All somatic cells in the body contain 23 pairs of chromosomes and approximately 100,000 genes that hold 3 billion bits of information. Protein-coding sequences in these genes vary from person-to-person, making every individual unique. Differential gene expression serves to control most cellular functions, including growth and differentiation.
Unfortunately.... mutations happen all the time. Mutations may occur in both germ cells and somatic cells through exposure to radiation, reactive oxygen species and/or chemicals. In most instances, these mutations are silent - the cells are not adversely affected.
However, mutations in genes controlling cell growth can lead to uncontrolled cell proliferation. For this reason, the critical genes that code for growth-regulating proteins are referred to as proto-oncogenes.
Cells that acquire one or more "activated" oncogenes may proliferate autonomously, forming a neoplasm that can either expand locally or spread throughout the body. Benign tumors do NOT spread, whereas malignant tumors invade and form distant metastatic colonies.
Here are the Key Points:
Cancer is the uncontrolled proliferation of cells that express varying degrees of fidelity to their precursors.
By definition, benign tumors do not penetrate adjacent tissue borders or metastasize to distant sites.
Histologic features of malignant cells include anaplasia, pleomorphism, mitotic activity, and evidence of invasion.
Cancer cells commonly invade thin-walled capillaries, venules and lymphatic vessels. This process of invasion requires the expression of appropriate cell adhesion molecules and extracellular matrix-degrading proteases (for example collagenase).
Staging is a means of predicting the clincial behavior of cancer, based on the extent of tumor spread within the body.
The following images illustrate Key Morphological Concepts:
1. This is an example of ACTINIC KERATOSIS, a pre-neoplastic condition of the skin that is associated with long-term exposure to sunlight. Microscopic examination of the affected skin would show severe dysplasia.
2. This is an example of a UTERINE LEIOMYOMA, a benign tumor of smooth muscle cell origin. The uterus and tumor have been bisected. Note that this tumor is encapsulated and well-circumscribed, as would be expected in any benign neoplasm.
3. This is an example of a PLEOMORPHIC ADENOMA OF THE SALIVARY GLAND. An adenoma is defined as a benign tumor of glandular epithelial cell origin.
4. This is an example of a MENINGIOMA, a benign tumor arising from mesenchymal cells of the arachnoid. This case illustrates the point that even benign tumors in critical locations can kill by compressing vital centers in the brain.
5. This is an example of a maligant UTERINE ADENOCARCINOMA. This tumor is derived from glandular epithelium of the endometrium. The prognosis for patients with uterine endometrial cancer depends on tumor stage.
6. This is an example of MALIGANT MELANOMA of the skin. Note the variagated appearance of the skin lesion.
7. This is an example of RETINOBLASTOMA, a malignant tumor arising from immature neurons in the retina. The pathogenesis of retinoblastoma involves mutations in the Rb gene. The Rb gene codes for a tumor suppressor protein. Without a functional Rb tumor suppressor protein, immature neuronal cells proliferate continuously forming a tumor.
8. This is an example of METASTATIC LIVER CANCER. Note the multiple "cannonball" metastases that have seeded the liver. The portal vein brings blood to the liver from the gut and other internal organs. For this reason, the liver is a major site for the development of metastatic cancer.
