Cell growth phases Death phase

Cell growth phases comprise lag phase, exponential or log growth phase, stationary or plateau phase, and senescence or death phase, as shown in Figure 2.5. Cell growth can be mathematically represented by the following general equation ... 

We also investigated the effect of 1-7 compotmds on the cell cycle (Fig. 11.33). The synthesis of enzymes for DNA replication takes place in the G1 phase, the replication of DNA in the S phase, the synthesis of proteins in the G2 phase, and finally, the cell division in the M phase. Cancer cells proliferate when the regulation of this cell cycle malfunctions. There are two paths in which antitumor agents act on cells one is associated with cell death, and the other one simply slows cell growth. Cell death is classified into apoptosis and necrosis. The spontaneous removal of unwanted cells is called apoptosis, and necrosis refers to cell death related to inflammation caused by bums and poison. After the addition of 1-7, 78.6 % of cells were in the G1 phase compared to 59.8 % in the control group. On the other hand, the rate of apoptosis was not different. Thus, 1-7 slows the transition to the S phase from G1 phase, but it does not induce apoptosis. 

Once there is an appreciable amount of cells and they are growing very rapidly, the cell number exponentially increases. The optical cell density of a culture can then be easily detected that phase is known as the exponential growth phase. The rate of cell synthesis sharply increases the linear increase is shown in the semi-log graph with a constant slope representing a constant rate of cell population. At this stage carbon sources are utilised and products are formed. Finally, rapid utilisation of substrate and accumulation of products may lead to stationary phase where the cell density remains constant. In this phase, cell may start to die as the cell growth rate balances the death rate. It is well known that the biocatalytic activities of the cell may gradually decrease as they age, and finally autolysis may take place. The dead cells and cell metabolites in the fermentation broth may create... 

Following the stationary phase the rate of cell death exceeds that of cell growth, and the cell number begins to decrease, resulting in the final death rate part of the curve. 

As mentioned previously, glucocorticoids promote apoptosis and reduce survival, differentiation, and proliferation of a number of inflammatory cells. While there is an increase in the number of polymorphonuclear leukocytes in the circulation, corticosteroids cause the involution and atrophy of all lymphoid tissue and decrease the number of circulating lymphocytes. The striking lymphocytopenia is caused in large part by an inhibition of lymphocyte proliferation, although diminished growth with preferential accumulation of cells in the Gi-phase of the cell cycle is followed by cell death. These effects are mainly mediated by alterations in cytokine production and action. 

The presence of air m the bioreactor will not jeopardize the cells in the initial phase of growth, but as the cells become established and increase in numbers, the bioreactor must be kept full or cell death of adherent cells will occur in the air pockets. 

Culture conditions influence the survival of cells submitted to cryopreservation. To be frozen, cells should be in an active growth phase, with a viability greater than 90% and free of contaminants. The optimal cryopreservation conditions are different for each cell line. When a cell is exposed to low temperatures, ice crystals are generally formed and can disrupt the cell membrane, causing death. Therefore, cells should be treated with a cryoprotector to support the freezing and thawing processes. 

During the stationary phase, the growth rate is zero as a result of the depletion of nutrients and essential metabolites. Several important fermentation products (including most antibiotics) are produced in the stationary phase. The stationary phase is followed by a phase where cells die or sporulate. During the death phase, there is a decrease in live cell concentration, which results from the toxic byproducts coupled with the depletion of the nutrient. The number of viable cells usually follows an exponential decay curve during this period. 

Cell lysis generally becomes important when viability is low. Examples include continuous culture (with or without cell retention) at low dilution rates and the late stationary and death phases of batch culture. Cell lysis is also important in stirred reactors with very high agitation rates. In order to quantify cell growth parameters under these conditions, the lysed cells must be accounted for. One way to do this is to measure the amount of the cytosolic enzyme lactate dehydrogenase (LDH) released into the medium (see Chapter 2, section 2.5). A procedure for measuring LDH activity is described in Chapter 4, section 4.7. 

All the curves obtained in Figure 4.3.3, except for antibody production, show two distinct upper and lower parts the upper part corresponds to the initial lag phase, and the lower part to the following growth and death periods. This indicates that during the initial lag phase, at a given growth rate, cells exhibit a faster rate of nutrient uptake and metabolite production. 

The model was adapted from Kontoravdi et al. (2005) for cell growth/death, nutrient uptake, and major metabolism. The model was further developed to include description of cell cycle sub-populations. The cell cycle representation was based on the yeast model of Uchiyama Shioya (1999) and the tumour cell model of Basse et al. (2003). Eq.(l)-(4) express viable cell concentration(Xv[cell L" ]) in terms of cells in Gq/Gi, S, and G2/M phases. As a simplification in notation, Gq/Gi cells will be indicated as G unless otherwise stated. Xoi, Xs, X02/M [cell L" ] are concentrations of viable cells in Gq/Gi, S, and G2/M phase, respectively, whereas Fo ,[L h" ] is the outlet flowrate. F[L] is the cell culture volume b, ki, k [h" ] are the transition rates of cells from Gi to S, S to G2, and M to Gi respectively and /[Pg.110]

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