Q & A - Two

Q (Anonick) : Why do different cells in the body function differently? What governs which stem cells become neurons and which become RBC's?

On the onset, let me say that it would be impossible to go through all the aspects of this mechanism..so I'll just provide a snapshot. Also, the answer has been simplified for better understanding.

Now what is the difference between a brain cell and the liver cell? Both of them have originated from the same initial cell fertilized by the parent gametes and both have the same DNA as all the cells in the body. The difference lies in the fact that some parts of their DNA are silenced by proteins known as transcription factors (TF's). Let's say in the brain cells, the TFs prevent the liver proteins from being expressed.

Brain and liver cells could be said to be examples of terminally differentiated cells. They are committed to their function and under normal circumstances, they cannot revert back to the original stem cell form. But how is this terminal differentiation achieved?

Let us begin from the embryo-a mass of few cells. At the embroyonal stage, the central problem is that of alignment. In the nematode worm, this is achieved by asymmetric divisions. The asymmetry originates with a cue from the egg's environment: the sperm entry point defines the posterior pole of the elongated egg which will in future become the posterior of the worm.

Another fundamental aspect of cell differentiation is a morphogen. Morphogens are chemical inducers. Most often, as in the fruit fly Drosophila, they make chemical gradients. Keep in mind here that the morphogens diffuse from a localized source. Cells at different distances from the source are driven to behave in a variety of different ways, according to the signal concentration that they experience. Morphogens typically act through binding to specific protein receptors present on cells. This triggers biochemical pathways, ultimately leading to production of TF's which silence DNA.

A couple of points to bear while imagining this:
1) A morphogen typically acts over a distance of 1 mm which directs choices between no more than a handful of cells. But the organs that eventually develop and much larger and complex than these. Here's where sequential induction kicks in:

2) Signals such as these play a huge part in controlling development, but it would be wrong to imagine that every change needs an inductive trigger. Sometimes, the cell will step through it's development program even without any trigger. For example, in a mouse neural stem cells divide and generate neurons for just 11 cycles after which they stop. Further, at different stages in this program, different neurons are generated.

Well, that's about it. I tried to be non-technical.
Happy cell dividing!

Anne

References: 'Molecular Biology Of The Cell, 4th Edition' Alberts et al.
Wikipedia