October 2020
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Metastasis is a pathogenic agent’s spread from an initial or primary site to a different or secondary site within the host’s body.

The newly pathological sites, then, are metastases.

Cancer occurs after cells are genetically altered to proliferate rapidly and indefinitely.

This uncontrolled proliferation by mitosis produces a primary heterogeneous tumor.

The cells which constitute the tumor eventually undergo metaplasia (the transformation of one differentiated cell type to another differentiated cell type), followed by dysplasia (an abnormality of development and then anaplasia (loss of differentiation of cells and their orientation to each other, a characteristic of tumor cells), resulting in a malignant phenotype.

This malignancy allows for invasion into the circulation, followed by invasion to a second site for tumorigenesis.

Some cancer cells known as circulating tumor cells acquire the ability to penetrate the walls of lymphatic or blood vessels, after which they are able to circulate through the bloodstream to other sites and tissues in the body.

This process is known (respectively) as a lymphatic or hematogenous spread.

After the tumor cells come to rest at another site, they re-penetrate the vessel or walls and continue to multiply, eventually forming another clinically detectable tumor.

This new tumor is known as a metastatic (or secondary) tumor.

Metastasis is one of the hallmarks of cancer, distinguishing it from benign tumors.

Most cancers can metastasize, although in varying degrees.

Basal cell carcinoma for example rarely metastasizes.

When tumor cells metastasize, the new tumor is called a secondary or metastatic tumor, and its cells are similar to those in the original or primary tumor.

Metastatic tumors are very common in the late stages of cancer. The spread of metastasis may occur via the blood or the lymphatics or through both routes.

The most common places for the metastases to occur are the lungs, liver, brain, and bones.

Metastasis involves a complex series of steps in which cancer cells leave the original tumor site and migrate to other parts of the body via the bloodstream, via the lymphatic system, or by direct extension.

Introduction to Cancer Biology: Tissue Invasion and Metastasis

Metastasis

Metastasis and Angiogenesis

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