About Giant Eggs & Double Yolks

This week’s gift from a neighbor’s chicken coop included one extra-large egg. After hard-boiling and cracking open we found a double yolk, which some say is a good omen on our wedding anniversary. The following conversation over a meal would be unlikely in most homes, but perfectly natural in ours.

“Have you ever seen a giant human egg?” I asked my wife, Lucinda Veeck. I have only had a few hundred eggs under my microscope, but Lucinda has examined tens of thousands over a long career in her IVF lab.

Chicken egg with double yolkShe said it happened once. It was obviously immature because there were two nuclei in a cell enclosed by a membranous ‘box’ (the ‘zona pellucida’). A normal egg contains one nucleus and it vanishes shortly before ripening at ovulation by ejecting a surplus set of chromosomes in a cytoplasmic bleb. This elimination is so a fertilizing sperm can add a matched set to restore the pair.

Now here’s the thing. Her giant egg was not a microscopic version of the cooked egg. If a hen bird ovulates two yolky eggs simultaneously, which happens occasionally, they are quickly bathed in albumen, then enclosed in a common membrane and a shell during their journey down the oviduct, a process that takes about 24 hours to laying. As the nuclei are in separate yolks, the outcome is a normal genetic makeup with two chicks hatching from the same egg, although the cramped space affects their viability . The closest parallel is when non-identical human twins are conceived after a double ovulation.

But the nuclei in Lucinda’s giant shared the same cell, so their DNA would be inherited together. Had it been mature for fertilization the embryo would likely have three sets of chromosomes (two female and one male), called digynic triploidy, and fail to develop.

We explored explanations for its origin. If you have seen densely packed eggs in biopsies of young human ovaries you might wonder how they manage to grow independently instead of being swayed by neighbors, like people jostled in a football crowd. Sometimes they lose autonomy. I have seen two or more eggs combined inside follicles of every species examined, up to 14 in dog ovaries, although some looked unhealthy from the competition.

When boxed inside their own membrane eggs can’t fuse to make giant eggs. They have unique genetic makeups, just like eggs from separate follicles that go forward to make non-identical twins. But what happens when they coexist in a ‘box’ and don’t fuse to form a single cell?

Lucinda saw an example five days after an IVF procedure. It was a double embryo at the blastocyst stage with about 64 cells each. If separated for implanting in the womb, it seemed likely they could make non-identical twins, but if they had originated from a fertilized egg that had split instead of from two separate eggs they would make identical twins. It’s possible that they could unite (or reunite) to make a singleton pregnancy, and, if originating from two fertilized eggs, the baby would inherit mixed genetic lineages, a known condition called chimerism.

In the interests of being (somewhat) intelligible, I avoided more abstruse explanations and outcomes. With so many ways that development can go awry, it is a marvel that we turn out well, or mostly, and I am thankful for my genesis otherwise you wouldn’t be reading this post. I usually avoid writing technical stuff, but correspondence is welcome from readers who like to crack eggs.

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Human Egg Farming

We called the ambitious program, ‘Egg Farming,’ over 30 years ago when I worked at Edinburgh University. That goal drew a step closer today as my graduate student from so many years ago published results of growing human eggs in vitro, to a blaze of publicity.

We started by growing mouse and rat eggs in Petri dishes. They were tiny cells nested in follicles that nurse their development, and some reached full size in a week and even ovulated a fertile egg into the culture medium. Our sole competitor in this new science was in the USA where John Eppig developed a more efficient method, and he called the first pup conceived from an egg grown in vitro, ‘Egbert.’ He was a strange and obese animal, and we worried if egg farming was too risky to apply in veterinary or human medicine, but many healthy mice were born later.

Human egg (oocyte). Courtesy: Lucinda Veeck Gosden

It seemed a huge leap from mouse to clinic. Human eggs grow more slowly in the ovary, and we expected the cultures would take weeks or even months and be risky. Progress was agonizingly slow because human ovarian tissue is rarely available for research, especially at the premenopausal ages required. Instead of relying on rodents, we needed intermediate species to forge better methods, and turned to sheep and cow ovaries. Visits to the local slaughterhouse for discarded tissue from butchered carcasses were some of the most unpleasant experiences ever, and almost turned me vegetarian from hearing the crack of the bolt-gun followed by a heavy slump. But they helped us to gain new knowledge when human tissue was unavailable.

A view of Edinburgh and its castle

Professor Telfer’s team in Edinburgh has succeeded with human eggs through careful work with clinical colleagues who asked consent from women to donate tiny slivers of ovarian tissue during C-section. The biopsy doesn’t harm them.

They watched eggs growing in dishes from the earliest stages to full size when a nuclear division occurred and signaled they were ready for fertilization. It is puzzling why the growth is much faster in vitro than in the body, though it is welcome if the eggs are no less competent for making babies. They look normal, but we know from experience how deceptive appearances can be. The only sure test is to fertilize eggs to see if they create healthy embryos, and that will require a special license under UK law.

Why is farming eggs worth the trouble and long struggle?

Progress will continue at a dawdling pace because a mature egg is the rarest cell in the body. For every egg ovulated each month, a man makes 3 billion sperm cells which offer researchers enough material for galloping discoveries in testes. Animals that fertilize eggs outside the body, like frogs and most other species, release as many eggs as they can make because the cost to them is small while superovulation maximizes the chance of making babies. But ovulation in mammals with internal fertilization and pregnancy is restrained by the limited accommodation of the uterus. Nature has arranged a bottle-neck in the egg production line to ensure that few eggs, or only one or at most two in humans, ripen at a time. The rest are consigned to the trash, which egg farming aims to rescue.

If more eggs can be ripened in vitro there will be benefits for animal breeding technology and young patients will have a better chance of preserving their fertility after freezing ovarian tissue during sterilizing treatment for cancer and other diseases. Egg farming will work better for child patients with their larger stores of eggs than in adults. Nearly a quarter-century ago, our first patient at an infant age had ovarian tissue stored while she was treated for Wilm’s tumor. The news from Edinburgh today was welcome for her and others with investments in the frozen ovary bank.

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A Third Parent contributes her Mighty Mites (Mitochondria)

Two biological parents are enough for any baby, aren’t they? Maybe not: sometimes it’s better to have two mamas.

The media have hyped a story that broke in September about a boy who was conceived by “mitochondrial replacement therapy.” His parents had lost two previous children to Leigh syndrome, a rare but severe condition affecting the brain, nerves and muscles caused by mutations in mitochondria, the mites that energize every cell inherited from our mothers’ egg (never from our fathers’ sperm). Less than 2% of the boy’s mitochondria were damaged, which was well below the 18% threshold for disease. An anonymous egg donor has given him the prospect of a full and healthy life.

Mitochondrial transfer
Thousands of tiny mitochondria fluorescing brightly in a human egg and its moon-like polar body

John Zhang, whom I knew when I worked in NYC, led the clinical team and is director of a fertility service that is popular there among Asian Americans. But he carried out the procedure in Mexico where, according to reports of his words, there are “no rules (for assisted reproduction).” That statement was as responsible for the media furor as the procedure itself, and he may regret playing into the polemics of conservatives at a time when Mexico is considering legislation for assisted reproduction. Ahem!

His “breakthough” probably put noses out of joint in a Newcastle team in my mother country because they have done the painstaking research work. The UK has often been at the forefront of reproductive technology (clinical IVF, Dolly the cloned sheep, etc.), and has a well-respected system for ethical review (HFEA) which, admittedly, delays the launch of new treatments such as this projected to help 10-20 British families annually. The Geordies should have been first, but were scooped.

Two methods exist for switching mitochondria when a prospective mother carries a harmful load of mutations in her mitochondria. Zhang chose the spindle method in which maternal DNA is transferred to replace the nuclear DNA removed from a healthy donor egg before it is fertilized with her husband’s sperm. Only one of the five embryos thrived in vitro, but after placing in the uterus it conceived a healthy pregnancy.

In the 1990s, a New Jersey clinic transferred small fractions of cytoplasm from donor eggs to improve the quality of eggs for patients who repeatedly failed with IVF treatment. None of the women were carrying mitochondrial mutations, but the clinical team had a hunch that eggs often fail to develop if they are under-energized, but can be boosted with an infusion of fresh mitochondria. After treatment resulting in 17 viable pregnancies with two of the girls missing an X chromosome (Turner’s syndrome), the FDA set a bar too high for the program to continue.

There are now reports from other countries of imminent births after mitochondrial transfer, and these too are said to aim at improving embryo quality instead of correcting a genetic fault. And in Boston a company is offering an expensive treatment (‘Augment’) for improving fertility by transferring mitochondria from a type of ovarian stem cell that some of us doubt exists (my recent critique is available on request).

These therapies have been called ‘eugenic,’ which is a word that makes us shiver, but if they are safe and effective and purely aim to restore a healthy population of mitochondria, need we worry? Some commentators have called the boy in Mexico a GMO baby, but that’s strictly inaccurate, because the mitochondrial genome containing a mere 37 genes and 0.0005% of the nuclear DNA was switched, not chemically altered. Moreover, there is no basis for concern about transmitting his new mixture of mitochondria to future children, although a girl treated likewise could and that possibility requires deeper study. We can’t be sure how her mitochondria would segregate in the bottleneck when eggs inherit tiny numbers of them, possibly creating a very different mix compared to her grandmother and donor, even the remote possibility of bringing back the former disease to her descendants.

Robert Hughes coined the expression Shock of the New for a TV documentary about modern art, but it is an apt expression for public responses to the ongoing revolution in fertility treatment. I imagine hands lifted in horror after every new development is announced, but it would be a hard person who condemns joyful parents by saying their healthy children “should never have been born!” The arrival of healthy babies quenched most opposition to IVF in the 1980s, and I predict the same for the next generation of therapies provided they meet the same standard of ethics and safety.

Human fertility has been said to be the prerogative of God or the gods. The Ancient Greeks had a hoard of myths, and woe betide gods or mortals who broke the reproductive code! Hera was the patron of marriage and childbirth, yet she covered the shame of being raped by Zeus by marrying him. She ate lettuce to improve her fertility, probably because its white sap resembles semen, which is something to think about next time you are served a salad.

One of her sons was Hephaestus. The virgin goddess Athena fled when he lusted after her (he was ugly), but spilled his seed on the ground, which caused Gaia, the Mother of the Earth, to conceive Erichthonius (“Eric” for shorthand). Athena made no biological contribution to Eric, but she was the object of desire and necessary for his conception, and afterwards she helped to bring him up. She was the third parent, and no less for that.

Eric looked so monstrous that he alarmed people and they tried to conceal him (depicted by Peter Paul Rubens). The Greeks were pondering extraordinary conceptions long before we added a third parent by  surrogate pregnancies with IVF or mitochondrial transfer. They posed the question whether a parent is defined by nature or nurture, but the answer is, of course, nature and nurture, and three parents can be better than two.

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