Like a Virgin (15 page)

For every cell in your body that isn’t an egg (for a woman) or a sperm (for a man), making a new cell is a simple case of copying the chromosomes, separating the copies into two lots, and
then distributing the original set and its copy equally into two new cells. This process is called
mitosis.
Think of it as making a photocopy of some pages: you separate the original pages
from the copied pages, keep the originals for yourself, and give the photostats to a colleague.

Weismann realized that a different division must also occur in order to make a sex cell. Whatever number of chromosomes there were in the original cells, these would need to be halved, resulting
in that sex cell with only one set of chromosomes. He had discovered
meiosis
, a process that only ever happens in eggs and sperm, and the thing that makes sex exciting, in evolutionary
terms; meiosis ups the ante in the grand gamble of reproduction. The word ‘meiosis’ is derived from the Greek for ‘diminution’, because, as Van Beneden and Weismann observed
through their microscopes, duplicated DNA from a sex cell that is dividing is diminished by half in the new cells that are produced. (The devices available to them were not quite sophisticated
enough to demonstrate that, in reality, meiosis achieves far more than that.)

Whereas most cells of our body contain one maternal chromosome and one paternal chromosome, each copied as precisely as possible, an egg or sperm must contain only one chromosome strand, and the
copies of the chromosomes that end up in the egg or sperm are not simple duplicates of the strand in other cells. The process of meiosis physically shuffles and
exchanges
information between the two chromosome strands. To do this, the double helix of the chromosomes breaks, and the broken ends physically move across each other, swapping genes before the double helix
re-forms. This is a much greater challenge than the usual process of cell division, because genes must be matched, sorted, scrambled, redistributed, and realigned. From this, a unique combination
of genes is born. It is different to the gene combination found in either parent, different to the one inherited in every other body cell, and peculiar to the offspring that may be created when
this sexual cell fuses with a mate’s. It is for this reason that no two children born to the same parents, unless they are identical twins developed from a lone fertilized egg, are
genetically the same.

At the end of this complicated process, it is not just how the chromosomes are divided up that makes the egg especially unique. Inside the egg, there is also a cellular ‘soup’, which
separates into two grossly unequal parts. The disproportionately smaller of the two parts helps to reduce the number of chromosomes until only one set of the two is left. Ultimately, that smaller
part degenerates while the larger one sticks around to become the egg, ready and waiting to be fertilized.

This rudimentary, immature egg will then undergo a second meiosis, just as complicated as the first. Another unequal division of a cellular soup produces a tiny cell and a fully mature egg. Like
the last one, this tiny cell is usually destroyed, but not always. In the fruit fly
Drosophila melanogaster
, this tiny cell sometimes ‘fertilizes’ the larger, mature egg to
create virgin-born fly offspring, essentially using a part of the egg to stand in for sperm. In humans (and almost all vertebrate animals), right in the middle of this second meiosis, the egg stops
dividing and enters a biological holding period, known as
prophase I
, in which it can remain for an extraordinarily long time. In frogs, this phase can last several years; in humans, several
decades.

When a girl enters puberty and starts ovulating, the egg will resume its monthly meiosis. But there is another catch: the egg will be blocked from maturing further or
transforming into an embryo until and unless some sperm show up. This block on development is dramatically named
metaphase II arrest
. Eggs need to be activated to start their dividing, and
activation usually happens with fertilization – the fusion of sperm and egg. At least, that is, when things are proceeding normally.

Moment by moment in the course of your life, cells in your body are dying off. Before they do so, they divide and give rise to ‘replacement’ cells just like
themselves – a skin cell divides into two new skin cells; a liver cell into two liver cells – which is how the body doesn’t dissolve into non-existence. But when eggs divide, they
can give rise to every cell type that exists in the adult, creating, over a series of cell divisions, a complete new individual – sometimes, in a matter of days. No other cell can match this
feat.

A beautifully orchestrated concert ensues in an animal egg after fertilization occurs. Like a pool bursting with the elegant and energetic motions of a team of synchronized swimmers, molecules
interact and cells cluster and move around to the very spot where they will be called upon to shape a new creature in early, miniature formation. Soon (in humans, about fifteen days later), the
early embryo organizes itself from a simple ball of cells into an organism made up into what are called
germ layers
: the
ectoderm
(the ‘outer skin’ in Greek), the
mesoderm
(the ‘middle skin’), and the
endoderm
(the ‘inner skin’) in all vertebrates. These skins are literally the layers that build us, and are responsible
for forming all the structures and organs present in a fully
developed animal body. It is now that a recognizable body plan begins to be laid out.

The endoderm, the innermost of the three layers, forms a simple tube, which will eventually become the digestive tract, connecting the mouth to the anus. The tube will differentiate into parts
as diverse as the pharynx, which helps us to speak; the oesophagus, the ‘entrance for eating’; the trachea, or windpipe; the salivary glands; the liver; the pancreas and certain glands
of the pancreatic system; and even the lungs – the respiratory and digestive systems being intricately connected. The mesoderm gives rise to the muscular and fibrous tissues – the
muscles, including the heart; connective tissues, cartilage, bones, bone marrow, blood, and the epithelia that line the blood vessels; the lymphatic vessels and lymphoid tissues; the reproductive
organs and the urinary system; and the notochord, a column of tissue that bisects the embryo into left and right sides, and which later develops into the vertebral column. The ectoderm becomes the
brain and spinal cord, via a process in which a part of the layer rolls up into a tube and pinches itself off from the rest. As it pinches off, some ectodermal cells escape into the mesoderm, where
they form part of the nervous system as well as the pigment cells of the skin. The rest of the ectoderm envelops the embryo with what will become the epidermis – the outer layer of our skin
– complete with sweat glands, hair, nails, and teeth.

An egg trying to make all this stuff on its own is up against a number of natural obstacles. For one, an egg has only one set of DNA, but its offspring requires at least two, to get that right
number of chromosomes. Second, to start the process of separating, copying, and dividing up its chromosomes, the egg needs some centrioles – barrel-shaped cellular structures, provided by
sperm, that help to move the chromosomes around during cell division. Third, at some point along the way to becoming an embryo, the egg will face the roadblock of metaphase II arrest.
And for mammalian and marsupial eggs, there is a fourth challenge: evolution has locked some genes so they just won’t work for creating offspring. Still, some human eggs have
gone solo – or perhaps it’s more precise to say that they have gone rogue.

The main evidence for the human egg’s capacity to develop on its own comes from
teratomas
– shocking, grotesque cell masses that appear to be an amalgamation of unfinished or
discarded body parts. Mature teratomas are a rare form of benign tumour made up of varying combinations of ectoderm, mesoderm, and endoderm tissues. They have been documented in guinea pigs, dogs,
cats, horses, marmosets, rhesus monkeys, baboons, and humans. Some teratomas are smooth, shiny balls of skin; others, a bloody fur ball of hair; yet others a lump of raw flesh spiked with perfectly
formed teeth. Often, under their skins, they also contain organ systems and major body parts. It may not come as a surprise, then, that
teratoma
comes from the Greek for ‘monstrous
tumour’.

Ovarian teratomas, which grow from egg cells, have been identified in girls as young as two and women as old as eighty-eight, but they mostly tend to develop in women in their twenties or
thirties, or ‘late’ reproductive age. Studies of twins indicate that the propensity to develop ovarian teratomas may be inherited. These teratomas are quite distinct from the more
highly developed growths known as
fetus-in-fetu
– malformed, parasitic twins that grow inside a living person’s body. Fetus-in-fetu are the product of normal conception, while
ovarian teratomas come from eggs that have never had a whiff of a sperm cell. Essentially, they are unfertilized eggs that didn’t or couldn’t respond to the signals to stop and restart
developing – the usual holding periods involved in readying an egg for reproduction.

The vast majority of ovarian teratomas recorded in humans have gone so far as to develop such features as skin, hair, and
teeth. In twenty-four known cases, ovarian
teratomas have contained a homunculus – a mini-human, or partial, foetus-like structure, something straight out of Paracelsus. The Latin term
homunculus
roughly translates as ‘a
structure resembling a miniature human body’ and today is used by doctors to describe a growth of tissue that has the features of a human being but which was not produced by pregnancy.

In 2002, a twenty-three-year-old woman was admitted to the Korean University hospital with a huge lump in her pelvic area. It was soft to the touch, and it moved around when prodded. The patient
had never been pregnant and had regular periods; her womb and her Fallopian tubes were normal. Doctors performed an ultrasound and found that, in fact, the woman had two lumps, one in each ovary.
The masses were removed and dissected. Both had smooth, glistening surfaces and measured about fifteen centimetres in diameter. One of the lumps contained another, smaller cyst, filled with hair
intermixed with a greasy yellow substance and some fluid. The other lump was more surprising. Encased in a tortuous network of blood vessels, the doctors uncovered another, smaller growth. There
were no muscles, ligaments, or organs inside the homunculus, but from the outside it looked eerily like a tiny, dismembered baby, lying face down, with only a hirsute head and part of its right arm
formed. The head was partially split open, and from it spilled a herniated brain. X-rays revealed an imperfect but impressively well-crafted skull, shaped somewhat like a cross between a Spartan
and a Samurai helmet. The skull bones included easily recognizable structures, including a cranium and a jawbone.

The following year, in 2003, Japanese doctors operating on a twenty-five-year-old virgin identified the most fully formed teratoma found to date. Once again the outer layer of the tumour was
filled with a mixture of hair and fat. Cutting
through this mess of cells, the woman’s doctors found a solid, hard lump. When the lump was cleaned up, the doctors could
see that it was a small, ‘doll-like’ body, mostly complete. Like any normal foetus, the body was covered with fine, downy hair, but the homunculus was unmistakably deformed. It appeared
to have spina bifida, a condition in which the ectoderm doesn’t quite finish rolling up into the spinal column (the name is Latin for ‘split spine’). Its head exhibited
malformations normally seen in babies with holoprosencephalia, which occurs when the forebrain of the embryo fails to divide fully into two normal hemispheres. In the centre of the forehead was a
single soft, spherical, fluid-filled ‘eye’ cloaked by thick, long eyelashes – a disorder named cyclopia, for the one-eyed Cyclops of Greek mythology. This strange foetus had one
ear, all its limbs, a brain, a spinal nerve, intestines, bones, and blood vessels – even a jaw, already ruptured by several teeth, emerging from beneath the skin. Paradoxically, it also had
what looked like a phallus, positioned neatly between its legs.