Steen M. Willadsen, Ph.D., D.Sc. 

Steen Willadsen was born in 1943 in Copenhagen, Denmark. He graduated from the Royal Veterinary School of Copenhagen (1969), receiving a PhD in reproductive physiology (1973) for experiments on in vitro maturation and transfer of bovine follicular oocytes, and a Doctor of Science degree for experiments involving micromanipulation and in vivo culture of cleaving embryos of the large domestic species (1985).

Working at the British Agricultural Research Council's Unit of Reproductive Physiology and Biochemistry, at the Animal Research Station, Cambridge (subsequently a department of the ARC Institute of Animal Physiology), Steen developed methods for freezing sheep and cow late morulae and early blastocysts (the stages of development used in routine embryo transplantation) (1973 – 78). It was a procedure developed by Steen that was used to produce Zoe, the first baby from frozen embryo transfer in humans. 

Next, Steen became interested in micromanipulation of eggs and embryos, and devised a procedure for producing genetically identical animals, primarily identical twins in sheep, cattle, pigs, and horses, and for producing mammalian chimaeras, including interspecies chimaeras (~1976 – 83). These experiments were made possible by a procedure he invented for in vivo culture of cleavage stage embryos. Various micromanipulative procedures used in human embryology owe much to the techniques described by him. 

While still in Cambridge, Steen started working on nuclear transplantation (~ 1979 – 1985). By 1984 he had pieced together a procedure which he used to clone, first sheep embryos, and the following year, cattle embryos, by nuclear transplantation. The procedure involved fusion of cells from early embryos into enucleated unfertilized eggs. The key to the success of Steen’s method was the use of unfertilized eggs as nuclear recipients. The procedure was essentially the one used a decade later to produce Dolly the Sheep, although in the latter case, nuclei from a mature sheep, i.e. not from sheep embryos, were used for cloning.

These experiments, plus others which Steen conducted or took part in, opened up areas of mammalian embryology and reproductive physiology which until then had been virtually inaccessible for study or practical exploitation. 

Most of Steen’s ground-breaking work took place before the digital revolution, but the profound impact of his work and his many contributions to the field cannot be overstated.