by Sue Black
Every single minute about 300 million of our cells will die, many of which are simply replaced. Our bodies are programmed to know which cells to replace, and when and how, and by and large they just get on with it. Each cell, tissue type and organ has its own life expectancy, which is managed like stock turnover in a supermarket based on a ‘best before’ date. Somewhat ironically, those with the shortest shelf life are the ones that start it all off: sperm survive for only three to five days after formation. Skin cells live for a mere two to three weeks and red blood cells only three or four months. Not surprisingly, there is greater longevity in the tissues and the organs. The liver takes a full year to replace all its cells and the skeleton almost fifteen years.
The charming myth that, because we replace so many of our cells on a regular basis, every decade or so we become an entirely new physical person is, sadly, misguided. No doubt it has its roots in the Theseus paradox – the question of whether an object that has had all of its components replaced at some stage remains fundamentally the same object. Can you imagine how we might be able to play with this fantastical concept in a court of law? Picture a wily old defence barrister holding forth in a murder trial. ‘But Your Honour, my client’s wife died fifteen years ago, so even if the person he once was killed her, he is no longer physically the same person because every cell in his body has now died and been replaced. The man before you could not have been at the crime scene because he did not exist.’
I don’t believe such an argument has yet been rehearsed in court, but I would dearly love to be the Crown witness if ever it is. It would be fun to engage in these metaphysical musings with a barrister. It does, though, raise a question: just how much alteration can a biological entity sustain while remaining recognisable as the same individual and maintaining its traceable identity? Take the physical changes witnessed in the late Michael Jackson over the years. Much of the child star of the Jackson Five was almost unrecognisable in the adult he became, but there would have been other components that persisted throughout his life and which continued to anchor his physical identity. Our job is to find such components.
There are at least four cell types in our bodies that are never replaced and which can live to be as old as we are – technically even longer, in the case of those formed before we are born. Perhaps these cells might be cited as the unlikely seat of our corporeal biological constancy to confound that tricky defence lawyer’s argument. The four permanent cell types are the neurons in our nervous system, a tiny little area of bone at the base of our skull called the otic capsule, the enamel in our teeth and the lenses in our eyes. Teeth and lenses are only semi-permanent as they can be removed and substituted by modern dentistry or surgery respectively without harming the host. The other two are immovable and therefore truly permanent, remaining locked in our bodies as irrefutable evidence of our biological identity from before our birth until after our death.
Our neurons, or nerve cells, are formed in the very early months of embryonic development and by the time we are born we will have as many as we are going to have for the rest of our lives. Their axons, which resemble long, extending arms, branch out like a motorway system, conveying traffic from north to south and vice versa. They carry motor commands south from the brain to our muscles and sensory information in the opposite direction from our skin and other receptors. The longest are those that transmit pain and other sensations along the full length of the body, from the tip of the little toe, all the way up through the foot, leg and thigh, up the spine and the brain stem and on to the sensory cortex of the brain at the top of the head. If you are six feet tall, each single neuron on that pathway may be close to seven feet long. So when we stub our little toe on the wardrobe, the message takes a moment to reach our brains, which is why we may have a pain-free split second before we feel the ‘ouch’ we know is coming.
It is the persistence of these cells within the brain that gives rise to the interesting question of whether there is an aspect of our identity to be found there. It is possible that the pattern of communication between them could be mapped to show how we think and how the higher functions of reasoning and memory come about. Recent research has demonstrated that with the help of a fluorescent protein we can now see a memory being formed at the single synapse level. Practical application may yet be a little too much science fiction for us to embrace fully, although I am tempted to predict that an understanding of the key role neurons may play in establishing identity might not be so very far away.
The second seat of cellular permanency is in the otic capsule, situated in the very depths of the skull around the inner ear. This is part of the petrous temporal bone, which houses the cochlea, the organ of hearing, and the semi-circular canals responsible for balance. As the inner ear forms in the embryo and fetus, it does so to full adult size immediately and remains insulated against growth and remodelling through the production of high levels of osteoprotegerin (OPG), a basic glycoprotein that suppresses bone turnover. It does not, in normal circumstances, remodel because if it were permitted to grow, it would interfere with the intricacies of our hearing and balance. Even though the otic region is already adult-sized in newborn babies, it is very, very small, representing, in volumetric terms, only around 200 microlitres – about the size of four raindrops. Unlike the neurons, the cells locked in this little bone already offer us opportunities to recover information about individual identity.
To understand what value any cell may have in the process of human identification we need to know how they form – be they bone cells, muscle cells or those that line the gut. At its most basic level, every cell in our body is comprised of chemicals. Their formation, survival and replication are dependent on a supply of elemental building blocks, an energy source to bind them together and keep them alive and a waste-disposal outlet for their by-products. The main opening in our bodies by which the building blocks for future cellular construction can enter is the mouth, leading on to the stomach and the gut system – our food-processing factory. So the core components of every single cell, tissue and organ can be obtained only from what we ingest. We are, literally, what we eat. Refuelling, then, is vital to survival, and the maxim that we cannot survive without air for more than three minutes, water for more than three days and food for more than three weeks, while not entirely accurate, is pretty close to the truth.
In utero, before we can ingest food independently, we source our fuel from our mother’s diet via the placenta and umbilical cord to enable us to go about the business of developing and organising our own cellular construction. While it is a fallacy that a pregnant woman eats for two, she does need to ensure that her diet is sufficient to meet not only her own needs but also those of a very demanding passenger.
The nutrient building blocks required to construct our otic capsule were supplied by Mum from what she was eating around sixteen weeks into her pregnancy. So within our head, in that minute piece of bone just big enough to hold four raindrops, we will perhaps carry for the rest of our lives the elemental signature of what our mother had for lunch when she was four months pregnant. Proof, if any were needed, that our mums never leave us, and a whole new perspective on the mystery of how they manage to get inside our heads.
We believe we have a cosmopolitan diet but in reality, much of our water and food intake is very local to where we live. As water percolates through various geological formations, it will take up isotope ratios of elements specific to that location and when we ingest it, its signature will be transferred into the chemical make-up of all our tissues.
The chemical composition of our tooth enamel remains largely unchanged throughout life, which is why decaying teeth cannot repair themselves. The crowns of all our deciduous teeth (milk or baby teeth) are formed before we are born and their composition is therefore also directly associated with maternal diet, as is that of the crown of our first adult molar. The rest of our permanent teeth are made by us and reflect our diet during childhood.
As well as our �
��permanent’ tissues, hair and nails are rich sources of information about diet as their structure is laid down in a linear fashion and they grow at a relatively regular rate. They provide a potential chemical timeline for the deposition of metabolised ingested nutrients that can be read almost like a barcode.
So how can forensic anthropologists use this amazing information offered by our cells to unlock a part of an individual’s life story and help confirm their identity? Stable isotope analysis is a good example of one of the scientific techniques that can assist us. The ratio of carbon-and nitrogen-stable isotopes in our tissues may tell us something about diet: whether a person was a carnivore, a pescatarian or a vegetarian. The oxygen isotope ratios may reveal more about the source of water in the diet, and from the stable isotope signature associated with water, we may be able to deduce where they have been living.
If you move to another geographical area, the signature you lay down will alter because of changes in the chemical composition of the food you eat and the water you drink. Analysis of hair and nails can produce a sequential timeline for geographical relocation. This can be extremely useful in trying to identify an unknown deceased person, or to track the movements of criminals. A terrorist suspect, say, who is falsely insisting he has never left the UK may be found out by a sudden change in hair stable isotope ratios that now conforms to a signature you’d expect to find in Afghanistan. Hair analysis can also tell us about persistent consumption of a variety of substances, including drugs such as heroin, cocaine and methamphetamine. And it was, of course, the favoured method of proving arsenic poisoning in Victorian murder mysteries.
So we could, in theory, look at the remains of an individual and, from the isotopic signatures in the otic capsule and first molar, discover where in the world their mother was living when she was pregnant with them and the nature of her diet. We could then analyse the remainder of the adult teeth to establish where the deceased person had grown up, and then the rest of their bones to determine where they had lived for the past fifteen years or so. Finally, we could use their hair and nails to locate where they spent the last years or months of their life.
The complexity of managing the human cell mass is staggering. As a cell factory, the body works incredibly smoothly most of the time when we are at the peak of our fitness, efficiently replacing the majority of those 300 million cells we lose every minute. But as we grow older and degeneration sets in, we become less able to produce new cells. Signs of ageing start to appear: hair becomes thinner and loses its colour, eyesight fades, skin wrinkles and stretches, muscle mass and tone are lost, memory and fertility decline.
These are all evidence of a normal slowing process, or senescence, and clear indicators that we are now probably nearer to the end of our life than its beginning. Being told by your doctor that a condition you have is normal for your age is of little comfort when you realise that death is also normal for your age. To compound the problem of ageing, some cells may ‘go rogue’ and start to grow and replicate abnormally, tissues damaged by environmental toxins or an abusive lifestyle may cease to function and organs under stress may stop operating effectively. We can extend the longevity of many of our body functions through medical or surgical intervention and pharmacological support, but in the end, when they cannot continue unassisted, we, the organism, will die.
According to one medico-legal definition, organismal death occurs when ‘the individual has sustained either irreversible cessation of circulatory and respiratory functions or irreversible cessation of all functions of the entire brain, including the brain stem’. The word ‘irreversible’ is key. Reversing the irreversible is viewed by the medical community as the holy grail of combating death.
It seems that the activities of those five vital organs define our life and therefore perhaps ultimately our death. The wonders of modern medicine make it possible for us to transplant four of them: heart, lungs, liver and kidneys. But the ‘big one’, the brain – the fundamental command control centre for every other organ, tissue and cell in our body – has never been successfully replaced. The pact between life and death seems to lie in those neurons (I told you they were special).
◊
Our bodies change not only throughout our life, but also in death. As the processes associated with organismal and cellular deconstruction begin, so we start to break down into the chemical components that were used to build us in the first place. There is an army of volunteers waiting to lend a hand, including the 100 trillion bacteria within the human biome, no longer restrained by an active immune system. Once the dynamic of the environment shifts catastrophically against the likelihood of successful revival or resuscitation of the organism, the bacteria can take over. Death is now confirmed by the fact that life cannot come back.
In most cases, for example, when we die at home with our family around us, or in hospital, or with emergency services in attendance, recording a reliable time of death is relatively straightforward. However, when someone dies alone, or a body is discovered unexpectedly in possibly suspicious circumstances, we must estimate the time and date of death to fulfil the legal and medical requirements. We try to establish a time death interval (TDI) from information that the body releases. So forensic anthropologists need to understand not only how the body is built, but how it deconstructs.
There are seven recognisable stages of postmortem alteration. The first is pallor mortis (literally, ‘paleness of death’), which starts within minutes and remains visible for about an hour. It’s what we are referring to when we say that someone who isn’t well looks like ‘death warmed up’. As the heart ceases to beat, capillary action is halted and blood drains away from the skin surface and begins to settle at the lowest gravitational level within the body. As it happens so early in the postmortem process, this pallor is of very little value in establishing a TDI. It is also a subjective feature and therefore difficult to quantify with any degree of certainty.
The second stage, algor mortis (‘coldness of death’, or the death chill) follows quickly as the body starts to cool down (in some instances it may warm up, depending on the ambient temperature). The temperature of the body is best recorded from a rectal reading as the skin surface will generally cool, or heat up, more quickly than the deeper tissues. Although the rate of cooling recorded for rectal temperature is relatively constant, it cannot be assumed that the temperature of the body was normal at death. Many factors can influence core temperature, including age, weight, illnesses and medication. Certain infections or drug reactions can raise it, as can exercise or extensive struggling before death; lower than normal readings may be attributable to other physical states such as deep sleep. So this is not an infallible indicator of the TDI.
When someone is found dead, the environment in which their body is discovered will also affect the rate of body cooling. For example, in a location warmer than 37°C a body will not cool and so a calculated TDI based on temperature will be irrelevant. Obviously, if a person has been dead for some time, assessing algor mortis will also be irrelevant as the body temperature will eventually adjust to the ambient temperature.
Within a few hours of death, muscles start to contract and the third and temporary postmortem condition, rigor mortis (‘stiffness of death’), will set in. Rigor starts in the smaller muscles first, usually inside five hours, and then spreads to the larger muscles, peaking between twelve and twenty-four hours postmortem. When we die, the pump mechanism that keeps calcium ions out of the muscle cells ceases to operate and calcium floods across the cell membranes. This causes the actin and myosin filaments within muscle to contract and then lock, shortening the muscle. Because muscles cross joints, the joints may also contract and fix into a rigid position in the hours immediately after death. In due course, the rigid muscles will begin to relax due to natural decomposition and chemical alteration and so the joints, too, will become moveable. This is the explanation for the rare but recorded phenomenon of a dead body appearing to twitch or move. But I promise you,
the dead do not sit up and moan – that really is something that only happens in horror movies.
Signs of initial flaccidity, rigor and then secondary flaccidity can be used to assist with establishing a TDI but many variables will affect how long rigor lasts and indeed whether it occurs at all. For example, it is quite common for newborn babies and the elderly not to exhibit rigor. In higher temperatures onset is more rapid; lower temperatures delay it. Other factors that will have a bearing include some poisons (strychnine hastens the rigor process, whereas carbon monoxide slows it down). Rigor is also brought about more quickly by intense physical activity before death and may never take place in cases of cold-water drowning. So again, it is not an incontrovertible indicator of TDI, regardless of what may be said in television crime dramas.
With the heart no longer pumping, the body enters the fourth phase of postmortem alteration, livor mortis (‘blue colour of death’). The blood will have begun to pool at the lowest gravitational level of the body almost immediately after death, at the pallor mortis stage, but lividity may not be visible for a couple of hours.
The heavier red blood cells sink through the serum and accumulate in the lower gravitational regions. Eventually the skin here will turn a darker red or purple-blue colour as a result of the concentration of red blood cells, in striking contrast to the pallor of the skin at the higher levels. Where the skin comes into contact with a surface (for example, if the body is lying on its back), blood is pushed out of the tissues into adjacent areas where there is no contact pressure. So the contact areas will appear paler compared to the darker surrounding areas of lividity.
Maximum lividity is generally reached within twelve hours. The livor colouration then becomes fixed and can be a useful indicator in the investigation of suspicious deaths. It may reveal how a body was positioned in the hours immediately after death and help us to evaluate whether it might subsequently have been moved. A body with marks of lividity on its back, but found lying on its face, has clearly been turned over. If someone has died by hanging there will be a pooling of blood in the lower segments of all four limbs and this fixed-lividity pattern will remain in the distal arms and legs even after the body is cut down.