The human body is a complex organism. It consists of trillions of cells interacting with each other producing life as we know it.
One of the more complex questions for biologists has been just how many cells are present. You would think this would be a straightforward question to answer - just take a body and count each individual cell - but the number of cells is prohibitive.
You could just measure the volume of a cell and then divide that into the volume of a typical human body to get a number but which cell would you use?
Based on the volume of a red blood cell, the answer would be 276 trillion cells but using a skin cell you would get only 15 trillion.
Alternatively you could weigh cells but the same problem arises - different cell-types are not all the same size or weight.
In 2013, researchers reporting in the Annals of Human Biology conducted a cell-type by cell-type analysis of all the major organs and tissue types in the human body using existing literature. With a 70 kg male as their standard, they concluded the body has 37.2 trillion cells (plus or minus 0.81 trillion) with the caveat cell counts might vary for individuals depending upon various factors such as health and age.
By any measure, 37.2 trillion is a lot of cells. Each communicates with its neighbour through chemical compounds. Each receives nutrients and oxygen via the circulatory system. Each has a role to play in the overall community.
Recognizing we are a community or collective organism is starting to take hold in some circles as it has implications for disease as well as other aspects of our nature. In particular, we also carry within us a separate community of other organisms. It is estimated the ratio between our cells and those of the bacteria, archaea, fungi, and viruses co-habiting our bodies is around one to 1.3. That is, we are host to somewhere close to 50 trillion other organisms.
One of the implications of this communal structure is only about 50 per cent of the DNA in our bodies is "ours" although our DNA dominates any other single species of organism. The plethora of organisms of different species means our bodies are "ours" but increasingly scientists are recognizing our bodies and all of these other organisms do talk. This has led to philosophical discussions about what constitutes a collective organism such as ourselves.
It has also generated discussions about the nature of the interaction between our cells and our guests. In 1885, Louis Pasteur correctly speculated animals lacking in bacteria would die. Our relationship is one of mutual benefit. Our microbiome, as it is called, provides us with a wide variety of useful compounds while we provide it with shelter and raw materials.
For example, it is estimated there are upwards of 10 trillion organisms living in our digestive tract. They breakdown complex molecules into simpler, more readily absorbed compounds. They modify proteins, fats, and carbohydrates into other useful compounds. They do a lot of the "grunt" work in providing us with a nutritious diet. And to some extent, they protect us by attacking foreign organisms within our food. It is a mutually beneficial relationship.
But the influence of the microbiome may not stop there. Studies seem to indicate a connection between our gut microbes and mental health. In 2004, Nobuyuki Sudo's group at Kyushu University reported microbe-free mice reacted more strongly to stress than regular mice. While this paper didn't initially garner a lot of attention, it is now viewed as the start of a new field of research into the gut-brain connection.
In 2009, neurobiologist John Cryan reported that when young rat pups were separated from their mothers, the stress could lead to long-term changes in their microbiome. Further studies on mice showed that giving the animals the pro-biotic bacterium Lactobacillus rhamnosus JB-1 resulted in both a decrease in stress levels and changes in brain chemistry.
Much of this research has focused on the vagus nerve which is a superhighway for information between the brain and the gut, among other regions of the body. When the vagus nerve is severed in mice and they are given L. rhamnosus JB-1, changes in stress levels and brain chemistry didn't occur.
Similarly other diseases may have a brain-gut connection. For example, epidemiological studies suggest if a mom is obese, there is a higher possibility of the offspring developing autism. And there may be other gut-organ connections. A recent study has demonstrated an increase in the frequency of heart attacks among women who have accumulated two months of time on antibiotics. The risk for women over 60 doubles compared to the control group, albeit from three in 1,000 to six in 1,000.
There is much we still don't know about the complex human body and all its inhabitants.
