sugars (Whitten, 1957), Anne and John made the pioneering discovery that cultured mouse embryos could be transplanted to the uteri of foster mothers, with successful full-term devel- opment as normal mice (McLaren and Biggers, 1958). Their publication opened the field of experimental mam- malian embryology, with its ultimate culmination in chimeras, embryonic stem cells, gene targeting, and, in the clinical arena, in vitro fertilization and regenerative medicine. In 1959, Anne moved to Edinburgh with Donald and their family, where she joined C.H. Waddington’s ARC Animal Genetics Unit. Anne collabo- rated further with Donald, despite their divorce, but greatly expanded her own interests in mouse reproductive biol- ogy, establishing much of the now- standard base of knowledge for this species. Her dynamic approach ex- tended beyond the specific findings of embryology. In 1960, the Polish em- bryologist A.K. Tarkowski discovered that two mouse embryos could be ag- gregated to generate a single chimeric conceptus and result in a single mo- saic mouse with four parents; Anne led the way to teaching the method to other scientists, obviating the diffi- culties of travel to and from the Soviet bloc and circumventing a developing monopoly on the chimera approach by a leading U.S. researcher. In addi- tion to her own numerous research contributions using the chimera ap- proach, in 1972 Anne published an in- fluential paper on the interpretation of chimera coat color patterns, correct- ing popular misconceptions on the number of founder cells in stem cell populations for the developing organ rudiments (McLaren, 1972). Thus, it became evident that Anne’s interests included both the factual substance of science and its political environ- ment, and her influence expanded far beyond her scientific publications. In 1974, Anne moved to University College London as director of a unit on mammalian development, which the Medical Research Council convened around her. There she continued her distinguished research career, further developing an interest in primordial germ cells that had seem- ingly arisen during her stud- ies of the inheritance pat- terns of mouse chimeras (McLaren et al., 1972; McLa- ren, 1975). In 1976, Anne summarized then-current concepts in early mamma- lian development in her sem- inal Mammalian Chimaeras, a diminutive monograph that, like its author, spoke volumes in a few carefully chosen words (McLaren, 1976). In her 18 years as unit director, Anne was to in- fluence the lives of countless junior researchers, develop- ing their careers as students, fellows, and group leaders. Her contributions to under- standing the mechanisms of mammalian sex determina- tion, using a combination of genetic and embryonic ap- proaches, laid the foundation for the definitive insights later gained by her colleagues. Typically, she minimized her Anne McLaren Cell Stem Cell 1, August 2007 ª2007 Elsevier Inc. 137