The Modern Laboratory · Room 4 of 8

Room 4

Day five

Our embryo becomes a blastocyst, and we meet the people who first helped it get there in a dish.

By the fifth day our embryo has changed beyond recognition. The little cluster of cells has become a hollow ball filled with fluid, called a blastocyst. Look closely and you can see two kinds of cell. Gathered in a small knot on one side is the inner cell mass, the cells from which the baby will grow. All around the outside runs a thin wall, the trophectoderm, which will help to form the placenta. The whole ball still sits inside its protective shell, the zona pellucida. As the blastocyst grows it swells and stretches that shell, and then it begins to squeeze its way out. This is called hatching, and only a hatched blastocyst can settle into the lining of the womb.

Three people, and a short letter to Nature. Long before day five became part of everyday IVF, Patrick Steptoe, Robert Edwards and Jean Purdy wanted to know whether a human embryo made in the laboratory could keep on growing. In January 1971 the three of them reported in Nature that it could: human embryos, fertilised outside the body, had grown in culture to become blastocysts. It was one more patient step on the long road that led to Louise Brown in 1978.

Counting the cells. In 1989 Kate Hardy, Alan Handyside and Robert Winston at Hammersmith Hospital in London looked closely at human blastocysts grown in culture, staining the inner cell mass and the outer wall in different colours so that each could be counted. Of the normally fertilised embryos they followed, 42 percent reached the blastocyst stage on day five or six, and a newly expanded blastocyst on day five held around 58 cells. Two years later Virginia Bolton and her colleagues at King's College School of Medicine and Dentistry in London reported three births, one of them twins, after transferring embryos on day five.

Yves Ménézo gave embryos company. In the early 1970s the French biologist Yves Ménézo studied the fluids of the oviduct and the womb, and in 1976 he described a synthetic medium for culturing eggs and embryos. Working in Lyon, he then found that embryos grew far better when they shared their dish with a living layer of helper cells. In 1990 he and his colleagues placed spare embryos, judged too poor in quality to freeze, onto a layer of kidney-derived cells of a kind already trusted for making vaccines. After five days, 61 percent had become blastocysts, compared with 3 percent of those cultured in medium alone, and many were expanding and hatching.

Co-culture, as it was called, allowed Ménézo and his team to transfer embryos on day five, and by 1992 to freeze blastocysts and achieve pregnancies after thawing them. The Lyon team saw pregnancy rates rise among patients who had faced repeated failed transfers, though they asked, as good scientists do, for a proper randomised trial to be sure.

Knowing when to move on. Co-culture was a bridge. Once scientists understood what a growing embryo needs, as we saw in Room 3, simpler, fully defined media could do the same work without helper cells. In 1998 Ménézo himself, with colleagues, wrote that it was time to switch from co-culture to sequential defined media for transfer at the blastocyst stage. So today our embryo reaches day five in a clear, carefully balanced medium, ready at last to hatch.