A very good question indeed. Related to the kind of neural cell (microglia) that I am working on, I would say that 'quiescence' in my context refers to a state of 'inactivation'. Upon stimulation, these cells immediately get 'activated' and respond to the stimuli. The cell type that I investigate also undergo 'senescence" which again refers to the state of these cells when they are morphologically and metabolically incapable of performing their usual functions and could be a leading contributor of pathogenesis.

I think that term quiescence is not so correct for cells undergoing death. To my opinion, quiescence mean dormancy in de-differentiated stage, when cell cycle have a very long G1 and G2 phase and under appropriate conditions were capable to re-enter cell cycle. In plants as example of quiescence we can consider QC in the root tissue.

Quiescent cells can undergo cell death or reenter cell cycle depending on the stimulating signal. I always associated quiescence with G0 not G1. So, cells growing in a plate don’t enter G0 until confluence I guess or maybe not. To describe quiescence as the cells whose fate is death is not completely accurate because even if quiescence applies also to terminally-differentiated cells, some of them still can de-differentiate to reenter the cell cycle (hepatocytes, etc...). How long G0 is is not an easy question. Sometime it is not easy to maintain cells in G0 (in their differentiated state). Depending on the cell type, there are markers which tell you if the cell has entered G1 for example by the activation of some early genes which are under the control of the E2F transcription factor family.

I agree with Abdelhalim: quiescence is something different than cell per preparing to death. I think it is important to look on chromatin structure. In stem cells (which we can consider as quiescent) chromatin structure is different form one in the terminally differentiated one. Differentiated cells continue to perform their main functions, but for re-enter cell cycle back require significant re-programming. In contrary to this, quiescent cell do not require significant re-programming, only exit from dormancy. One can look on my papers about cell reprogramming in the case of plant cell.

In recent studies, statin, a nuclear envelope protein, has been used as a marker for quiescent (G0) cells. In previous studies, chromatin assembly factor was used as well (cancerres.aacrjournals.org/content/64/7/2371.full.pdf). The following is a nice review article that discusses quiescence in the context of cancer stem cells: http://www.hindawi.com/journals/jo/2011/396076/

Cellular quiescence is as reversible growth/proliferation arrest (=G0). Nothing to do with cell death or senescence (irreversible proliferation arrest). Also, not necesarely linked to differentiation. You may find many different scenarios that apply to quiescent cells (e.g. most cells in adult mammals are quiescent). The molecular definition is also obscure since it is an state defined by the "lack" and not "presence" of something (in this case proliferation). I would suggest to check publications by J.M. Roberts (PLoS Biol 2006; Science 2008, Trends Mol Med 2010)

I mostly agree with Marcos regarding definitions. Although he's right concerning the (lack of) positive biochemical markers one can find some. In human fibroblasts, in which quiescence and senescence are widely studied, while most of the cell cycle regulators are downregulated, hypophosphorylated form of pRB-like p130 pocket protein strongly accumulates in both situations. On the other hand, while D-type cyclins are strongly downregulated in quiescent cells, cyclin D1 (and cyclin E1) is stabilized and cyclin D2 even increases in senescent fibroblasts. This is also true for primary human T-cells. Regarding CDK inhibitors, p27 protein is upregulated/stabilized in quiescent cells, while p21 is usually upregulated in senescent cells and low in quiescent cells. But, as often in biology, there are exceptions.

I agree with Marcos regarding definitions: Cellular quiescence, defined as reversible growth/proliferation arrest. We used TK1 (thymidine kinase 1), a proliferating marker, to detection of the TK1 activity and TK1 concentration in quiescennt cells. Cells that are rendered quiescent by serum deprivation contain low levels of TK1. When

such cells are induced to re-enter the cell cycle by addition of fresh serum, the level of TK1 increases started the later G1, and accumulates rapidly at the beginning of S-phase and the highest level was foundin latter S /G2 phase of cells.

In plant cells as a marker of quescent cell one can use duration of G1 phase of cell cycle and, hence, low abundace of S-phase. The last can be quantified by frequiency of EdU or BrDU incorporation. The additional marker of quescnce is a formation of protien storage vacuole and specific chromatin structure (compact). Both markres can be easy detected. Also, in plant cells quesnce (dormancy) can be induce by certian conmibation of growth regulators and stress factors. Once after stress factor has been removed, quescent cells fast became cycyling again. Quescent cells can formed both in planta and in cell culture.

I agree with Taras Pasternak in that quiescence could also be regarded as an extended G1. However people like to use G0. I think that quiescence is an exit from cell cycle and doesn’t need to be reversible. Terminally-differentiated cells are also considered to be quiescent. Senescence relates to the aging process, hence dividing cells also senesce. For example they suffer some shortening of their telomeres with each division, epigenetic alterations, etc….

I don’t confuse quiescence with cell differentiation. I think my comment has been a bit misunderstood. All cells are differentiated or locked in a pre-stage of differentiation that can be more or less distant from full pluripotency.

G1 is a phase of the cell cycle and quiescent cells are NOT in the cell cycle

I agree with the Marcos. At least in plant cell biology there is a confusion between G0 and G1. G0 is not cycling cells...

Hi Marcos! You are saying that because you referring exclusively to mammalians cells. If in plants they use the term extended G1 to refer to that interval of time whereby the cell cycle is shutoff then where is the problem? As far as they have markers for it.

Whether the corner at the exit door of a subway station belongs or not to the circuit is a matter of scientific refinery, which is very useful. We can ask the guard :(

The problem to compare G0 as an extended G1 is that, at least in mammals, not all G0 exits happen at the entrance of G1. Some cells (polyploids) cycle between S phase and G2/M after which they exit the cell cycle in G2/M (in the previous station). So, saying that those cells are in G1 would be incorrect. That why G0 is more appropriate. So, in mammals I agree with you, we should use G0. I am not sure cancer cells in culture even with serum starvation ever enter G0.

However, I think G0 is also part of the cell cycle. But you are right to disagree with me. I consider the ON/OFF states as integral part of the same phenomenon.

Hi, Abdelhalim, I agree with you, in plant cells also many cells exit cell cycle after DNA reduplications, so, they are G2. However, you can distinguish quiescent cells and G0 cells by dramatical differences in chromatin packing.

Hi Taras! That is fine! We all know that cell cycle phase subdivisions are arbitrary because their boundaries were based on observations made by the microscope. Later on, molecular characterization of the cell cycle circuitry has kept that tradition, which is very confusing even with the existence of checkpoints.

I have consulted Wikipedia on G0 and it is the same as what we all are saying. We are all right, so a bit wrong too, but not more than the guy who wrote the reference on wiki, which is a bit contradictory. He considers terminally differentiated cells to be quiescent but at the same time quiescence as a reversible growth/arrest. He adds more confusion too. .Also, when considering some cells which are heterogeneous in term of polyploidy, some cells will reenter the cell cycle while highly polyploid cells will be prevented. So, how do we stand there? Those cells are quiescent or not or do we have to check their polyploidy and separate them to be able to label them as quiescent? To make the situation more complex, non-dividing hepatocytes are in G0 but some are blocked at the entrance of G1 others at the G2/M!

Hi, Abdelhalim, I agree, there are a lot of confusion in Wikipedia about cell cycle exit and about G0. Lets say that G0 is a terminally differentiated cells, what perform only specific function (like photosynthesises in mesophyll cells) with Rubisco as 50-60% of total protein contents. I am not sure we can consider quiescent cells as G0 as well, because they have different protein profile. I would suggest to use for G0-G1marker flow cytometry picks position: differentiated, quiescent and cycling cells have different nuclear DNA/RNA stain-ability. This may serve as excellent marker of quiescence and cell differentiation and allow us to distinguish all thee stages. .

Hi, Abdelhalim,

as a marker of cell quiescence one can use flow cytometry as a method. Actually, flow cytometry shown us not DNA contents, but DNA stain ability, ea. amount of dye, what can bind to DNA with specific conformation. One of the quiescent cells feature is a much more packed DNA to compare with non-quiescent cells. You can easy find this differences if you have 2 cells population: quiescent and non-quiescent. On the FACS you will find different position of the G1 peak for these populations. My be, to clearly see differences, after running nucleus form both population separately, you can mix both and subject to FACS. In that case you will have 2 distinguish peaks of G1, corresponding to quiescent and non-quiescent cells. Even more differences will be if you perform double labeling of the nucleus with DAPI (first) and than with propidium iodine. You can find some more details in my paper.

Hi Taras!

It is very kind from your part. I am not working actually on cell cycle. We come across cell cycle because of the regulation of a gene in which we were interested. Could you send me the ref? I will have a look to your paper. I find what you are suggesting very interesting and one never knows what kind of information will be useful. I must say, I didn’t know you can use the degree of chromatin compaction as a marker for quiescence.

Thanks, I think that beside DNA stain-ability, RNA level in the nucleolus even more better marker. Quiescence it mean low level of transcription, ea. low RNA level as well. You van use FACS for quantification as well. But some details of sample preparation are crucial. Here is citation: T. Pasternak, P. Miskolczi, F. Ayaydin, T. Mészáros, D. Dudits, A. Fehér (2000) Exogenous auxin and cytokinin dependent activation of CDKs and cell division in leaf protoplast-derived cells of alfalfa Plant Growth Regulation Volume 32, Issue 2-3, pp 129-141

Cheers Taras!

Dear Bridget, I work with adult-derived mammalian stem cells, i.e., mesenchymal, pluripotent, and totipotent [Minerva Biotech 17:55-63, 2005]. We have found them in mice, rats, rabbits, cats, dogs, sheep, goats, pigs, cows, horses, and humans. Our stem cells remain quiescent (do not divide, do not differentiate, do not do much of anything) in the absence of activation factors (proliferative or inductive) or in the absence of inhibitory factors. As long as we feed the stem cells when the medium turns color we have kept the stem cells going for as long as a year in cell culture and we stopped the experiment they did not. I suspect they would have been content to keep going had we not needed the incubator space, so we just froze them down (plating medium + 7.5% ultra-pure DMSO [i.e., 99.99% pure]). In the higher order organism the stem cells I investigate can last in a quiescent state for years.

For cells whose fate is cell death, I would and have called them senescent. For example, progenitor cells and differentiated cells have a defined biological clock before pre-programmed senescence and cell death. Rodent cells have a biological clock of 8-10 population doublings (Rohen), for human cells it is 50-70 population doublings (Hayflick), afterwhich they senesce, become apoptotic and die. For human cells you can check this with CD95 which is a marker for apoptotic cells.

We have taken both rodent and human cells and induced them to become progenitor cells for specific cell types. At that point they assumed the characteristics of primary isolates of progenitor cells and displayed the 10 population doublings for rodent cells and 70 population doublings for human cells. Whereas in the "stem cell state", i.e., uncommitted to any progenitor cell type of mesenchymal stem cells have doubled 690+ times while maintaining a normal karyotype and differentiation potential; our pluripotent stem cells have doubled 400+ times while maintaining a normal karyotype and differentiation potential; and our totipotent stem cells have doubled 300+ times while maintaining a normal karyotype and differentiation potential. The differences in time refer more to when we discovered the stem cells rather than their actual doubling time frame. All three stem cells contain the telomerase enzyme (until they become progenitor cells) and therefore we would propose that they have essentially unlimited proliferation potential.

And to answer Abdelhalim before he asks, our stem cells are propidium iodide positive by flow cytometry.

Hi Henry! OK! If you are making the point that stem cells do not senesce then I think the myth of the fountain of eternal youth has turned to be true. I wish other nice myths could also turn true. :)

I am not sure but if senescence refers to the incapacity of cell to deal with it normal functions as a result of accumulated damages, then yes, stem cell have no functions at all except waiting to be assigned one or to engage into one. In real life, unemployed people also senesce (it is a joke). I don’t have now in mind what are the most reliable markers for senescence apart from telomeres shortening that could be applied to stem cells. I have no problem to accept that stem cells may not senesce.

I will only ask you if you were to perform a hypothetical stem cell therapy on a patient would you take a fresh isolate or inoculate him with your frozen multi-replicated stock? (Suppose it is from the same patient). Wouldn’t you doubt? Wouldn’t you suspect that the ones which have undergone +600 replications may have experienced some kind of genotypic changes that may be harmful? I agree, those changes if they happen are random and may affect only a small fraction, but you know 1 bad cell could initiate a disease. I also agree that this is a different subject which has nothing to do with senescence.

Normal karyotype, viability, differentiation potential, etc... may hide hazardous changes. For example some changes may affect the type of cells in which they differentiate into. A random mutation may have affected a gene which is only expressed in neurons. So, your stem cells may be OK if you differentiate them into myocytes.

Finally, Senescence is also studied in other microorganisms such as yeast, which have indefinite replicative capacity.

SUGGESTION: This has nothing to do with the discussion. I think you should keep replicating those stem cells and freeze aliquots once every year (just to say a number). I think that those samples may be very precious for a wide number of different studies. I think, you can always find a place for a plate or two in the incubator. They could be worth a lot!

Dear Abdelahim, When we tested our cells every hundred doublings or so we tested all the parameters that were checked initially, see Minerva Biotech 17:55-63, 2005 for a table listing of comparison / contrast data for the different cell types examined. We have found the stem cells in primary isolates from mice, rats, rabbits, cats, dog, sheep, goats, pigs, cows, horses, and humans. You can download the article from my list of publications posted on the Research Gate web site. By the way, the totipotent stem cells, pluripotent stem cells, and mesenchymal stem cells are all telomerase positive, hence the ability for long term replicative capabilities.

Cell surface markers for the totipotent stem cells are CD66e, CEA, HCEA, etc., some variation of carcinoembryonic antigen, which is one way to identify them. Also, be forewarned, the totipotent stem cells are Trypan blue positive. That may sound strange because one is always taught in cell culture that Trypan blue negative means live cells and Trypan blue positive means dead cells. The reasoning is that dead or dying cells that have corrupted cell membranes will let in the dye. The actual reasoning is the ability of the cell to pump the dye out. Live cells will pump the dye out, while dead cells won't. That holds for most cell types except the totipotent stem cells which do not contain the cellular machinery to pump out the dye. We looked with transmission electron microscopy. The totipotent stem cells are basically a compacted VERY heterochromatic nucleus surrounded with a plasma membrane with enough cytoplasm for 2 small lamellar mitochondria and that is all. But they will differentiate into at least 68 different cell types across all three primary germ layer lineages (ectoderm, mesoderm, and endoderm) as well as form spermatogonia. From this data we changed the name from "very small entities" to totipotent stem cells. The totipotent stem cells do not synthesize their own substrate and need to be given type-I collagen as a substratum for cell growth, will survive post-confluence as multiple cell layers if fed fresh medium whenever the medium changes color, are unresponsive to progression agents (such as insulin, IGF-I or IGF-II), are responsive to inductive agents. They optimally freeze at -80C with 7.5% v/v ultra-pure (99.99%) DMSO, contain the telomerase enzyme and thus have almost unlimited proliferation potential and are the precursor cells for pluripotent stem cells.

Cell surface markers for pluripotent stem cells are CD10 (human, neutral endonuclease) and SSEA (stage specific embryonic antigen). The cells are Trypan blue negative. The pluripotent stem cells do not synthesize their own substrate and need to be given type-I collagen as a substratum for cell growth, will survive post-confluence as multiple cell layers if fed fresh medium whenever the medium changes color, are unresponsive to progression agents (such as insulin, IGF-I or IGF-II), are responsive to inductive agents They will differentiate into at least 63 individual cell types across all three primary germ layer lineages [ectoderm, mesoderm, and endoderm] as assessed by cell specific phenotypic expression markers. They optimally freeze at -80C with 7.5% v/v ultra-pure (99.99%) DMSO, contain the telomerase enzyme and thus have almost unlimited proliferation potential and are the precursor cells for ectodermal stem cells, mesenchymal stem cells, and endodermal stem cells.

Mesenchymal STEM cells are CD90 & CD13 positive (human) or Thy-1 positive (animal), do not synthesize their own substrate and need to be given type-I collagen as a substratum for cell growth, will survive post-confluence as a single cell layer if fed fresh medium whenever the medium changes color, are unresponsive to progression agents (such as insulin, IGF-I or IGF-II), are responsive to inductive agents and will form any cell type of mesodermal origin (i.e., 3 types of muscle, 2 types of fat, 5 types of cartilage, 2 types of bone, endothelial cells, dermis, tendons. ligaments, trabeculae, capsules, scar tissue and the entire hematopoietic lineage of cells), optimally freeze at -70C with 7.5% v/v ultra-pure (99.99%) DMSO, contain the telomerase enzyme and thus have almost unlimited proliferation potential and are the precursor cells for the mesenchymal progenitor cells. And once committed to becoming a progenitor cell will lose all their unique stem cell characteristics and acquire the progenitor cell characteristics as part of their differentiation process.

Mesenchymal PROGENITOR cells are CD105, CD117 & CD166 positive (human) or SH1, SH3, & SH4 positive (animal), will synthesize their own substrate for cell growth, will die when reaching contact inhibition, are responsive to progression agents (i.e., can be accelerated to express a differentiated phenotype using 2 micrograms per ml insulin in your culture medium), are unresponsive to inductive factors outside their respective tissue type (i.e., myoblasts are unresponsive to BMP-2 that stimulates cartilage and bone formation), optimally freeze in liquid nitrogen (-196C) with a cryoprotectant, do NOT contain the telomerase enzyme and thus have a defined biological clock and are the precursor cells for differentiated mesodermal cell types.

And we have already performed the hypothetical experiment you suggested twice, once on a series of studies on animals using our genomically-labeled and expanded pluripotent stem cell clone during an IACUC-approved animal study [J Cell Molec Med 9:753-769, 2005] and also during an IRB-approved Parkinson disease Phase-0 efficacy clinical trial (everyone got treatment). We used autologous pluripotent and totipotent stem cells from the human participants. However, instead of culturing ex vivo (too great a chance for contamination), we discovered a method where we could "tweak" the stem cells while still in the participants' bodies to both expand the stem cells in vivo and reverse diapadese the stem cells into the vasculature. We harvested their blood using standard venipuncture techniques, separated the stem cells from the blood cells, activated the stem cells and returned the autologous stem cells to each respective participant. Utilizing this technology we were able to isolate and transplant upwards of 5 billion stem cells per participant. While the results were not what I would call stellar, they were positive, enough so that we are planning to apply for an FDA IND to run an expanded Phase-I clinical trial with placebo controls. The participants are out well over a year from their single transplant and all are still doing well, with some smiling after a long time of "masked faces". That group plus others are having their medications reduced. I am putting the results together for a publication, "Parkinson disease: from bench top to bedside".

With respect to innoculating clinical patients with multiply expanded stem cells, I would only attempt that with allogeneic stem cells into someone with a genetic disorder. There ae so many parameters that must be tested before that particular experiment can move forward, i.e., does the donor carry any genetic diseases themselves, do they carry any uncovered inborn errors of metabolism, etc., etc., etc. Once we find a "clean" donor then it will be a mater of expanding the population (inside the individual) and checking for any genetic drifts during the propagation process. If and when that proves successful and we obtain approval from the FDA, those clinical trials will move forward as well.

Thank you very much Henry! I get your point. The truth, I am impressed by the work you have done to characterize these cells. I understand that you have had to analyze so many parameters. I may start working on stem cells soon but only for chromatin research and epigenetics, but one never knows. I agree that stem cells therapy bears hope at all fronts of medicine and we should invest and investigate as much as it is possible. I wish you get the approval from the RDA.

If I understood well, the biological clock imposed on a differentiated cell doesn’t vary if it originated from a stem cell that has been left to replicate +600 times or a couple of times. So, from a practical point of view, yes your argument is valid. Stem cell doesn’t senesce.

My argument is also philosophical. The capacity to divide indefinitely is perfection. Biological systems are built on not so perfect machineries. The DNA replication machinery has an intrinsic rate of error; low but if we project it in the infinite it become a source of trouble. You have to add to this spontaneous mutations which are only the result of the chemistry, lesions to the DNA from the oxidative byproducts of the cell own metabolism that may or may not be repaired, etc… From an absolute point of view and taking into account all these considerations make any living organism perishable or transforming into something else

Dear Abdelhalim,

I am always looking for collaborators. I would be interested in a joint project looking at the epigenetics of stem cell differentiation from the small totipotent blastomeric-like stem cell (0.2 - 1.0 micron) to the large totipotent blastomeric-like stem cell (1-2 microns) to the pluripotent halo-like stem cell (2-3 microns) to the pluripotent corona-like stem cells (3-5 microns) to the pluripotent epiblast-like stem cell (6-8 microns) to the germ layer lineage stem cell (7-9 microns) to the ectodermal stem cells (8-10 microns), mesodermal stem cells (8-10 microns) and endodermal stem cells (8-10 microns) and so on an so forth until one reaches cell/tissue-committed progenitor cells. If you are interested, I will send the isolation, cultivation, replating, cryopreservation, induction and characterization protocols. Most are already present in the publications listed on Research Gate under my name.

With respect to your biological clock question, we took primary isolates of stem cells and stem cells with doublings of 50, >100, >200, >400 and >690; induced them to commit to a differentiated cell types with multiple inductive agents (one of the parameters we check to see if our cells are going "bad"); and assayed them for population doublings. In all instances, once the cells committed to particular cell-tissue types, they lost the expression of the telomerase enzyme and displayed a committed biological clock of 8-10 population doublings for rodents and 50-70 population doublings for humans.

I agree that biological systems are not perfect. That is one of the reasons that we are using individuals as their own sterile bioreactors rather than expanding their cells ex vivo for transplant. The body knows far more about normal and aberrant cells than I could ever possibly know in multiple lifetimes. Therefore we are using the "body" as a checks and balance system to rid itself of aberrant stem cells before we harvest them. In vitro (culture), the rate of cell division for totipotent stem cells and pluripotent stem cells is 12-14 hours (by using PDGF-BB as a single inductive proliferative stimulatory source). That is just the lower limit for the genomic repair mechanisms to correct any mutations before cell division. In vivo (in the individual), the proliferation rate for these stem cells is more like every 24 hours, more time to check for potential mutations and either fix the mutation(s) or apoptose the stem cells. We have had some individuals on the "tweaking" compound 8 months before transplant. That is roughly 2^240 power for each stem cell. And when we assayed 20 non-tweaked humans for base line levels of stem cells in their blood they ranged from a low of 70M cells/ml to a high of 800M cells/ml. [So much for one of my hypotheses. I had postulated that all humans would have a similar number of primitive stem cells if standardized for height and weight (we already knew that age was not a factor in the equation for stem cells: progenitor cells - Yes, stem cells - No). From the experiments we concluded that my hypothesis of equality in stem cell numbers in humans was obviously wrong].

In your last paragraph you discuss mutations occurring from metabolic and/or environmental sources. Are you referring to their effects on differentiated cells, progenitor cells, or what type of stem cells? The most primitive of the stem cells exist within a quiescent anaerobic state within connective tissue niches. Only when there is damage to the organism will they become activated and migrate towards the site of damage [Amer Surg 73:1106-1110, 2007] to form either scar tissue or restore the histoarchitecture and thus function to the tissue [Cell Biochem Biophys 40:1-80, 2004]. So philosophically we may be talking about apples versus oranges at this point in time.

Thank you Henry! I sincerely appreciate your offer and I will consider it seriously when time will come. The decision is not mine for now but I find it interesting to investigate if there are epigenetic differences between populations of stem cell which have duplicated a couple of times (once isolated) versus populations of stem cells which have duplicated +600 times. Don’t know of sure yet but I will let you know. Cheers.

Going back to the original question. Neither quiescence (non-dividing state) nor senescence (irreversible arrest) have nothing to do with cell death. These are independent terms. The quiescent G0 phase can be as long as the cell life span.

Hi Marco,

I am aware that you have more scientific credibility in this field, so I will take it on your words …. It is just to make sure that I properly understand the definition of fundamental concepts regarding cell cycle and the implications they have. I have always associated senescence with aging. Does it mean that dividing cells do not senesce?

Most of proliferating somatic cells do senescence: as they divide they approach the end of their lifespan, as defined by B. Hayflick. By contrast, most cell lines used in the lab are immortal, i.e., they do not senesce by propagating in the culture but they could be driven into senescence by different means. There are plenty of reviews on that topic now.

As a matter of fact, evidence connecting senescence and aging are just emerging.

@Bridget, @Taras, @Abdelhalim, @ Marcos, @ Henry.

Dear colleagues, I agree with you: quiescence is certainly more like a simple dormancy or a simple growth/proliferation arrest. Quiescent cells require significant re-programming and complex molecular mechanisms for cell cycle

exit and cell cycle re-enter. Here are some personal comments to quiescence and differentiation concerning ancient stem cells (Abdelhalim, Nov 8,2012),

in regard to interrelations and mechanisms common to both processes

Ancient stem cells give rise by asymmetric divisions to different daughter cells: a self renewing cell (SR), identical with the mother cell, and a non-identical daughter, exiting cell cycle as mitotically repressed cell. Depending of the mother’s cell type and their decisions, mitotically repressed cells evolve either to a state of reversible quiescence (Q-cell) or to an irreversible stage of differentiation, that is in the case of a single-celled organism the totipotent progenitor, namely the terminal differentiated cell (TD-cell).

Both quiescence and terminal differentiation are transitions patterns from the mother’s cell type to another, with more distinct functions. Quiescent cells persist for a long time as endocytotic cells in the G0 compartment reservoir (Bridget and Nov 5 and Abdelhalim Nov 6 2012), while precursors for terminal differentiation converted after division rapidly into TD cells. Appropriate hypoxic stimuli return Q cells in the cell cycle and convert them either to cycling cell (SR cell) or to precursors for terminal differentiation (PTD cells).

A justified question is: are quiescence and terminal differentiation together more related as believed and are the molecular patterns for cell cycle exit the same? The second question is: could be quiescence be an optional pre-stage for differentiation? I don’t think so.

In my opinion quiescence and terminal differentiation are alternative states, not directly correlated to one another. For terminal differentiation Q cells end quiescence and must return to the S-phase, before stepping up to differentiation. Quiescent, cells have different DNA levels, in contrast to cycling and differentiated TD cells, and SR cells are tetraploid (Taras, Nov 12, 2012), gained by whole-genome duplication (WGD). In ancient systems, daughter stem cells become tetraploid by asymmetric division, but only cells entering to the tetranucleate TD differentiation patern profit from the tetraploid DNA content. Q cells undergo in the G0 phase DNA -deletion and diploidization. Responding to differentiation inducers, Q cells step out from G0, re-enter the cell cycle and accomplish necessary rounds of DNA synthesis. Therefore, quiescence is not part of the differentiation path but single-celled organisms take away quiescent cells for differentiation, whenever environment provide this.

Quiescent cells are withdrawn from the cell cycle and do not proliferate except after exposure o certain conditions like hormones and growth factors. they are in the G0 phase.

Quiescent cells are different from cells that have undergone terminal differentiation. Terminally differentiated cells cannot be stimulated to proliferate by growth factors or transduction of cellular oncogenes. 

The queiscence is not exit of cell cycle, but rather resting state, with significant extension of cell cycle duration (G1) , but not cell cycle exit. Cell ba be easy back to rapid cell cycle when conditions will change. Exit from cell cycle is differentiation, what is completeky different.