13. TYPICAL DISORDERS OF TISSUE GROWTH. TUMORS

Typical forms of abnormal tissue growth include:

• pathological hypertrophy and hyperplasia;

• pathological hypotrophy and atrophy;

• metaplasia;

• dysplasia;

• benign tumor;

• malignant tumor.

TUMORS

The essential property of all tumors may be defined as excessive growth of abnormal (atypical) cells.

Abnormality of tumor cells takes various forms.

• Abnormal proliferation.

• Abnormal differentiation.

• Abnormal metabolism or energy production.

• Physico-chemical abnormality.

• Abnormal functioning.

• Antigenic abnormality.

• Morphological abnormality.

• Abnormal cell-to-cell interactions.

The principal targets of genetic damage during neoplastic transformation of a cell are three classes of normal regulatory genes:

• the growth-promoting protooncogenes;

• the growth-inhibiting cancer-supressor genes (antioncogenes);

• genes that regulate programmed cell death (apoptosis);

• genes that regulate repair of damaged DNA.

Mechanisms of transformation of protooncogene to oncogene.

• Integration of proviral DNA near protooncogene.

• Insertion of retroviral promoter in the close vicinity of protooncogene.

• Mutations of protooncogenes (including genes for growth factor, or growth factor receptor).

• Overexpression or amplification of protooncogene.

Differences between benign and malignant tumors

Three main concepts of carcinogenesis:

1) chemical carcinogenesis;

2) viral carcinogenesis;

3) radiation carcinogenesis.

Chemical carcinogenesis

Chemicals that initiate carcinogenesis fall into one of two categories:

1) direct-acting compounds, which do not require chemical transformation for their carcinogenicity;