Thread: Cells

So we learned in class today that some organs and tissues etc. regenerate after a while. So we get a 'new' heart every X years, a 'new' arm every Y years. (this wasn't the case for the eyes and some other things). This because our cells split, creating new cells, and the old ones die after a while.

Now as I was wondering, if you have some chronic disease, and an infected cell splits, would the freshly created cells be healthy or would they still be infected as they have identically the same DNA as their mother cell?

A virus would spread to the daughter cells through the cytoplasm, or in the DNA if it's a retrovirus.

As for other chronic diseases, that is a broad category of diseases. Most aren't caused by viruses. You have things like arthritis and lupus which are chronic but are autoimmune diseases so the cells involved don't really matter.

Often in chronic diseases (not caused by an infectious agent like a virus) you actually are having degeneration of tissue either by inflammation/ immune reactions or by some other process obscuring cell survival/function. In an case, the opposite of what you are saying is happening, cells are dying, not reproducing.

How is this normal renewal cycle (which gives you mostly a "new" body about every seven years time AFAIK) different from organs that can re-grow themselves, like the liver for instance?

Normal repair is simply the slow duplication of existing cells and/or structures. Regeneration requires the complete re-growth of structures that have been lost - a differentiated cell can't just divide and replace itself, new cells have to differentiate correctly and in concert with the other cells in the structure.

So the exposed tissue simply revert back to unspecialized form, i.e stem cells? Or does it still retain a measure of specialization, albeit reduced, during the regeneration process? Do we know what triggers this in humans? I suppose this has already been looked at for neuron regeneration and such.

Well, the answer is - it depends. From Liver Regeneration:

Originally Posted by Fausto et al 2006

Regeneration Is Compensatory Growth of the Liver

In biological terms, regeneration means the reconstitution of a structure that has been excised, such as the complete re-growth of the limb of a newt, including skin, muscle, and digits. Regeneration of a lost limb starts with the formation of a blastema at the cut surface, which contains dedifferentiated cells with broad differentiation potential. (6) Liver “regeneration” after PH is a very different process, in which the excised parts do not grow back. Rather, the remaining liver expands in mass to compensate for lost tissue. Thus, liver regeneration is technically a process of compensatory growth rather than regeneration. As such, it does not follow the same general steps involved in true regenerative processes, and formation of a blastema containing dedifferentiated cells does not occur.

An important distinction must be made regarding the origin of the cells that replace missing hepatocytes after PH and in the growth processes that follow parenchymal cell necrosis.(7,8 ) After PH or CCl4-induced injury, liver mass is replenished by replication of existing hepatocytes, without activation of a progenitor cell compartment. (9) In the regeneration of the liver that follows loss of parenchymal-cells induced by other toxins, such as galactosamine, replication and differentiation of intrahepatic progenitor cells occurs. (10-13) The extent to which these cells contribute to regeneration varies according to the nature of the injury, doses of inducing agents, or other experimental variables. It is important to mention that we do not find any compelling evidence that bone marrow cells generate significant numbers of hepatocytes in in vivo physiological or pathological hepatic growth processes. (8 ) Conversely, cells originating in the bone marrow may generate 20% or more of the endothelial cells and other nonparenchymal cells (NPCs) during liver regeneration. (14)

In other words, most cases of liver regeneration are simply the increased duplication of existing liver cells; stem and/or dedifferentiated cells only become involved if the injury causes damage to specific parts of the liver. Bone marrow will also only contribute cells when the liver is recovering from physical damage, but not during normal growth or recovery growth from a bacteria or virus.

Cool, thanks :wink:

Wow
great answers, i guess my question is solved

Cells regenerate and completely replace "last year's cells" in organs and bones, etc. So why does a liver transplant recipient still need anti-rejection medications after a few years of regeneration? Is he somehow replicating the liver donor's DNA?

Originally Posted by prenderm

Cells regenerate and completely replace "last year's cells" in organs and bones, etc. So why does a liver transplant recipient still need anti-rejection medications after a few years of regeneration? Is he somehow replicating the liver donor's DNA?

In a sense, yes. The liver belongs to the donor, and when the cells in it replicate to replace themselves, that creates more cells with the donor DNA. It's not as though donor cells kick out their own DNA and suck up the DNA of the host.

OK. New cells are created to replace aging cells. But a liver transplant recipient produces those cells from his own native resources, does he not? Is the liver donor's organ reproducing the donor's DNA within the recipient? PS: I am a liver recipient and I find this perplexing.

Originally Posted by prenderm

OK. New cells are created to replace aging cells. But a liver transplant recipient produces those cells from his own native resources, does he not? Is the liver donor's organ reproducing the donor's DNA within the recipient? PS: I am a liver recipient and I find this perplexing.

Yes. It is somewhat the same as keeping the liver in a jar and feeding it with enough blood to sustain it. Regeneration means that a cell divides by mitosis (in two), so the original DNA of the mother cell is retained. As paralith indicated, this regenerative mitosis involves specialized cells, so only liver cells can make new liver cells in an adult. Since the only liver cells in your body are those of the donor, your liver will always have your donors DNA in it, as far as I understand.

Here is one fact that I learned in my molecular biology course and which I like to share [a little off topic]:

Abasic sites are well known DNA lesions that occur spontaneously via depurination at a frequency of ~10 000 per genome per day in mammalian cells

Source:Lindahl, T.; Nyberg, B. Rate of depurination of native deoxyribonucleic acid. Biochemistry. 1972;11:3610–3618.

Link to citation of source

Thats what the DNA repair system in our body has to deal with, in every cell every day. Its simply amazing how numerous the possibilities for error are.