 We are made up of more than 35 trillion cells. They have their differences, some are brain cells, some are skin cells, others are intestinal cells. But despite these differences, they have a common lineage since they all came from one single cell. The process by which one cell makes 35 trillion cells that do everything from digesting food to killing pathogens and creating memories is a complex one. But understanding this process is helping scientists treat disease. One example is diabetes, which affects about 10% of the US population. Scientists at the Harvard Stem Cell Institute are making diabetes and metabolic disease a primary research focus. Diabetes is caused by the loss of functional beta cells. Beta cells are found in the pancreas and are responsible for controlling blood sugar levels. The body needs a certain level of blood sugar to stay healthy. Here's how it works. When we eat the carbohydrates or sugars, enter the blood and this activates beta cells to start producing insulin. Insulin is a hormone that allows your body to make use of and store sugar. In doing so, it maintains a balanced blood sugar level. Beta cells respond to blood sugar levels similar to a thermostat. When the temperature gets uncomfortably high, the air conditioner turns on to bring the room back to a normal temperature. When blood sugar levels get too high, beta cells turn on and produce insulin, which will reduce blood sugar levels. In type 1 diabetes, beta cells are destroyed by the body's immune system. In type 2 diabetes, beta cells become dysfunctional and cannot produce enough insulin. In both cases, the loss of functional beta cells lead to elevations in blood sugar levels that damages organs and if untreated can lead to death. At present, diabetics inject insulin with a syringe or pump, but providing insulin by that means is nowhere near as exact and effective as having functional beta cells. To treat diabetes, scientists are trying to replace the missing or dysfunctional beta cells. To do this, they have turned back the clock to the time when we were all single cells to understand how beta cells are naturally produced. The development of beta cells in the embryo involves the coordinated activation of a very specific set of genes in a very specific sequence. Scientists have been able to map out these genes and have been using this information to create their own beta cells from stem cells. By replicating exactly what happens in normal development of the embryo, scientists are creating beta cells in a petri dish. And these new beta cells are functional. They can produce insulin in response to blood sugar levels. Scientists are now solving a new challenge. How do we introduce these cells into the body and keep them alive? One challenge is to protect them from the immune system, which is responsible for killing beta cells in type 1 diabetes. One possible solution is to use encapsulation or modify the surface of the cells to make them invisible to the immune system. Scientists also want to make sure they can produce enough viable beta cells to make sufficient insulin when the body needs it. These are the next set of challenges on our road to treating and maybe even curing diabetes. Scientists at the Harvard Stem Cell Institute have been working for more than a decade on these challenges and are developing more effective techniques to use stem cells to cure diabetes. But we wouldn't be here without the deep knowledge of how that one single cell produces 35 trillion other ones that make up a living human being. Observing how normal development happens helps scientists produce not only beta cells but many other cell types relevant for all kinds of diseases by creating brain cells, liver cells, or lung cells.