The typical morphology of a tumour spheroid develops due to the limited nutrient supply through diffusion into the tumour. Glucose availability, for example, is decreasing with growing distance to the tumour boundary. Therefore, while cells on the boundary are well-fed and continue to multiply, cells with greater distance to the boundary are either growing much slower or are forced to enter quiescence at the restriction point. This effect gives rise to a quiescent cell core during the early stage of tumour spheroid growth.
During later stages of growth, the depletion of nutrients in the core of the tumour reaches a level upon which survival becomes impossible for the tumour cells. A necrotic core starts to develop which leads to inflammation, attracting immune agents into the core and leading to the excretion of growth-inhibiting factors. At this stage the final morphology of the avascular tumour is present. A core of necrotic cells is surrounded by a layer of quiescent cells with actively dividing cells at the boundary of the tumour spheroid (see figure 1.2 for an illustration of the spheroid growth).
Multicellular tumour spheroids (MCTS) are a three dimensional cultures of cancerous proliferating cells. They are often studiedin vitro (see for exampleCasciari et al.(1988), Wehrle et al. (2000),Freyer (1998)) or in silico (e.g. Ward and King (1997), Drasdo and Hohme (2005)) as a model system of avascular tumour growth, due to the fact that they are simple in their cultivation but at the same time able to capture the relevant processes at the heart of cancerogenesis (see figure 1.2). MCTS show the distinct tumour topology which develops as described above. This topology should be reproducible by an agent-based model as a basis for further developments.
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Figure 1.2: Commonly accepted morphology of a MCTS showing the outer proliferating region, the intermediate quiescent layer and the necrotic core. These distinct regions form as the MCTS grows in size. Nutrients can only diffuse into the spheroid volume through the boundary, thus, with growing size, the nutrient concentration inside the tumour decreases.
Upon reaching a critical level cells in the tumour core enter quiescence. This core of quiescence cells increases in size with the tumour until the available nutrient concentration in the core drops to a level where cells are dying through necrosis.
Figure 1.3: Histological structure of spheroids from differently transformed rat embryo fibroblasts.
Representative 5µm-thick median, hematoxylin and eosin-stained paraffin sections of Rat1 (A) and M1 (B) aggregates with a diameter of 150–200µm, and Rat1-T1 (C) and MR1 (D) spheroids with a diameter between 1200 and 1300µm(sector magnification). Bar size is 100µm. Figure and caption fromKunz-Schughart et al. (2000).
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Tumour angiogenesis / Vascularisation
In the beginning tumours cannot grow beyond a certain size, generally 1−2mm3, due to a lack of oxygen and other essential nutrients. Therefore tumour growth would be rather slow or even reaching a saturated state due to nutrient limitation and mechanical inhibition of cell growth. However, tumours may induce blood vessel growth (angiogenesis) by secreting various growth factors (e.g. Vascular Endothelial Growth Factor or VEGF) to ensure a continuous supply of nutrients. Growth factors, such as bFGF and VEGF can induce capillary growth into the tumour, thereby delivering required nutrients into quiescent regions and serving as a waste pathway for end products of the rapid division taking place inside the spheroid.
Angiogenesis is a key step for the transition of a tumour from a small harmless cluster of cells to full scale cancer. Furthermore, angiogenesis also opens up a pathway for the spreading of the tumour via metastasis, where single cancer cells can break away from an established solid tumour, enter blood vessels, and be carried to a distant site, where they can implant and begin the growth of a secondary tumour.
Mutations inside the tumour
In cancerous cells some of the DNA repair mechanisms are no longer working. If, in addition, the apoptotic pathway is blocked, mutations rapidly accrue within cancerous cell populations. These mutations allow the cancer cells to develop drug resistance and escape various therapy attempts. They also lead to the rise of multiple sub-populations in one tumour, which may show different reactions to pharmaceuticals, further complicating the treatment. This diversity can be easily captured in an agent-based model.
Hypoxia in tumour tissue
According to the Warburg hypothesis, cancer is a problem of mitochondrial deregulation.
It was postulated by the Nobel laureate Otto Warburg in 1924, when he hypothesised that cancer is caused by the fact that tumour cells mainly generate energy by non-oxidative breakdown of glucose. Since this breakdown takes place within the mitochondria, accord-ing to Warburg, cancer should be interpreted as a mitochondrial dysfunction. Warburg reported a fundamental difference between normal and cancerous cells to be the ratio of glycolysis to respiration; this observation is also known as the Warburg effect.
Today this hypothesis is generally assumed to be wrong. Nevertheless, anaerobic metabolism might play an important part in tumour growth and its interaction with its environment.
Under hypoxic conditions the only possible solution for the survival of cells is a change in the metabolism to anaerobic combustion of glucose. However, as discussed in section 5.3, this leads to a production of lactic acids. If large parts of a tumour use anaerobic glycolysis as part of their metabolism this leads to an acetous court around the tumour spheroid. Depending on the amount of acid produced and the tolerance of the surrounding tissue this has a strong impact on the interaction between the tumour and its surrounding.
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