Growth fraction

In the steady state, additions to the population are balanced by subtractions from it and the age distribution is rectangular (Fig. 10.6), i.e. the fraction of cells in a phase is now directly proportional to the duration of that phase e.g. 

Further information on cell cycle kinetics can be found in Cleaver (1967) and in Aheme et al. (1977). 

Tritiated thymidine pulse method (Howard and Pelc, 1953) 

This is perhaps the earliest method used, and the following description is for cells growing in small dishes or on coverslips. The method, 

In practice, because of the variability between cells and difficulties in producing short pulses all boundaries are blurred. However, by taking 50% values for the various transitions a reasonable approximation to the duration of the cell cycle phases can be obtained. 

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The limited efficacy of classical anticancer diugs can be explained in part by the compartment model of dividing (growth fraction, compartment A) and nondividing (compartment B) cells. The majority of antineoplastic diugs acts upon cycling cells and will hit, therefore, compartment A only. 

The rate of growth of human and experimental cancers is initially quite rapid (exponential) and then slows until a plateau is reached. The decrease in growth rate with increasing tumor size is related both to a decrease in the proportion of cancer cells actively proliferating (termed the growth fraction) and to an increase in the rate of cell loss due to hypoxic necrosis, poor nutrient supply, immunological defense mechanisms, and other processes. 

Many proliferation-associated antigens have been reported as clinically useful indicators of proliferative activity (1). Of these, the so-called proliferating cell nuclear antigen (PCNA) and Ki-67 have been identified as the most useful in both immunohistochemistry (see Chapter 27) and flow cytometry (FCM). PCNA is an auxiliary protein to DNA polymerase 8 (2,3) and is intimately associated with DNA replication, but also DNA repair (4,5). Ki-67 is a large protein associated with nuclear nonhistone proteins (6,7), and is expressed in all actively proliferating cells (8,9). Expression of these two proteins, in a cell population should equate to the growth fraction, i.e., the proportion of cells involved in an active cell cycle. However, there are apparent inconsistencies when these two proteins have been compared with one another (10) and with other methods of assessing cell proliferation (11). 

Scott, R J., Hall, P A, Haldane, J S, van Noorden, S, Price, Y, Lane, D P, and Wright, N. A. (1991) A comparison of immunohistochemical markers of cell proliferation with experimentally determined growth fraction J Pathol 165, 173-178. 

Because MIB-1 monoclonal antibody is used extensively to determine the cell proliferation index, its applications are discussed below. This antibody detects the nuclear antigen Ki-67 expressed in proliferating cells but not in resting cells. The antibody reacts with the nuclei of cells in mid-Gj (first gap), S (DNA synthesis), G2 (second gap), and M (mitosis) phases, but not in the G0 or quiescent phases. The use of MIB-1 antibody is one of the simplest and most reliable labeling techniques for assessing the rate of proliferation of a neoplastic cell population. Thus, the antibody can be used to assess the growth fraction (i.e., the number of cells in cell cycle) of normal, reactive, and neoplastic tissues. 

It is apparent from the above discussion that Ki-67 is present in the nucleus of proliferating cells and is an indicator of the growth fraction in tumor cells. It is primarily a DNA-binding protein that plays a crucial role in the maintenance or regulation of cell division. This protein may also function as a matrix for chromosomal DNA or contribute to the condensation of the chromosomes or be involved in breakdown of the nuclear membrane before mitosis (Duchrow et al., 1994). The association of Ki-67 with RNA in the nucleoli and with the DNA with nuclear matrix suggests that the antigen plays a role in transcriptional processes as a structural protein by mediating between nuclear DNA and nucleolar RNA. 

Table 10.1 presents recent examples of the usefulness of Ki-67 antigen-MIB-1 antibody complex as a marker of cellular proliferation (growth fraction). However, the usefulness of this antibody is restricted to certain species and is not applicable, for example, to rat tissues (personal communication, K.-H. Wrobel). 

Birner, P., Ritzi, M., Musahl, C., Knippers, R., Geredes, J., Voigtlander, T., Budka, H., and Hainfellner, J. A. 2001. Immunohistochemical detection of cell growth fraction in formalin-fixed and paraffin-embedded murine tissue. Am. J. Pathol. 755 1991-1996. 

Concepts of Growth Fraction and Total Cell Kill... 

If the cells do not all have the same generation time then the time taken to double the number of cells (TD the cell doubling time) will be slightly shorter than the generation time (7 ). Alternatively, if not all the cells in the population are growing i.e. the growth fraction is less than 1 (see below) then Tu will be longer than T. 

In most cell populations not all the cells are proliferating, there being a proportion of non-proliferating cells. These may alternatively be described as being in GO or in the A-state. The proliferation index of growth fraction is given by... 

The time taken for the total number of cells to double (cell doubling time) is therefore not equal to the cell cycle or cell generation time. It is important, therefore, when doing cell cycle analyses to carefully distinguish between T and TD and preferably maintain the growth fraction as high as possible. 

Low growth fractions are common in vivo (even some tumours may have a growth fraction less than 0.1), and very often in vitro in... 

Indications for treatment Chemotherapy is indicated when neoplasms are disseminated and not amenable to surgery. Chemotherapy is also used as a supplement to surgery and radiation treatment to attack micrometastases. [Note Tumors most susceptible to current chemotherapy are undifferentiated and have high growth fractions.]... 

Effective for high growth fraction malignancies, e.g., hematoiogic cancers... 

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