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cells that can act as sperm but are not, strictly speaking, sperm. This makes a difference: the sperm ‘actors’ are basically little packages of DNA, and those sorts of packages can be extracted easily and directly from an infertile man’s testes. There’s little point in going through all the trouble of making sperm if they can’t do more than that.

Sheffield University is home to one of the few labs around the world where artificial sperm is being manufactured properly. As part of this quest, Dr Allan Pacey, a male fertility specialist at the university, looks at semen on a day-to-day basis. In a very small room with a folding bed – installed for research purposes – Pacey explained the rigorous quality tests that are being used to understand what makes great sperm. In addition to the bed, Pacey’s lab keeps a supply of porn magazines in its efforts to collect semen samples. Two thousand men from the Sheffield area have donated sperm to the study, which has verified that a count of twenty million sperm per one millilitre of semen is the norm for a fertile man – and that some men’s semen contains no sperm whatsoever.

In the lab next door to the bedroom, a student scientist was busy with a microscope. Images of three or four sperm were projected from the microscope on to a computer screen. Seen this way, the sperm appeared huge and robust – around ten thousand times their true size. Normal sperm contain just one set of DNA, and all of these checked out as normal; none of them displayed the characteristic signs of carrying two sets: a grossly enlarged head or a head split in two. The day I visited, the lab was focused on something quite basic to reproductive function, and central to making artificial sperm that do more than serve as a container for DNA: they were measuring the length of normal sperms’ tails.

The definition of a ‘normal’ tail length is not yet understood. Though the length of a sperm’s tail affects how efficiently the sperm swims to the egg, one millilitre of a man’s ejaculate will contain sperm displaying a huge variety of different tail lengths. So what length should an artificial sperm’s tail be in order to have all the functionality of a real sperm? Making sperm, after all, is not just a case of making a cell with half as much DNA as the rest of our cells. ‘What you are trying to replicate in the lab is not just a case of taking a cell and letting it divide. A skin cell could easily be made like that,’ Pacey explains. To be a ‘true’ sperm, the cell has to move, and that requires a certain size and shape. In contrast, eggs, which start off round and remain that way, may be easier to construct. Pacey admits that, in theory, any cell might be altered to carry one set of DNA and be used to fertilize an egg in vitro, but that, he says, does not make a sperm or allow for natural conception using an artificial cell.

Another issue involves replicating how sperm develop in the body. Sperm do not develop in one spot but evolve progressively more mature forms as they move through the tubes of the testicles. It is in the final stages of sperm development that the head and tail are ‘finished’ and which pose such a challenge to researchers in the lab. The Sertoli cells, also known as nurse cells, guide the proper development and maturing of sperm in the testes, no matter where they are. In fact, scientists have managed to insert immature human and rat sperm into mice testes and observe it mature there, alongside the mouse’s own sperm. Because human, mouse, and rat sperm look quite distinct, the researchers were able to confirm that nurse cells from one species can be used to mature sperm from another. In Pacey’s view, Sertoli cells are an exciting avenue for investigation – they offer a way to use something like an ‘artificial’ testis to produce real human sperm.

These challenges are not impossible to overcome, and chances are that one or more of the three labs based in the UK and Japan that are competing to solve the problem will do it within a generation. Scientists expect that in vitro-derived germ cells will be ready to test in clinical studies within thirty years, and possibly sooner.

Using in vitro-manufactured eggs and sperm could remove a significant issue that arises when a child is conceived from donor material: every bit of information that we can now glean from our genetics will be available to the parents to judge and assess and decide what to pass along to their children, including such things as whether or not they will be more susceptible to a disease than the general population. The egg and sperm banks that now offer their services often provide some information, particularly around superficial characteristics such as a donor’s appearance and perceived intelligence: hair colour, eye colour, skin colour, and educational attainment. One solo parent specifically said that she had chosen a sperm bank because it released information about the donors’ looks, character, and health. But often, people do not know their own family medical history – a mother or grandmother may have died long before the BRCA1 gene could set her body on a path towards full-blown breast cancer. An artificial egg or sperm, in comparison, would come with a full genetic profile.

Using lab-made germ cells might also make us think more biologically about the family. When a person or a couple turns to a donor’s egg or sperm, there’s a niggling sense that biology has some lurking trump card to play. Will the child’s genes harbour some undesirable disease or temperament? Will the child feel the