Stages of the metastatic cascade
The process begins with the emergence of a metastatic subclone. Cells undergo the epithelial-mesenchymal transition: they alter the properties of the plasmalemma, lose intercellular contacts, and acquire the ability to actively migrate.
- Invasion and intravasation. Using integrins, cells attach to matrix components (laminin, fibronectin). Enzymes (collagenase, plasmin, cathepsin, etc.) degrade the basement membrane. The cell penetrates the vessel, downregulates adhesion molecules, and detaches from the endothelium.
- Circulation. In the bloodstream, cells are exposed to mechanical destruction, immune attacks, and anoikis (apoptosis due to loss of cell-cell and cell-matrix contact). To survive, they aggregate with leukocytes and platelets, forming thromboemboli coated with a protective layer of fibrin.
- Extravasation. Emboli become trapped in the capillaries of distant organs. Tumor cells actively utilize CD44 molecules (normally required by T lymphocytes) to bind to venule endothelium and enter the parenchyma.
- Development of a secondary tumor. Once in the perivascular tissue, the cells must initiate angiogenesis, begin proliferation, and suppress local anti-tumor defense.
Where do tumors metastasize?
As a general rule, most metastases occur at the first capillary bed encountered in the bloodstream (the natural drainage mechanism). However, many neoplasms exhibit organ tropism—specificity for certain tissues. This is explained by several factors:
- Specific adhesion. The endothelium of target organs possesses ligands that ideally match the adhesion molecules of specific tumor cells.
- Chemokines. Target organs secrete substances that attract cancer cells. For example, breast cancer cells express CXCR4 and CCR7 receptors, while their ligands (CXCL12 and CCL21) are abundantly present in the lungs and lymph nodes.
- Chemoattractants. The lungs and liver actively produce growth factors (IGF-I and IGF-II), attracting metastases.
- Paget's "seed and soil" theory. Even with a rich blood supply, certain tissues remain an "unsuitable soil." Therefore, metastases are extremely rare in the heart, spleen, and skeletal muscles.
Colonization and the phenomenon of dormancy
Detaching from the primary node is easy, but establishing a foothold in a new location is a formidable challenge. Millions of cells enter the bloodstream daily, but visible metastases develop in only a fraction of patients.
The phenomenon of dormancy frequently occurs—the latent existence of micrometastases without progression. This dormant state is typical of melanoma, prostate, and breast cancer.
To emerge from "hibernation" and begin successful colonization, tumor cells must interact with local stromal cells. By secreting cytokines, extracellular matrix components, and growth factors, the tumor induces the stroma to form a comfortable "niche" for its survival and growth.
Main theories of metastasis
Modern pathological anatomy recognizes four main perspectives on the nature of this process:
- Clonal evolution theory. Due to genetic instability, the tumor becomes heterogeneous. Only a small subpopulation of cells accumulates the full complement of mutations required to overcome all steps of the cascade.
- Early "metastatic signature" hypothesis. The capacity for dissemination is established at early stages in the majority of cells within the primary tumor. The outcome depends on microenvironment characteristics and host anti-tumor resistance mechanisms.
- Pre-existing genetic variability hypothesis. The propensity for metastasis is dictated by the initial genetics of the patient (proven in experiments on mice with identical induced oncogenes).
- Cancer stem cell hypothesis. Secondary lesions are the result of the spread and division of individual cancer stem cells.
Briefly: answers to topical questions
What are the main pathways of malignant tumor metastasis?
The main pathways of malignant tumor metastasis are lymphatic, hematogenous, perineural, and implantation.
- Lymphatic pathway — spread via lymphatic vessels (retrograde metastasis against the lymph flow is possible), most characteristic of carcinomas.
- Hematogenous pathway — spread via blood vessels, characteristic predominantly of sarcomas.
- Perineural pathway — spread of tumor cells along nerve trunks.
- Implantation pathway — contact spread along serous membranes.
To which organs does lung cancer metastasize?
Lung cancer metastasizes via lymphatic and hematogenous pathways.
- Lymph nodes — the first metastases appear in regional lymph nodes; in advanced stages, bifurcation, paratracheal, mediastinal, and cervical lymph nodes are affected.
- Hematogenous metastases — liver, bones, adrenal glands, brain.
- The development of carcinomatosis of the lungs, pleura, and peritoneum is also possible.
To which organs does prostate cancer metastasize?
Prostate cancer metastasizes to bones and lymph nodes.
- Bones — indicated as a characteristic direction of prostate cancer metastasis; prostate cancer is also listed among primary tumors when metastasizing to the jaws.
- Lymph nodes — regional pelvic lymph nodes; metastases to the inguinal, iliac, and para-aortic lymph node groups are also described.
Additionally, the tumor is capable of local invasion, growing into the seminal vesicles, and in advanced stages, into the urinary bladder.
Which adhesion molecules are involved in the extravasation of tumor cells?
Integrins, laminin receptors, and CD44 molecules are involved in the extravasation of tumor cells.
- Integrins — used during migration through the matrix and attachment to basement membrane components.
- Laminin receptors — used during the exit of tumor cells through the basement membrane into the organ parenchyma.
- CD44 molecules — enhanced expression of CD44 by tumor cells promotes their interaction with venule endothelium and metastasis.
To remember the factors of organ tropism, use the mnemonic AHAP: Adhesion (specific ligands), Chemokines (directed movement), Attractants (IGF growth factors), Pasture/Soil (Paget's theory).
Class and exam questions
What is anoikis and how does a tumor evade it?
Anoikis is a form of apoptosis that is triggered when a cell loses adhesion to other cells or the extracellular matrix. To survive in the bloodstream, tumor cells aggregate with platelets and circulating leukocytes, forming fibrin-protected emboli.
What role do CD44 molecules play in metastasis?
Normally, CD44 is required for T lymphocytes to migrate into lymphoid tissue. Tumor cells upregulate the expression of these molecules to efficiently bind to venule endothelium and enter the tissues of distant organs.
Why do metastases rarely affect the heart and skeletal muscles?
According to Paget's "seed and soil" theory, despite abundant vascularization, the microenvironment of these organs is an "unsuitable soil" for the survival and colonization of tumor cells.
What else to study on this topic
- Mechanisms of epithelial-mesenchymal transition in tumor cells
- The role of integrins and proteolytic enzymes in basement membrane degradation
- Interaction between tumor and stromal cells during the formation of the metastatic "niche"
- Mechanisms of host anti-tumor resistance (stromal reaction, immune defense)
- The influence of the microenvironment on the activation of latent micrometastases (emergence from dormancy)
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