Decomposition begins several minutes after death, with a process called autolysis, or self-digestion. Soon after the heart stops beating, cells become deprived of oxygen, and their acidity increases as the toxic by-products of chemical reactions begin to accumulate inside them. Enzymes start to digest cell membranes and then leak out as the cells break down. This usually begins in the liver, which is enriched in enzymes, and in the brain, which has high water content; eventually, though, all other tissues and organs begin to break down in this way. Damaged blood cells spill out of broken vessels and, aided by gravity, settle in the capillaries and small veins, discolouring the skin.
Body temperature also begins to drop, until it has acclimatised to its surroundings. Then, rigor mortis – the stiffness of death – sets in, starting in the eyelids, jaw and neck muscles, before working its way into the trunk and then the limbs. In life, muscle cells contracts and relax due to the actions of two filamentous proteins, called actin and myosin, which slide along each other. After death, the cells are depleted of their energy source, and the protein filaments become locked in place. This causes the muscles to become rigid, and locks the joints.
During the early stages of decomposition, the cadaveric ecosystem consists mostly of the bacteria that live in and on the human body. Our bodies host huge numbers of bacteria, with every one of its surfaces and corners providing a habitat for a specialised microbial community. By far the largest of these communities resides in the gut, which is home to trillions of bacteria of hundreds or perhaps thousands of different species.
The so-called gut microbiome is one of the hottest research topics in biology at the moment. Some researchers are convinced that gut bacteria play essential roles in human health and disease, but we still know very little about our make-up of these mysterious microbial passengers, let alone about how they might influence our bodily functions.
We know even less about what happens to the microbiome after a person dies, but pioneering research published in the past few years has provided some much needed details.
Most internal organs are devoid of microbes when we are alive. Soon after death, however, the immune system stops working, leaving them to spread throughout the body freely. This usually begins in the gut, at the junction between the small and large intestines. Left unchecked, our gut bacteria begin to digest the intestines, and then the surrounding tissues, from the inside out, using the chemical cocktail that leaks out of damaged cells as a food source. Then they invade the capillaries of the digestive system and lymph nodes, spreading first to the liver and spleen, then into the heart and brain.
Once self-digestion is under way and bacteria have started to escape from the gastrointestinal tract, putrefaction begins. This is molecular death – the break down of soft tissues even further, into gases, liquids and salts. It is already under way at the earlier stages of decomposition, but really gets going when anaerobic bacteria get in on the act.
Putrefaction is associated with a marked shift from aerobic bacterial species, which require oxygen to grow, to anaerobic ones, which do not. These then feed on the body tissues, fermenting the sugars in them to produce gaseous by-products such as methane, hydrogen sulphide and ammonia, which accumulate within the body, inflating (or ‘bloating’) the abdomen and sometimes other body parts, too.
This causes further discoloration of the body. As damaged blood cells continue to leak from disintegrating vessels, anaerobic convert haemoglobin molecules, which once carried oxygen around the body, into sulfhaemoglobin. The presence of this molecule in settled blood gives skin the marbled, greenish-black appearance characteristic of a body undergoing active decomposition.
As the gas pressure continues to build up inside the body, it causes blisters to appear all over the skin surface, and then loosening, followed by ‘slippage,’ of large sheets of skin, which remain barely attached to the deteriorating frame underneath. Eventually, the gases and liquefied tissues purge from the body, usually leaking from the anus and other orifices, and often also from ripped skin in other parts of the body. Sometimes, the pressure is so great that the abdomen bursts open.
Bloating is often used a marker for the transition between early and later stages of decomposition, and another recent study shows that this transition is characterised by a distinct shift in the composition of cadaveric bacteria.
When a decomposing body starts to purge, it becomes fully exposed to its surroundings. At this stage, microbial and insect activity reaches its peak, and the cadaveric ecosystem really comes into its own, becoming a ‘hub’ not only for insects and microbes, but also by vultures and scavengers, as well as meat-eating animals.
Two species closely linked with decomposition are blowflies, flesh flies and their larvae. Cadavers give off a foul, sickly-sweet odour, made up of a complex cocktail of volatile compounds, whose ingredients change as decomposition progresses. Blowflies detect the smell using specialised smell receptors, then land on the cadaver and lay its eggs in orifices and open wounds.
Each fly deposits around 250 eggs, that hatch within 24 hours, giving rise to small first-stage maggots. These feed on the rotting flesh and then molt into larger maggots, which feed for several hours before molting again. After feeding some more, these yet larger, and now fattened, maggots wriggle away from the body. Then they pupate and transform into adult flies, and the cycle repeats over and again, until there’s nothing left for them to feed on.
Under the right conditions, an actively decaying body will have large numbers of stage-three maggots feeding on it. This “maggot mass� generates a lot of heat, raising the inside temperature by more than 10°C. Like penguins huddling, individual maggots within the mass are constantly on the move. But whereas penguins huddle to keep warm, maggots in the mass move around to stay cool.
The presence of blowflies attracts predators such as skin beetles, mites, ants, wasps, and spiders, to the cadaver, which then feed on or parasitize their eggs and larvae. Vultures and other scavengers, as well as other, large meat-eating animals, may also descend upon the body.
In the absence of scavengers though, it is the maggots that are responsible for removal of the soft tissues. Carl Linnaeus, who devised the system by which scientists name species, noted in 1767 that “three flies could consume a horse cadaver as rapidly as a lion.� Third-stage maggots will move away from a cadaver in large numbers, often following the same route. Their activity is so rigorous that their migration paths may be seen after decomposition is finished, as deep furrows in the soil emanating from the cadaver.
Bodies are, after all, merely forms of energy, trapped in lumps of matter waiting to be released into the wider universe. In life, our bodies expend energy keeping their countless atoms locked in highly organized configurations, staying composed.
According to the laws of thermodynamics, energy cannot be created or destroyed, only converted from one form to another, and the amount of free energy always increases. In other words, things fall apart, converting their mass to energy while doing so. Decomposition is one final, morbid reminder that all matter in the universe must follow these fundamental laws. It breaks us down, equilibrating our bodily matter with its surroundings, and recycling it so that other living things can put it to use.
https://www.theguardian.com/science/neu ... fter-death
