A metastasis in the medical sense is understood to be two different clinical pictures with a similar background: the splitting off of tumor cells from the primary tumor and colonization of tumor-derived tissues and the settling of bacteria from the original site of inflammation. In the following, the former will be discussed here.
A metastasis is a daughter tumor originating from a primary tumor, which has separated from the primary tumor by spread through blood and lymph channels, but is still similar to identical in cell type and cell function. The development of metastases is a very complex process, which has not yet been clarified in detail, but is currently a major focus of medical research in the context of cancer treatment. One of the main characteristics of tumor cells, apart from “degeneration”, i.e. the functional profile that differs from that of the original tissue, is the massively increased rate of cell division.
Tumors therefore grow faster and have a different function than the surrounding tissue. Furthermore, most tumor cells have fewer so-called adhesion molecules (“adhesive proteins“, which anchor cells firmly in their natural environment) than their original cells, i.e. they form less stable cell complexes. If a primary tumor expands and makes contact with the blood or lymphatic system, it can lead to the spread of tumor cells into foreign tissue sections and the settlement and development of a secondary tumor, a metastasis.
This process is called metastasis. If tumor cells enter other tissues through the blood, this is called “hematogenic” spreading; the equivalent for the lymphatic system is lymphogenic spreading. When spreading through the blood and lymphatic system, the tumor cells benefit from the fact that they are basically of “endogenous” origin and are therefore not recognized by the immune system as foreign and pathogenic.
Of course, the spread tumor cells also need other properties to settle in the new tissue, such as the ability to integrate into the new tissue sections, to adhere and to adapt to the prevailing conditions. If a transplanted tumor cell possesses these properties, it enters a new body compartment and can settle there. Once again, it multiplies faster than the host tissue, stimulates the formation of small blood vessels (capillaries) in the area of influence (increased angiogenesis) and, over time, displaces the actual functional tissue.
A distinction is made between local metastases, regional metastases and distant metastases. Local metastases develop in direct proximity to the primary tumor. They enter the neighboring organ through small gaps in the cell structure and settle there.
Regional metastases refer to tumor cells that have been transported by the lymphatic system and are deposited in the following lymph nodes and their surrounding tissues. Depending on the organ of origin of the primary tumor, there are typical deposition sites for regional metastases in the lymphatic system. If tumor cells enter the bloodstream and are carried away, this is called distant metastasis. Here, too, there are specific sites of distant metastases for various primary tumors.