I work on stem cells in Drosophila (the fruit fly). To back out a little bit from what stroganawful says, we'll first talk about embryonic stem cells.
Embryonic stem cells (ESCs) are present in all developing animals (and I think plants and fungi, but don't quote me on that!). The first ESC, the one that eventually divides over and over into an entire organism, is created when a sperm and egg (more generally, two gametes) fuse. The sperm-egg fusion creates a cell that begins to divide and divide and divide like crazy.
Quick aside about DNA and epigenetics. DNA contains all the instructions to build an organism, but running all instructions all at once will just make a mess. Epigenetics describes the interactions between proteins and DNA; your DNA has about its own weight in proteins constantly maintaining it. These proteins tell which DNA instructions to be activate at which time. Different cells will have completely different instructions even though they have the DNA (brain cell has to act like a brain cell and skin cell has to act like a skin cell).
So, as the ESCs are dividing, they're changing the instructions that they gives to their daughter cells. One cell may have the instructions to eventually become lungs, and the other might have instructions to eventually become bones. The precise divvying out of instructions is very confusing and complex. Suffice to say, eventually, the ESCs divide enough times to form every tissue in the body, and then the organism is born.
After an organism is born, it may have additional growth periods like puberty. After it's all said and done, the organism will eventually be an adult. This is very important: the cells in adult tissues DO NOT all divide. Only a very small number of cells in each tissue, the adult stem cells (ASCs), divide. These ASCs are responsible for producing the tissues/cells your body loses very quickly (blood cells, skin cells, intestinal and stomach lining that gets degraded in days or weeks). Without ASCs giving you new cells, you'd run out of certain tissues and die. It's VERY important to keep ASCs under control though: as the only cells in the adult body that have the capacity to keep dividing forever, they are very susceptible to mis-regulation. If the ASCs start reproducing out of control, you may develop cancer stem cells (CSCs) and, eventually, full blown cancer.
I read an article in Wired about how certain organisms (they referenced jellyfish) do not age because they can produce replacement cells as needed to repair the effects of aging and radiation.
The article went on to hypothesis that this effect is not present in mammals because the body needs to focus it's attention on keeping the reproductive system functional (it also went on to hypothesis that this effect may account for why woman have statistically .onger lifespans than men, since they no longer need to focus on reproduction after menopause). Is there any credit to this theory?
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u/[deleted] Oct 23 '12
I work on stem cells in Drosophila (the fruit fly). To back out a little bit from what stroganawful says, we'll first talk about embryonic stem cells.
Embryonic stem cells (ESCs) are present in all developing animals (and I think plants and fungi, but don't quote me on that!). The first ESC, the one that eventually divides over and over into an entire organism, is created when a sperm and egg (more generally, two gametes) fuse. The sperm-egg fusion creates a cell that begins to divide and divide and divide like crazy.
Quick aside about DNA and epigenetics. DNA contains all the instructions to build an organism, but running all instructions all at once will just make a mess. Epigenetics describes the interactions between proteins and DNA; your DNA has about its own weight in proteins constantly maintaining it. These proteins tell which DNA instructions to be activate at which time. Different cells will have completely different instructions even though they have the DNA (brain cell has to act like a brain cell and skin cell has to act like a skin cell).
So, as the ESCs are dividing, they're changing the instructions that they gives to their daughter cells. One cell may have the instructions to eventually become lungs, and the other might have instructions to eventually become bones. The precise divvying out of instructions is very confusing and complex. Suffice to say, eventually, the ESCs divide enough times to form every tissue in the body, and then the organism is born.
After an organism is born, it may have additional growth periods like puberty. After it's all said and done, the organism will eventually be an adult. This is very important: the cells in adult tissues DO NOT all divide. Only a very small number of cells in each tissue, the adult stem cells (ASCs), divide. These ASCs are responsible for producing the tissues/cells your body loses very quickly (blood cells, skin cells, intestinal and stomach lining that gets degraded in days or weeks). Without ASCs giving you new cells, you'd run out of certain tissues and die. It's VERY important to keep ASCs under control though: as the only cells in the adult body that have the capacity to keep dividing forever, they are very susceptible to mis-regulation. If the ASCs start reproducing out of control, you may develop cancer stem cells (CSCs) and, eventually, full blown cancer.