Multistep Carcinogenesis

Tumor development in the whole animal is thought to be a multistep process (1-4). This process, which could be defined as a bad situation becoming worse, has been termed tumor progression and is crudely diagrammed in Fig. 1. A normal cell with the burden of its inherited genetic makeup, which may predispose it to cancer, is subject to successive environmental insults (5), causing a gradual accumulation of genetic changes that may result in cancer. Such mutant cells are influenced by many complex host responses and influences that can either encourage or retard tumor growth. Crucial to the development of malignancy is angiogenesis—the cancer obtains a blood supply and can thus expand in size, since it is not restricted to obtaining nutrients by diffusion (6). The tumor finally becomes a killer when it

From Methods in Molecular Biology, Vol 18 Transgenesis Techniques Principles and Protocols

Edited by D Murphy and D A Carter Copyright ©1993 Humana Press Inc , Totowa, NJ

TUMORS IN ANIMALS

METASTASIS

Fig. 1. Diagramatic representation of tumor progression m vivo and its mimicry in culture.

TUMORS IN ANIMALS

METASTASIS

Fig. 1. Diagramatic representation of tumor progression m vivo and its mimicry in culture.

becomes invasive—it eats into surrounding normal tissue, and when it metastasizes—that is, it spreads to other parts of the body (7).

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