Dying to Make Life
As you read these words, hold out your hand. Notice the space between your fingers, the fragments of MacBook you can see through these windows. Think about how these digits formed, precisely arrayed and extended. Imagine them sprouting from the nub of your wrist. Imagine your entire body swelling and branching out from a single fertilized egg.
Now stop imagining this and consider an alternative: your conception as a block of flesh the size of a refrigerator, dropped off by a stork as big as a killer whale. From this template, pieces slough off or are chiseled away, and the sculpted remnant is you.
The reality of development is some amalgamation of these fantasies. The process of growth takes a single cell and turns it into trillions, but a developing organism is always subtracting as well. The final shape of our fingers and toes are the result of subtraction, function carved into an initially featureless mass.
This fact of our development means that cells know how to die. We can identify two types of cell death, called necrosis and apoptosis.
Necrosis is cell death induced by some kind of external trauma, perhaps a virus infecting, replicating, and bursting out of a cell, or a simple physical blow.
Apoptosis, on the other hand, is a cellular self-destruction mechanism, the willful result of a careful calculation. Like all cellular information systems, its complexity exceeds our understanding, but headway is being made here at UCSF.
While apoptosis has long been known to carve out tissues and organs during the later stages of development, work done in the UCSF laboratories of Dr. Scott Oakes and Dr. Robert Blelloch has shown that apoptosis also regulates development at its very earliest stages.
Their recent paper, led by graduate student Eric Wang and published in the December 7 edition of Current Biology, shows that apoptosis has a critical function in embryonic stem cells (ESCs), each of which has the ability to generate every cell in the body.
Technological advancements over the last few decades have allowed researchers to culture embryonic stem cells from many organisms, including mice, primates, and humans. Not only can scientists keep them alive in a laboratory dish, but - by the addition of different chemicals and proteins - they can push these cells to differentiate into a variety of cell types. A few examples include neurons, the heart muscle cells known as cardiomyocytes and the keratinocytes that form the fabric of your skin.
This differentiation process takes the same core cell and adds any number of functional costumes. The process both remodels how the cell looks and reconfigures the operations it can perform, like training a doctor and giving her a white coat in the same instant.
To study whether apoptosis plays a role in early embryonic stem cell differentiation, Wang and company removed two genes essential in the apoptotic pathway, Bax and Bak, from mouse embryonic stem cells, and then pushed the cells to differentiate with retinoic acid, a very powerful inducer of differentiation with the ability to turn on or off over 500 genes.
When removing Bak and Bax, ESCs were delayed in their differentiation. This suggested that apoptosis is critical for embryonic stem cells to efficiently differentiate.
The researchers looked at normal (i.e. still containing Bax and Bak) mouse ESCs and saw that differentiation triggered apoptosis in about 12% of the cells. So when a group of ESCs is triggered to differentiate, most of them will, but a certain subset decides to self-destruct instead. When this self-destruction mechanism is removed, differentiation is delayed.
Wang et al hypothesized that the slowly differentiating cells are the ones to get consciously removed, like cutting straggling runners from a varsity cross-country team. Indeed, a critical experiment in the Wang study suggested exactly this.
Mouse ESCs were modified to have a fluorescently tagged version of a protein called Oct4, which is present in ESCs, but is quickly lost when cells differentiate. This tool allowed the researchers to locate the stragglers when the whole group was pushed to differentiate.
Compared to the rest of the pack, the fluorescent stragglers were preferentially removing themselves through apoptosis, which can be visualized with other fluorescent markers. The slow cells also displayed higher levels of another protein that is a marker for DNA damage.
It is possible that DNA damage leads to a delay in differentiation (i.e. it is the cause of the slowness), and that these cells are killed off since they are damaged. Conversely, it is also possible that a delay in differentiation causes DNA damage, which then triggers apoptosis. Either way, the slowly differentiating and damaged cells get expunged, ensuring that the embryo develops in a timely and uniform manner.
We are noticing cell death to be an integral part of the creation of an organism. It is not just a carving tool for adding in the finer details, but also a checkpoint that ensures rapid and efficient development.
Death is not just part of life, but somehow is life, an essential component of the cooperation and complex orchestrations that are so beautiful at all scales.