Fetal Development

[leftrightleftrightleft]

I know this is going to sound like an evolution-bashing intelligent-design thread but I'm honestly just curious and I don't have some secret ulterior motive to jump out and say "Ah ha! Gotcha!" I'm really just curious.

Lets say you're looking at a fetus that's a few weeks old. Its already composed of millions of cells. It has a crude head, the rough formation of the spine and a few stubs for legs and arms.

Now, there is going to be a cell (Cell A) somewhere near the spine that has to start forming a nerve that will one day allow the fetus to feel a soft breeze on its right elbow (when its all grown up). And there is another cell (Cell B) near the spine that has to start forming a nerve that will one day allow the fetus to flex its right bicep.

What information is in Cell A that is uniquely different from Cell B that causes one cell to divide and become one thing and the other to become another different thing?

Logically, there must be something uniquely different between the two cells in order for them to divide and develop into unique parts and forms. If they were the same (physically identical and also containing the same information), then I don't see how they could logically become two different things over time (except via random mutation which wouldn't result in the complex organization of organs and large-scale structures).

Where did this uniqueness originate?

Isn't all the DNA in the fetus genetically identical (since it initially came from a sperm and egg source)? And doesn't DNA contain the genetic information for coding the division and growth in fetal development?

Where is the uniqueness in the information contained in cells and where does it originate?


(Apologies if this is a simple question, I am no biologist….far from it).

 

[Loudmouth]

I know this is going to sound like an evolution-bashing intelligent-design thread but I'm honestly just curious and I don't have some secret ulterior motive to jump out and say "Ah ha! Gotcha!" I'm really just curious.

Lets say you're looking at a fetus that's a few weeks old. Its already composed of millions of cells. It has a crude head, the rough formation of the spine and a few stubs for legs and arms.

Now, there is going to be a cell (Cell A) somewhere near the spine that has to start forming a nerve that will one day allow the fetus to feel a soft breeze on its right elbow (when its all grown up). And there is another cell (Cell B) near the spine that has to start forming a nerve that will one day allow the fetus to flex its right bicep.

What information is in Cell A that is uniquely different from Cell B that causes one cell to divide and become one thing and the other to become another different thing?

Logically, there must be something uniquely different between the two cells in order for them to divide and develop into unique parts and forms. If they were the same (physically identical and also containing the same information), then I don't see how they could logically become two different things over time.

Where did this uniqueness originate?

Isn't all the DNA in the fetus genetically identical (since it initially came from a sperm and egg source)? And doesn't DNA contain the genetic information for coding the division and growth in fetal development?

Where is the uniqueness in the information contained in cells and where does it originate?


(Apologies if this is a simple question, I am no biologist….far from it).

All cells have the same DNA. The difference is which parts of the genome are expressed. Cells interact with each other by the proteins on their surface and with chemicals/proteins that they release. This causes cells around them to express different genes, and ultimaly producing different cell types and tissues.

You might think of it as a cookbook. When you make a meal you don't cook everything in the cookbook. You only use certain recipes. This is what cells do. They each have a full cookbook, but they only make certain courses or certain side dishes.

 

[leftrightleftrightleft]

All cells have the same DNA. The difference is which parts of the genome are expressed. Cells interact with each other by the proteins on their surface and with chemicals/proteins that they release. This causes cells around them to express different genes, and ultimaly producing different cell types and tissues.

But where does the uniqueness come from? How does one cell express one piece of DNA while another expresses a different part? Where do the unique proteins come from that tell one cell to do one thing and another set of unique proteins to tell another cell to do another thing?

You start with one cell and it divides into two cells. Are the two remaining cells unique or not? And if not, where did the non-uniqueness come from?

You might think of it as a cookbook. When you make a meal you don't cook everything in the cookbook. You only use certain recipes. This is what cells do. They each have a full cookbook, but they only make certain courses or certain side dishes.

But how does one cell know to make one side dish and another cell to make a different side dish if they both have the same cookbook?

I thought DNA was where the genetic information was stored, but in your analogy, the cells actually require another piece of outside information beyond the cookbook.

This analogy doesn't answer the question of uniqueness because in this example we have an intelligent agent deciding what recipe to make. The cookbook itself does not provide the information about which recipe to make.

 

[andy b]

thats an interesting point in a sports sense who is the ref

 

[leftrightleftrightleft]

No biologists on the forum? No geneticists?

 

[jacks]

These are great questions, but its possible no one has the answer.

 

[Mystman]

There are all kinds of clever feedback mechanisms that help with this:
https://en.wikipedia.org/wiki/Cell_differentiation

Cells are never completely identical. Even if they have the exact same genes, the number of proteins that they have can differ, because their production, degradation, activation, etc all have an element of chance: the insides of cells are not fixed factories with production lines. Instead, a lot of the elements are randomly bouncing around, and stuff only gets done when the right components meet up.

Positive feedback loops can amplify these small differences between cells, and lead to cell differentiation.

As an example, I'm currently studying the role of a positive feedback loop in blood vessel formation, where there are two types of cells "tip cells" and "stalk cells". At the start, all the cells are the same. Then, a growth factor called VEGF binds to a receptor (VEGFR) to tell the cells they need to become active. This leads to the production of a signalling protein called Dll4. And here is the crucial part: Dll4 is stuck to the cell's own membrane, and can bind to receptor called Notch on the membrane of the neighbouring cell. And finally: if Notch is activated in this way, it slows down the production of VEGFR.

What does this accomplish? It makes a positive feedback loop. At the start, all the cells are activated by approximately equal VEGF, which will activate approximately equal VEGFR, which will produce approximately equal Dll4, which will activate approximately equal Notch...

..but because the Notch activation in one cell will be just a bit more, that cell will end up with just a bit less VEGFR. And less VEGFR means that it can produce less Dll4, so that it won't activate as much Notch in the neighbour. And if the neighbour has less activated Notch, it can produce MORE VEGFR, and more Dll4, and activate more Notch in the first cell, which leads to the first cell being able to active less Notch in the neighbour cell, which leads to the neighbour cell being able to active more Notch in the first cell, etc.

You end up with a situation where you get an alternating pattern of cells, one having a lot of Notch-activation, and the other having very little. If activated Notch then also has the "side effect" of making a cell into a "stalk cell" by upregulating or downregulating the activity of other proteins (it does!), cell differentiation has been accomplished.

 

[Dizredux]

A lot of it is epigenetics which is a bit too complex to present in a forum context.

A book I enjoyed on the subject is The Epigenetics Revolution by Nessa Carey.

Written to be accessible to an interested layperson and well recommended.

Dizredux

 

[leftrightleftrightleft]

There are all kinds of clever feedback mechanisms that help with this:
https://en.wikipedia.org/wiki/Cell_differentiation

Cells are never completely identical. Even if they have the exact same genes, the number of proteins that they have can differ, because their production, degradation, activation, etc all have an element of chance: the insides of cells are not fixed factories with production lines. Instead, a lot of the elements are randomly bouncing around, and stuff only gets done when the right components meet up.

Positive feedback loops can amplify these small differences between cells, and lead to cell differentiation.

As an example, I'm currently studying the role of a positive feedback loop in blood vessel formation, where there are two types of cells "tip cells" and "stalk cells". At the start, all the cells are the same. Then, a growth factor called VEGF binds to a receptor (VEGFR) to tell the cells they need to become active. This leads to the production of a signalling protein called Dll4. And here is the crucial part: Dll4 is stuck to the cell's own membrane, and can bind to receptor called Notch on the membrane of the neighbouring cell. And finally: if Notch is activated in this way, it slows down the production of VEGFR.

What does this accomplish? It makes a positive feedback loop. At the start, all the cells are activated by approximately equal VEGF, which will activate approximately equal VEGFR, which will produce approximately equal Dll4, which will activate approximately equal Notch...

..but because the Notch activation in one cell will be just a bit more, that cell will end up with just a bit less VEGFR. And less VEGFR means that it can produce less Dll4, so that it won't activate as much Notch in the neighbour. And if the neighbour has less activated Notch, it can produce MORE VEGFR, and more Dll4, and activate more Notch in the first cell, which leads to the first cell being able to active less Notch in the neighbour cell, which leads to the neighbour cell being able to active more Notch in the first cell, etc.

You end up with a situation where you get an alternating pattern of cells, one having a lot of Notch-activation, and the other having very little. If activated Notch then also has the "side effect" of making a cell into a "stalk cell" by upregulating or downregulating the activity of other proteins (it does!), cell differentiation has been accomplished.

I recognize you probably know a good deal more about this than me, but could you explain exactly how any of this explains blood vessel formation?

To me, you've explained how a "stalk" cell is created. You haven't explained how a "tip" cell is made (unless a "tip" cell is just the other set of cells in the alternating pattern between tip and stalk).

You also haven't explained how large scale structures like a large artery is differentiated from a small capillary. This alternating pattern of tips and stalks sounds very organized and nice, but how does it make anything other than a big blob of alternating tips and stalks?

Also, where does the growth factor (VEGF) come from to start the whole process?

Furthermore, how does the positive feedback loop stop?

 

[leftrightleftrightleft]

A lot of it is epigenetics which is a bit too complex to present in a forum context.

A book I enjoyed on the subject is The Epigenetics Revolution by Nessa Carey.

Written to be accessible to an interested layperson and well recommended.

Dizredux

Thanks! I will look into the book

 

[Mystman]

I recognize you probably know a good deal more about this than me, but could you explain exactly how any of this explains blood vessel formation?

It was just an (perhaps poorly chosen and explained -_-) example of a feedback loop that can result in cell differentiation. Blood vessel formation requires a number of other mechanisms, some pretty well described and some still up for debate. I don't know if you have access to scientific literature, but any recent review of angiogenesis should tell you everything I know about the subject.

But to answer your questions..

Tip cells and stalk cells are both types of endothelial cells. VEGF-activated endothelial cells that aren't Notch-inhibited to become stalk cells default to tip cells, yes.

VEGF exists in different forms and can come from different sources, but the classic example is a relatively nearby tissue that has a lack of oxygen. The tissue will secrete VEGF, the VEGF will diffuse, and the endothelial cells will want to grow towards the higher VEGF concentration, thus eventually forming a new blood vessel close to the tissue and supplying it with oxygen.

The positive feedback loop is probably stopped when one of the necessary components can't be produced any faster, or when a regulatory system kicks in. Many cellular component also have negative feedback loops: if there is too much of protein X, it will slow down its own production.

 

[Loudmouth]

But where does the uniqueness come from? How does one cell express one piece of DNA while another expresses a different part? Where do the unique proteins come from that tell one cell to do one thing and another set of unique proteins to tell another cell to do another thing?

That would require me to explain the entire field of developmental biology, which is usually covered in massive textbooks.

If this is something that you are interested in then I would urge you to pay a visit to our local library and find some books on the subject.

I thought DNA was where the genetic information was stored, but in your analogy, the cells actually require another piece of outside information beyond the cookbook.

Yes. Cell to cell communication is extremely important in embryonic development.

This analogy doesn't answer the question of uniqueness because in this example we have an intelligent agent deciding what recipe to make. The cookbook itself does not provide the information about which recipe to make.

All analogies break down at some point.