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| Year : 1984 | Volume
: 30
| Issue : 4 | Page : 199-206 |
The cytoplasmic basis of cellular differentiation--redressing the injustice done to the cytoplasm.
Kothari ML, Mehta LA
How to cite this article:
Kothari M L, Mehta L A. The cytoplasmic basis of cellular differentiation--redressing the injustice done to the cytoplasm. J Postgrad Med 1984;30:199-206 |
How to cite this URL:
Kothari M L, Mehta L A. The cytoplasmic basis of cellular differentiation--redressing the injustice done to the cytoplasm. J Postgrad Med [serial online] 1984 [cited 2023 May 31];30:199-206. Available from: https://www.jpgmonline.com/text.asp?1984/30/4/199/5444 |
The riddle of cellular differentiation continues to defy the conceptual and experimental ingenuity of all-logists. Man, like any other metazoic organism starts as a featureless zygotic cell to end up as a fully formed individual comprising the indivisible neurones, the peripatetic polymorphs, and such other "few hundred different types"[20] that refuse[28] to tell anything about how they came into being.
A typical mammalian cell, even in its patently differentiated, stable state, possesses an all-in-one versatility-an actor in the Shakespearean style, playing one role at a time, but capable of playing any role that any act may demand at any time. Any theory on differentiation must account for this seeming paradox of stability in the midst of total versatility. Such a theory must also explain some recent developments[1],[10],[11],[12],[13],[20] as the success of nuclear transplantation into somatic and zygotic cells to spawn some new cell types on the one hand, and some fully formed organisms on the other.
The hypothesis advanced in this paper rests on a radical departure[15] from the nucleus-oriented thinking on cytodifferentiation. It proposes that from the very start, namely, zygote-formation, it is the cytoplasm that governs and guides the nucleus to be this or that, do this or that, be "normal" or cancerous, and so on. The cytoplasm gives the lead which the nucleus follows.
The entire embryogenic blueprint lies in the ovular/zygotic cytoplasm. The sperm, at fertilization (more truly, it is diploidization) torpedoes into the ovum, to get decapitated at its border thus giving its nuclear head to the mature ovum and losing its cytoplasmic body and tail to the periovular void. We now have the zygocyte (=zygote) comprising its diploid "somatic" nucleus-formed by the fusion of the haploid female pronucleus of the ovum and the haploid male pronucleus brought in by the sperm-and the cytoplasm contributed exclusively by the ovum. Replacement of either the female haploid pronucleus of the ovum or the diploid nucleus of the zygocyte by the nucleus of a somatic cell of an adult animal-classically, the frog intestinal cell nucleus replacing the nucleus of the frog zygote[10],[11],[12],[22]-is unbelievably compatible with normal embryogenesis. Here, the ovular cytoplasm stands primarily important, unsubstitutable, and knowledgeable, while the "reprogrammed" nucleus plays second fiddle.
Even when the guest nucleus has been obtained from a cancer cell, the enucleated host-ovular-cytoplasm asserts its preprogrammedness on the guest nucleus to initiate and complete embryogenesis.[13],[21],[27] "The most extreme example of 'pre-programming' of the cell cytoplasm is perhaps to be seen in the ovum itself. A frame of reference in which the ovum is regarded as the undifferentiated cell has the disadvantage that it tends to encourage an underestimate of its structural and functional complexity... Although the data so far available are rather preliminary, they lead one to suppose that in a wide range of biological material all the organization necessary for the early stages of development is already present in the cytoplasm of the ovum before it is fertilized."[13] Cytologists have nowhere thought,of as to when, where, and most importantly why, should cytoplasm abjure its dominance over the nucleus later in life, following embryogenesis, in the somatic cells!
The avian erythrocyte is a nucleated cell wherein the nucleus is inactive and dormant, devoid of any synthesis of nucleic acid. Such a nucleus, freed from its own cytoplasm, can be made to resume the synthesis of nucleic acid and specific proteins at full steam when placed into the cytoplasm of cells from a wide variety of animal species ranging from mouse to man.[1],[4],[13] What holds true for avian erythrocyte nucleus also holds true for the frog erythrocyte. "We can therefore conclude that the signals emanating from human or mouse cytoplasm are understood perfectly well by the hen (or frog) nuclei."[13] We can further generalize that the dominance of cytoplasm over nucleus is a panvertebrate feature that even species-barriers cannot abrogate. The cytoplasm dictates; the nucleus deputizes.
The mammalian red cell is not the sole example of the active survival of the cellular cytoplasmic sac after the nucleus has been voided. The giant unicellular alga, Acetabularia, withstands enucleation, exhibiting thereafter growth and regeneration, the regenerative process producing an essentially normal Acetabularia, although, of course without a nucleus.[13] The cells of Spyrogyra, following nuclear excision, survive for more than 2 months during which time they grow, form new cytoplasm containing; organelles, synthesize proteins, and carry out all their normal physiologic functions.[13] The enucleated egg of the sea urchin Arbacia exhibits repeated cell divisions to the point of forming a blastula that may develop functional cilia.[13] The protein synthesis induced in frog oocytes by pituitary hormones is not abolished by enucleation; indeed, for many hours, the rates of synthesis in nucleate and enucleate oocytes are indistinguishable.[13] Enucleate fragments of cultured human cells survive in vitro for upto 4 days exhibiting movement, pinocytosis, and the incorporation of amino acids to form protein.[8] We can no longer deny the high degree of structural and functional autonomy that the cytoplasm enjoys regardless of the nucleus.
Cytologic truisms, meant to drive home the "central or crucial"[19] role of the nucleus, usually advance the argument that "enucleated cells will stop dividing and will eventually die."[19] But such assertions lack the mandatory follow-up explanation: What happens to the ecytoplasmed nucleus? Does it survive as long as, and as actively as, the enucleated cytoplasm? The answer, alas, seems to be a big "no"! Such a 'no', however, is not available in cytologic literature. Driven by the obsession about the 'obvious' superiority of the nucleus, nobody has bothered about excising the cytoplasm from about a nucleus and see if the latter really survives. From whatever is available from cytoplasmically-oriented enucleation experiments, one can summarise that: The moment the nucleus is divested of the cytoplasmic matrix, it loses weight and volume, collapses, turns pyknotic, afunctional, and then dies. The dependence of the nucleus on the cytoplasm is absolute; that of the latter on the former is relative. The indispensability of their polar coexistence is an example of mutualism, illustrated by such informational/ bioelectric units as the neurones and muscle cells, one sustaining and sustained by the other, by what can best be called[4] a "continuous interrelationship." The evolution of the nucleated cell itself shows that the nucleus was begotten of the cytoplasm-it was the anucleate procaryote that gave rise to its nucleate eucaryote descendent.
The sheer power of service, in a tennis final at Wimbledon, is traceable to the mitochondria of the mother of the tennis star, male or female. In all vertebrates, the mitochondria (or, shall we 'say, mightochondria?) are derived exclusively from the mother; to be precise, from the maternal cytoplasm comprising the ovum.[20] There is reason[1],[15] to believe that what is true of mitochondria is true of all organelles, the paternal contribution being just half of the zygocytic nucleus. And since such a nucleus itself is substitutable, one can say that inheritance is essentially a cytoplasmic affair, a maternal endowment supplemented by the paternal (hemi) nuclear contribution. The fact that cytoplasmic inheritance does not allow the application of Mendelian laws is no reason for denigrating it as non-Mendelian inheritance worthy only of passing mention."The basic embryonic plan of all mammals is inherently feminine."[26] Is this fundamental embryologic truth traceable to the supremacy that the zygocytic maternal cytoplasm has over the half-maternal, half-paternal nucleus, a state of cytoplasmic dominance from which hereditary transmission can have no escape?
The nuclear dogma is synonymous with assertive nucleism that allows a Nobel laureate[23] to describe, in a Dictionary of Modern Thought, nucleus as the administrative centre of the cell, being the repository of all the cell's genetic information, and of all the information. The same dictionary defines dogma[5] as a term used pejoratively to mean an opinion held on grounds, and propagated by methods, that are unreasonable. The central dogma of cytology is nucleism propagated despite overwhelming evidence to the contrary, vindicating the observation that consistency, especially in science, is the last refuge of the unimaginative.[29]
A story retold many times by speakers at scientific meetings illustrates the dogmatic befuddledness of cytologists. A man running down a dark street, dropped his coin in the process. He started looking for the coin under the bright street lamp and was joined in the search by a policeman. After several minutes of failure, the policeman asked, "Are you sure you dropped it here?" "No," the man answered, "I lost it back there in the dark street, but I can see better here." The unfathomable, diffuse complexity of the cytoplasm has been like the coin lost in the dark street, and the clear visibility/isolation ability/transplantability of the compact nucleus-constituting but 2% of the cell volume-has been like the bright lamp-shine where the search continues. "The development of cariology (nucleology) was somewhat detrimental to the study of cell as a whole."[4] And what has caryology studied but some aspects such as chromosomal structure and choreography, that are admittedly far removed from the cellular reality? A Textbook of Cytology[3] makes a confession: "The interphase nucleus is of great biological significance, as demonstrated by biochemists and others. Yet, cytologically, it is not very exciting. The chromosomes can hardly be seen or studied and the nucleus just sits there, as observably inactive as a sleeping dog and as static as the old term for this nuclear condition, 'resting stage,' indicates." If the nucleismically-biased cytology is not ready to do justice to active, significant, but (structurally) unexciting interphase nucleus, what could we expect of it vis-a-vis the largely featureless cytoplasm! No wonder that the father of modern genetics, Morgan[24] was led to pontificate that "the cytoplasm can be ignored genetically," a sermon that has been religiously followed right up to the recent[6] 2-volume, 1500+ pages tome on genetics. Even when experimental findings overwhelmingly suggests[17],[20] that the cytoplasm may be endowed with as much DNA and information as the nucleus, the suggestion is strongly opposed on the pragmatic ground that "one is forced to consider the possibility of a widespread duplication of the genes in both nucleus and cytoplasm, a state of affairs that makes nonsense of most forms of genetic analysis."[13] The epistemologic stumbling block to according cytoplasm its due is the Copernican change it would force upon "most forms of genetic analysis." This is yet another example of looking for the coin at a wrong place, for, today, "genes in mammals lie broken into pieces-and so do some of the basic assumptions of molecular biology."[2] The inherent nonsense of genetic analysis[16] is good enough reason to give cytoplasm its due, a state of nucleocytoplasmic equivalence-with the nuclear dogma dead-wherein the cytoplasm enjoys the status of primum inter pares, or, first among equals.
The cytoplasm, given its magnificent machinery of microtubules, actin, myosin, ergastoplasm, and so on, is the powerhouse, the workhorse, the active, feminine Yin, whose business it is to be "committed" to a cell type, varying from a zygocyte to a cancerocyte. It drives water and proteins into the nucleus to selectively hydrate, separate, rarify, and thus euchromatize specific areas of the nucleus.
The nucleus, necessarily diploid, is a long twine comprising two-maternal and paternal-chromatin elements which, because of inherent mutual attraction, remain fused (pyknotic, heterochromatic) except in areas where the two are separated by the cytoplasm driving water and proteins between them, constituting in these regions, the euchromatin. In this light, it becomes clear how a nucleus, when removed from the cytoplasmic bed, collapses and turns totally heterochromatic.
Free of the organellar complexity that the cytoplasm has, the nucleus is, for most of its part, the dormant Yang, this being its heterochromatin, save specifically activated areas called the euchromatin. That the distribution of euchromatin is widely scattered and stereotopic may be guessed from the diffuse rarefaction of the nucleus that euchromatization brings in. It has been estimated[10],[13] that an excised nucleus-that is thus totally heterochromatic-enlarges, on being put back into cytoplasm, 20-90 times in volume. This inflation is by hydration and activation of only 2-5% of the nuclear chromatin material. The fact that in any cell the quantum of euchromatin does not exceed the above figure shows that even the euchromatin/cytoplasmic ratio, like the nucleo/cytoplasmic ratio may be fixed for vertebrate cells.
The ultimate goal of all science, Hickman[14] generalizes, is to devise conceptual schemes to explain the nature of the universe we live in. Each of our cells-"a better chemist and physicist than all the Nobel prize laureates put together"[25]-is a nouniverse (from nou = knowledge universe) the wonders of which are briefly wondered at in this paper. A conceptual scheme, if governed by two important tenets-of Occam saying that the basic assumptions should be minimal, and of Feynman,7 that "Any discovery of a new law is useful only if we can take more out of it than we put in,"-acquires comprehensibility, viability, and wider applicability. Occam's razor stands satisfied, as the new hypothesis only asserts that cell differentiation, in a metazoan, is cytoplasmically determined. Feynman is respected by the ability of the hypothesis to explain, in some measure, the major conundrums of cytology, as touched upon below.
The cytoplasmic prerogative to call the nuclear tune allows nuclear equivalence or equipotenhality[14] of somatic cells-the zygotic nucleus, a normal diploid cell nucleus, and a cancer cell nucleus, belonging to an organism, are selfsame genetically. A corollary to this is the all in-one gestalt nature of the nucleus. The imposition by the cytoplasm, of a differentiating field, on the nucleus, activates widely scattered areas of the nucleus, beyond the coarser chromosomal, and the finer genetic barriers, thus accounting for the invention[2] of such apologetic terms as polygenes, split genes, jumping genes, and so on.
"In eukaryons, at any given time, most of the DNA is not genetically functioning."[4] A common lamentation in cytology and genetics is that eukaryocytes carry, in their nucleus, DNA "in vast excess of any genetic requirements[20]-probably 1000 times greater than is needed. "What all that extra DNA is doing is one of biology's great riddles."[9] It has been computed that each of our body cells does not use more than 2-5% of its genetic potential. Why?
Ask the nucleus to be all-in-one (see above), and then ask it to work with cytoplasm to produce one cell type-at-a-time and you have the answer. The cytoplasm, while differentiating the nucleus, activates only those areas oŁ the nuclear DNA as are specifically needed to produce a cell type-say, an epidermal cell. The rest of the DNA material-up to 98%-is not "excess" but purposedly quiet in favour of the cell-type in question. That the DNA of all cell types in a human being should exhibit this quality of quietitude more than supports the cytoplasmonuclear relationship proposed here.
"Differentiation is a strictly limited exercise. Once selected, the programme is normally remarkably stable and confusion with any other programme of gene expression is very rare."[20] The lack of confusion is because of "strong cytoplasmic commitment to the stable state"[20] The stability of the cytoplasmonuclear relationship is because of the differential response of nucleus to cytoplasm's command. As Maclean[20] generalises, "a differential cell is poised in a state of dynamic equilibrium between nucleus and cytoplasm, in which signals from the cytoplasm are necessary for the maintenance of nuclear gene expression which characterizes that state." The most irreversible differentiation is to be found in cancer cells, entirely because of the cytoplasm calling the cancer-cell-specific-nuclear-tune.
Nuclear-transfer experiments are, in a way, cytoplasmic-transfer experiments as well. But a cell, under such experimental conditions, is known and behaves by the cytoplasm it keeps. The nucleus, subservient to the differing calls of the cytoplasm in different cells, permits swappability by its ready adaptability to the new cytoplasmic host.
Cytological diagnosis of cancer has rested on the assumed aberrations of the nucleus-hyperchromatism, mitotic figures, pyknosis, and so on. But this has led cancerology nowhere. As of today, cancerology is undecided about what a cancer cell is.
Since even cancer cell nuclei are able to beget embryos on transplantation into enucleate zygocytes, a suggestion[27] has been made that cancer cell nuclei may be "perfectly normal." This exoneration of the nucleus from the cancerous crime makes cancer cytology more or less defunct, both microscopically and experimentally, since the cancerousness moves out from the easily describable nucleus into the amorphous, enormous cytoplasm. This explains why cells without a single microscopic aberration are known to behave most cancerously, whereas the most abnormal-looking cells may be most benignant.
A cell is an organ of behaviour, being known mostly by what it does or secretes rather than by what it looks. All lymphocytes or plasma cells are look-alikes but differ in their reactive or secretory behaviour. Within a given cancer, there may be a number of cell-lines, that looking-alike, have widely divergent behaviour. Save for such structurally specialized cells as neurones or muscle cells, the vertebrate body is composed of like cells-the liver or thyroid cell of one species or phylum refusing to differ structurally from its fellow in the other species or phyla. Differentiation, thus, is more functional than structural.
This state of "non-equivalence"[18] among cells "that look alike to the histologist"[18] is traceable to the fact that the individuality or the uniqueness of a cell is determined by the nebulous cytoplasm to which the nucleus follows suit. Szent-Gyorgyi's lament-"How can I differentiate between a normal cell and a cancer cell when I don't know what a cell is?"[30]-is a sweeping indictment of the failure of (even electron-microscopic) structuralism of modern cytology, readily understandable by the fact that a cell has never bothered to see how it looks as far as it did what it wanted to.
The paper would be incomplete if it did not point to areas of future research based on the new hypothesis. Hayflick limit is now a byword in cytology; future experiments would reveal whether this numerical limit to cell duplication resides in the cytoplasm or the nucleus. The tremendous specificity that a lymphocyte or a plasma cell exhibits against an antigen forms the basis of modern immunology. It will be a great day when an active, specific-antibody-secreting plasma cell will be given a nuclear swap, and the new nucleus would do exactly what its predecessor was doing. That would clinch the highly suspect superiority of the cytoplasm over the oversung nucleus.
The authors are deeply indebted to Miss Jayanthi Mani for illuminating help throughout the preparation of this paper.
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