Cancer is a disorder where a group of cells ignores normal cell division principles and grows uncontrollably. Because cancerous cells are self-sufficient, they do not respond to the signals that initiate the normal cell cycle, resulting in the uncontrolled development and proliferation of altered cells.
If malignant cells continue to increase, it can be deadly. 90% of cancer-related fatalities are caused by cancer cells spreading to other tissues; a process termed metastasis. During mitosis, normal cells develop in an interdependent way, reliant on external growth stimuli. As a result, when the availability of these growth signals becomes insufficient or ceases, cells cease to replicate.
By contrast, tumor cells proliferate in the absence of an external stimulus or signal. Additionally, normal cells possess the ability to prevent interaction. They halt cell division when a sufficient number of surrounding cells is present, i.e., when a certain threshold is reached. On the other hand, cancer cells lack this ability to prevent interaction, resulting in an undesirable mass of cells.
A typical cell’s existence is well-programmed; it multiplies around 50 times before dying of apoptosis and replacing a new cell. Multiplication is due to the restricted efficiency of DNA replication, as repeated replication results in telomere shortening. On the other side, cancer cells have increased telomerase activity, which continually replaces the lost, worn-out ends of the telomere, allowing for unrestricted cell growth.
Biology of tumors
When a cell divides independently of growth stimuli, it produces tumors formed in a sequence of stages. The first stage involves the formation of a huge mass of cells known as hyperplasia as a result of unregulated cell division. Hyperplasia is followed by dysplasia, a condition in which aberrant cell growth occurs.
Additional modifications occur in the subsequent stage when these atypical cells begin to expand throughout a small tissue region, therefore losing their original role. This stage is referred to as anaplasia. The tumor is not invasive at this stage and is considered benign.
Tumor cells gain the capacity to spread during the advanced stage. They begin invading both adjacent and distant tissues via circulation. This stage is considered malignant and is extremely difficult to cure. However, not all cancers that are detected early grow to this stage.
While tumor cells may grow without growth hormones, they require nutrition and oxygen to develop. Capillaries are abundant in all normal tissues, supplying nutrients and oxygen to each cell. Similarly, when tumors develop, they generate new blood vessels through a process called angiogenesis. New blood vessels allow nutrients to reach the tumor mass’s center cells that have access to regular blood arteries.
Tumors are classified according to the type of cell that was first changed. These include the following:
- Carcinomas are epithelial cell cancers. They have the greatest ratio of all cancer forms.
- Sarcomas are cancerous tumors that develop in the bone, muscle, fat, and connective tissue.
- Leukemia is a blood cancer that develops from malignant white blood cells.
- Lymphoma is a cancer that affects the lymphatic system or bone marrow cells (BM).
- Myelomas are malignancies of the white blood cells that produce antibodies.
Classification according to grade
The grade of a tumor is an anomaly in cells in comparison to their typical tissue surroundings. The grade is raised from 1 to 4 when the degree of irregularity rises. Well-differentiated cells have a high degree of resemblance to normal cells and are associated with low-grade malignancies. Improperly differentiated cells exhibit a significant degree of abnormality in comparison to the surrounding tissues. These are malignant tumors of a high grade.
Grade 1: This category contains cells that are highly differentiated yet have a minor irregularity.
Grade 2: These cells have a moderate degree of differentiation and are slightly aberrant.
Grade 3: The cells are poorly differentiated and extremely aberrant in terms of chromosomal mutations, and they generate toxic compounds that damage neighboring cells and may reach the bloodstream.
Grade 4: Cells are young, primitive, and undifferentiated.
Causes of cancer
Cancer develops and progresses due to several variables within the cell (mutations, immunological conditions, and hormones) and external elements from the environment (smoking, chemicals, infectious organism, and radiations). These combined factors result in aberrant cell behavior and uncontrolled growth.
The resulting abnormal cell mass develops and damages normal tissues in its immediate vicinity and occasionally spreads to other parts of the body (metastasis). According to the most widely recognized hypothesis of cancer etiology, mutations in tumor suppressors and oncogene genes are the primary cause behind the genesis of cancer.
Another hypothesis proposes that a mutation in a master gene that regulates cell division might also steer normal Neoplasm cells toward aberrant chromosomal replication, resulting in the duplication or deletion of whole chromosome regions.
This alteration in the genetic composition of the cells results in an abnormal quantity of a certain protein being produced regardless of the actual requirement.
Cancer may occur if a chromosomal abnormality has a quantitative or qualitative effect on a protein involved in the cell cycle. There is compelling evidence that the addition (hypermethylation) or deletion (hypomethylation) of methyl groups to genes involved in the cell cycle, DNA repair, and apoptosis is related to some malignancies.
It’s important to keep in mind that tumors might take months or even years to accumulate enough DNA mutations for the resulting tumor mass to be detected. Thus, various pathways may contribute to the development of cancer.
Establishing a link between tumor viruses and cancer
Viruses have a significant role in the development of human cancers. Indeed, oncogenic viruses are thought to cause 15% of cancer. These viruses include:
- Epstein–Barr virus (EBV)
- Hepatitis B and C viruses (HBV and HCV).
- Human papillomaviruses (HPVs)
- Kaposi’s sarcoma-associated herpesvirus (KSHV, also known as HHV-8)
- Merkel cell polyomavirus (MCPyV) was discovered as the cause of Merkel cell carcinoma, a rare but severe form of skin cancer.
Recent research on these cancer-causing viruses has aided in our understanding of the fundamental biology of the cell and how disruptions in cellular pathways result in the beginning and maintenance of cancer.