| Apoptosis is Aborted Cell Division at Regeneration |
| Continuing |
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Berezhkov N.V.
The body of adult man consists of 1013 cells and millions of them dividing and dying every second. These events lay in the basis of life of every multicellular being from the earliest stages of embryogenesis to the last ones of ontogenesis. The disturbances of balance between dying and functioning cells (existing and newly generated) are principle for different pathologic states, including so global and contradictory processes as aging and tumor growth. The basis of dead cells replacement is regeneration, going on constantly in common physiological conditions (physiological regeneration) and increasing in extreme states demanding extreme functioning (reparative regeneration). Elementary regeneration begins from intracellular molecular level as biological synthesis of main cell elements (mitochondrias, ribosomes, endoplasmatic reticulum). It leads not only to exchange of threadbare cell parts (regeneration) but to their hyperplasia as well. The last event is thought to be the most prominent in theory of intracellular regeneration as the material basis for mitosis in dividing cells, or for hypertrophy in non-dividing, but functioning hard at the moment [22]. Cell death is realized using two different ways. The first one is necrosis, which is well known and considered to be the only way of cell death some time ago. Necrosis is preceded by cell injury, accompanied by progressive decreasing of intracellular fermentative function with final autolysis based on membrane destruction. Necrosis is natural manifestation of passive or violated cell death. In 1972 J.F.R. Kerr, A.H. Wyllie and A.R. Currie [14] developed new concept of cell death basically different from necrosis by its functional sense. Apoptosis, as this type of cell death was called, has specific morphological and biochemical features. Between these two variants of cell death apoptosis is of a great interest, because it is controlled and active cell self-destruction. Apoptosis is programmed process of destruction and the majority of authors consider it to be synonymous to phrase programmed cell death [12]. However, the other point of view declares the apoptosis only as the part of wide range of events, called programmed cell death, observed during organism development (insects as example) and in human tumors (breast cancer). Using these examples, the authors shares out non-apoptotic variant of programmed cell death [1,7]. From the one side apoptosis is undoubtedly compensatory process in normal and pathological conditions, directed on support of certain quantity of cells in populations. Therefore, impossibility of its accomplishing because of some reasons serve as important pathogenic stage in oncology diseases, for example. At the same time, the necessity or excess of apoptotic events can be the independent central pathogenic factor in the development of other diseases. The first of them are autoimmune and immune deficient states. It was proved, that neuron death in CNS caused by so-called slow infections of prion etiology and other degenerative diseases is performed via apoptotic pathway also. The attempts to find out apoptosis in cardiac muscle and in its conducting system elements during ishemic heart disease are also undertaken [9,11,21]. So, the understanding of apoptotic mechanisms can provide the instruments for influencing different pathological process, first of all in oncology. The NLM site in Worldwide Web searches more than 50 thousands of scientific papers based on key word apoptosis and this data base replenishes constantly and fast. Among them there are a lot of reviews and discussions about mechanism of apoptosis and its significance. However, we find out a lack of understanding of close relations between apoptosis and regeneration. This study is devoted to more detailed explanation of these fundamental processes, based on new facts and authors own opinion. Before the exposing the facts we should refer to new methods of apoptosis revealing. Morphologically apoptosis can be easily detected in dividing cell populations using routine light or electronic microscopy, but it is necessary to perform additional contrasting for evaluation of early stages of apoptosis in quiet cells. Recently the most popular is immunohystochemistry, based on marking of fragmented DNA using terminal transferase (abbreviated as TUNEL - terminal deoxy nucleotidyl transferase-mediated dUTP-biotin nick labeling), which is able to incorporate marked nucleotides to sites of DNA rupture. Instead of transferase DNA-polymerase can be used (ISEL method - in situ end labeling), which, however, thought to be less specific for rupture marking [15,21]. At the same time cell necrosis is also accompanied by DNA fragmentation. Apoptosis is specified by strictly arranged cleavage of internucleosomic fragments by nucleases. It is revealed by chromatography and microscopy, when specific chromatin bodies can be visualized close to karyolemma. Different from apoptosis, necrotic DNA fragmentation is chaotic, while the terminal sites of DNA fragments possibly can be marked by he methods mentioned above. Taking in mind that DNA ruptures are not restricted to apoptosis, the data received by this method, should be critically commented. Few studies are also proving this fact. So, studying of morphology of TUNEL-marked myocardial cells in infarct zone and CNS neurons of aged dogs revealed that positive cells appeared to be necrotic when explored by electronic microscopy in details. The first authors called these cells oncotic, the second ones - autolytic[6,18]. The variety of factors, contributed to apoptosis initiation, can be divided on few groups. The first group includes the natural conditions when apoptosis occurs as normal event; the second group consists of experimental states and pathological processes when apoptosis exceeds normal values. Separately we should sign out some general conditions, which have to be maintained for apoptosis accomplishing independently to initiating reason. The last ones are playing the main role in programmed death realization. They include some synthetic processes resulted from activation of certain genes of ced-3 family, localization of cells in regenerative active zones, certain time of apoptosis initiation after sublethal injury. Therefore, apoptosis thought to be polyethiological, but monogenetic by its realization (Picture 1). Below we sign out predominantly those facts and data, which are proving apoptosis junction to regeneration processes. Light microscopy finds out that apoptotic cells located in proliferative active (regenerative) zones where less differentiated for each tissue cells exist. In gut they can be find on crypts close to stem elements, in flat epithelium - on basal layers, in testis - in spermatogonium, in lymph nodes - in germinogenic centers, in tumors - close to blood vessels, in liver - in the first zone of hepatic acinus [4,2,19]. Regeneration (physiological and reparative) can be discussed using the liver model. Liver possesses different functional activities and it is the most well learned organ with capacity to regeneration process. One of the inductors of reparative regeneration is acute blood loss, or post-hemorrhagic anemia. This event stimulates regeneration not only in injured hepatic tissue but of lost blood proteins as well. It was revealed that the apoptotic hepatocytes have morphogenetic relation to dark hepatocytes, which demonstrate the morphological signs of functional activity. Apoptotic hepatocytes can be found in 10-24 hours after blood loss, in the stage of stabile compensation (by Selyes theory), which is characterized by DNA and RNA synthesis and mitotic activity. However, in third zone of hepatic acinus with low capacities to regeneration because of blood supply peculiarities, collicvative necroses appeared in normal aged rats (30-38months) and in anemic young animals (8-10 months) soon after blood loss (in 2,5 hours) [3]. This data are also true for partial liver resection (classical model of reparative regeneration) when coexistence of DNA synthesis activation and apoptotic wave was noted in 60-96 after injury [20]. Liver regeneration in pathological conditions of cirrhosis leads to forming of post necrotic nodules, where one can observe a lot of Councilman bodies (apoptotic hepatocytes). The process of body growth and development is close related to regeneration. The difference is that cell structure synthesis is performed not for change of threadbare particles, but for forming of new ones. Apoptosis was revealed during ontogenesis of multicellular beings from flat warms to mammalians. A lot of studies are devoted to programmed cell death and apoptosis during prenatal ontogenesis because of prominent apoptotic activity at this period, characterized by cell proliferation leading to growth and development. Particularly a lot of apoptotic cells were registered in nervous system, especially in CNS neurons of embryos. Apoptosis was also studied in late ontogenesis. The relative increase in apoptotic hepatocytes was revealed in 24-month old rats compared to 8-month ones. At the same time the liver of 30-month and especially of 38-month old rats, characterized by decrease in regenerative capacity, contained no apoptosis, but demonstrated a lot of monocellular collicvative necroses [3]. The same data were registered during evaluating of the morphology of neuron aging in men and dogs [6]. Speculating with the data of increasing apoptotic activity in early ontogenesis and first stages of late ontogenesis we can suppose the possibility of active neuron death in hypothalamus, which is known as neuro-humoral regulative center. The following elevation of sensitivity to some hormones in feedback mechanism leads to realization of genetic development program. We thought aging not to be programmed but gradually transforming from development process. So, according to data mentioned above, aging should be also connected to apoptosis of neurons in hypothalamic regulative centers. Tumor growth reminds defective reparative regeneration which occurred in wrong place on wrong time, so it attracted our attention. Differentiated and less differentiated tumors posses different apoptotic activity. We observe more apoptotic cells in differentiated adenocarcinomas of stomach and gut as well as in endometrial adenocarcinomas (please see picture below), compared to non-differentiated and diffuse cancers. Other investigators also confirm this data [23]. Adenocarcinomas also demonstrate more prominent mitotic activity then less differentiated tumors and it matches the logic of apoptotic process. Moreover, it is possible to reveal morphogenetic connection between irregular mitoses characterized by fully condensed chromatin in chromosomes and apoptotic cells. We can conclude that malignancy of tumor is defined not only by mitotic activity, but by apoptotic activity as well. Plenty of apoptotic cells in highly differentiated adenocarcinomas of stomach and gut can signify relatively low malignancy of these tumors from one side and create the evidence of tissue similarity based on the type of self-destruction from another side. Special attention should be paid to lymphocyte-associated apoptosis. It can be observed during interaction of T-cells and large granulated lymphocytes (Pit-cells with NK-activity) with target cells. Stimulating proliferation, lymphocytes mediate increasing intracellular regeneration in target cells and can induce apoptosis, when combined with mitotic block. In tumors with decreased ability of modified cells to apoptosis lymphocytes predominantly induce proliferation and stimulate tumor growth [5]. New morphological and molecular data concerning apoptosis are presented in reviews elsewhere [12,21]. Below we shall look through those facts about apoptotic process, which are can be also observed in mitotic cells during proliferation and synthesis (regeneration) caused by different factors. |
