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Sperm, eggs and embryos: how do we define them as science offers new possibilities?

18 December 2017
By Dr Avi Lerner
Research Associate, Institute of Reproductive and Developmental Biology, Imperial College London
Appeared in BioNews 931

The Progress Educational Trust (PET)'s Annual Conference 'Crossing Frontiers: Moving the Boundaries of Human Reproduction' discussed some of the most important ethical and scientific questions facing human reproduction. The first session, chaired by Sarah Norcross, the Director of PET, tackled the very fundamentals. What is a sperm? What is an egg? And what is an embryo?

Opening the first session, Allan Pacey - Trustee of PET and Professor of Andrology at the University of Sheffield - reminisced about the first time he pondered 'what makes a sperm a sperm?'. It was back in 2009 when Professor Karim Nayernia, then working at Newcastle University, announced he had produced human sperm from stem cells in the laboratory. Professor Pacey went on to describe the key elements that make up a sperm, illustrating some of the cell's complexity through scanning electron micrographs that showed in incredible detail the '9+2 arrangement' of microtubules that exists in the tail.

One thing that he made clear was the importance of separating 'payload' - ie, the DNA - from the vehicle. He was wary of putting too much emphasis on sperm morphology or even the ability to swim when defining a sperm. Many members of the animal kingdom have wildly different sperm morphologies and every day during ICSI (intracytoplasmic sperm injection), sperm perform their function without the need to swim. In fact, with the increasing complexity of assisted conception, the sperm is required to do less and less. Professor Pacey concluded that at a bare minimum, a sperm requires a healthy haploid genome, a pair of centrioles to help the chromosomes divide and a smattering of calcium to trigger fertilisation and embryo development.

Moving onto the female gamete, the second speaker, Professor Richard Anderson at the University of Edinburgh, gave a counterpart talk on 'What is an egg? And what makes a good egg?'. In contrast to sperm, he explained, we have known for over three hundred years that it is all about the egg and that essentially all an egg really needs to do is provide a vehicle for fertilisation and subsequent development.

One of the biggest issues that eggs have, according to Professor Anderson, is the early commitment they make to their final fates. The business of laying down your store of eggs so early in life is something that really differentiates the female reproductive system from the male reproductive system. He pointed out that 'before a woman had even felt her future daughter give her a first kick, her daughter's germ cells have stopped replicating and entered meiosis' and how strikingly few eggs you have when you want to use them compared with when you were 18 weeks old.

Professor Anderson then explained that alongside quantity, is the important issue of quality. Keeping a cell in near-suspended animation until they are ready to be released, meet the sperm and turn into a baby is a really difficult thing to do. This has all become more of a problem as society has dramatically changed over the past forty years with women having children at increasingly older ages. Concluding, Professor Anderson, summarised that overall, being a good egg in the 21st century is a real challenge!

The third and final talk in this session was by Dr Sue Avery, Trustee of PET and Director of the Birmingham Women's Fertility Centre, who brought the first two talks together to speak about 'how to make an embryo'. She started with an historical perspective that illustrated just how little we knew about human embryology back in the 1980s but then with the invention of IVF, we started to become very aware of what embryos look like. She stressed that after the birth of Louise Brown, it was really important to understand what we were doing biologically and legally, as new legal frameworks were coming into play.

While early definitions of an embryo revolved around fertilisation, this became more complicated, as in 2001 arrived the possibility of a human embryo developing by some process other than fertilisation. This led to some confusion in the law, Dr Avery explained, as these embryos no longer fell under the Human Fertilisation and Embryology Act 1990 and were potentially unregulated. Dr Avery pointed out it often seemed 'the law was always running to keep up with the science'.

We have now arrived at even more ways to make a human embryo. Entities like hybrid embryos are not humans according to the Act. Being 'slightly' human doesn't count. Dr Avery concluded that we need to be slightly less wary about interfering with human development and that a little bit of tinkering may be in order.

The discussion that followed started with a question from the floor asking, especially in light of artificial gametes, what exactly Professor Pacey meant regarding a 'healthy haploid genome'? Professor Pacey replied that the challenge is how we define that? How do we measure that? And how do you measure it in a non-destructive way? The discussion went on to mention the animal kingdom and their different reproductive systems, sperm quality and sexual monogamy as well as issues of law and legal terminology. The session finished with Sarah Norcross asking who is going to win in the great egg and sperm race? The answer it seems, is that artificial gametes, at least in some form, have already been created.

PET would like to thank the sponsors of its conference - the Anne McLaren Memorial Trust Fund, the Edwards and Steptoe Research Trust Fund, the ART Institute of Washington, Ferring Pharmaceuticals, the London Women's Clinic and Vitrolife.

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