What Are Cells?
Ans: Life is both wonderful and majestic. Yet for all of its majesty, all organisms are composed of the fundamental unit of life, the cell. The cell is the simplest unit of matter that is alive. From the unicellular bacteria to multicellular animals, the cell is one of the basic organizational principles of biology. Let's look at some of the components of this basic organizer of living organisms.

Figure: The Cell
Cancer Cell:

Figure: The Cancer Cell
A cancer cell is a cell that has achieved a sort of immortality. Unlike normal cells that stop growing at a certain point, cancer cells continue to divide out of control. And unlike normal cells that remain in the region where they began, cancer cells have the ability to both invade nearby tissues and spread to distant regions of the body.

Differences Between Cancer Cells and Normal Cells:

There are many differences between cancer cells and normal cells. Some of the differences are well known, whereas others have only been recently discovered and are less well understood. You may be interested in how cancer cells are different as you are coping with your own cancer or that of a loved one. For researchers, understanding how cancer cells function differently from normal cells lays the foundation for developing treatments designed to rid the body of cancer cells without damaging normal cells.

** Growth :–
Normal cells stop growing (reproducing) when enough cells are present. For example, if cells are being produced to repair a cut in the skin when the repair work is done, cells are no longer reproduced to fill in the hole.

In contrast, cancer cells don’t stop growing when there are enough cells present. This continued growth often results in a tumor (a cluster of cancer cells) being formed. Each gene in the body carries a blueprint that codes for a different protein. Some of these proteins are growth factors, chemicals that tell cells to grow and divide. If the gene that codes for one of these proteins is stuck in the “on” position by a mutation (an oncogene) – the growth factor proteins continue to be produced. In response, the cells continue to grow.

** Communication :–
Cancer cells don’t interact with other cells as normal cells do. Normal cells respond to signals sent from other nearby cells that say, essentially, “you’ve reached your boundary.” When normal cells “hear” these signals they stop growing. Cancer cells do not respond to these signals.

** Cell repair and cell death :–
Normal cells are either repaired or die (undergo apoptosis) when they are damaged or get old.

Cancer cells are either not repaired or do not undergo apoptosis. For example, one protein called p53 has the job of checking to see if a cell is too damaged to repair and if so advise the cell to kill itself. If this protein p53 is abnormal or inactive (for example, from a mutation in the p53 gene,) then old or damaged cells are allowed to reproduce. The p53 gene is one type of tumor suppressor gene that code for proteins that suppress the growth of cells.

** Stickiness :–
Normal cells secrete substances that make them stick together in a group.

Cancer cells fail to make these substances, and can “float away” to locations nearby, or through the bloodstream or system of lymph channels to distant regions in the body.

** Ability to Metastasize (Spread) :–
Normal cells stay in the area of the body where they belong. For example, lung cells remain in the lungs.

Cancer cells, because they lack the adhesion molecules that cause stickiness, are able to travel via the bloodstream and lymphatic system to other regions of the body. Once they arrive in a new region (such as lymph nodes, the lungs, the liver, or the bones) they begin to grow, often forming tumors far removed from the original tumor.

** Energy Source :–
Normal cells get most of their energy (in the form of a molecule called ATP) through a process called the Krebs cycle, and only a small amount of their energy through a different process called glycolysis. Whereas normal cells produce most of their energy in the presence of oxygen, cancer cells produce most of their energy in the absence of oxygen. This is the reasoning behind hyperbaric oxygen treatments that have been used experimentally (with disappointing results thus far) in some people with cancer.

** Appearance :–
Under a microscope, normal cells and cancer cells may look quite different. In contrast to normal cells, cancer cells often exhibit much more variability in cell size – some are larger than normal and some are smaller than normal. In addition, cancer cells often have an abnormal shape, both of the cell, and of the nucleus (the “brain” of the cell.) The nucleus appears both larger and darker than normal cells. The reason for the darkness is that the nucleus of cancer cells contains excess DNA. Up close, cancer cells often have an abnormal number of chromosomes that are arranged is a disorganized fashion.

** Rate of growth :–
Normal cells reproduce themselves and then stop when enough cells are present.
Cancer cells reproduce rapidly before the cells have had a chance to mature.

** Maturation :–
Normal cells mature. Cancer cells they grow rapidly and divide before cells are fully mature, remain immature. Doctors use the term undifferentiated to describe immature cells (in contrast to differentiated to describe more mature cells.) Another way to explain this is to view cancer cells as cells that don’t “grow up” and specialize into adult cells. The degree of maturation of cells corresponds to the "grade" of a cancer. Cancers are graded on a scale from 1 to 3 with 3 being the most aggressive.

** Evading the immune system :–
When normal cells become damaged, the immune system (via cells called lymphocytes) identifies and removes them.

Cancer cells are able to evade (trick) the immune system long enough to grow into a tumor by either by escaping detection or by secreting chemicals that inactivate immune cells that come to the scene. Some of the newer immunotherapy medications address this aspect of cancer cells.

** Blood supply :–
Angiogenesis is the process by which cells attract blood vessels to grow and feed the tissue. Normal cells undergo a process called angiogenesis only as part of normal growth and development and when new tissue is needed to repair damaged tissue.

Cancer cells undergo angiogenesis even when growth is not necessary. One type of cancer treatment involves the use of angiogenesis inhibitors – medications that block angiogenesis in the body in an effort to keep tumors from growing.

** Mortality/Immortality :–
Normal cells are mortal, that is, they have a lifespan. Cells aren’t designed to live forever, and just like the humans they makeup, cells grow old. Researchers are beginning to look at something called telomeres, structures that hold DNA together at the end of the chromosomes, for their role in cancer. One of the limitations to growth in normal cells is the length of the telomeres. Every time a cell divides, the telomeres get shorter. When the telomeres become too short, a cell can no longer divide and the cell dies. Cancer cells have figured out a way to renew telomeres so that they can continue to divide. An enzyme called telomerase works to lengthen the telomeres so that the cell can divide indefinitely – essentially becoming immortal.

** Evading growth suppressors :–
Normal cells are controlled by growth (tumor) suppressors. There are 3 main types of tumor suppressor genes that code for proteins that suppress growth. One type tells cells to slow down and stop dividing. One type is responsible for fixing changes in damaged cells. The third type is in charge of the apoptosis noted above. Mutations that result in any of these tumor suppressor genes being inactivated allow cancer cells to grow unchecked.

** Functioning :–
Normal cells perform the function they are meant to perform, whereas cancer cells may not be functional. For example, normal white blood cells help fight off infections. In leukemia, the number of white blood cells may be very high, but since the cancerous white blood cells are not functioning as they should, people can be more at risk for infection even with an elevated white blood cell count. The same can be true of substances produced. For example, normal thyroid cells produce thyroid hormone. Cancerous thyroid cells (thyroid cancer) may not produce thyroid hormone. In this case, the body may lack enough thyroid hormone (hypothyroidism) despite an increased amount of thyroid tissue.

** Genomic instability :–
Normal cells have normal DNA and a normal number of chromosomes. Cancer cells often have an abnormal number of chromosomes and the DNA becomes increasingly abnormal as it develops a multitude of mutations. Some of these are “driver” mutations, meaning they drive the transformation of the cell to be cancerous. Many of the mutations are passenger mutations, meaning they don’t have a direct function for the cancer cell.

** Invasiveness :–
Normal cells listen to signals from neighboring cells and stop growing when they encroach on nearby tissues (something called contact inhibition.) Cancer cells ignore these cells and invade nearby tissues. Benign (non-cancerous) tumors have a fibrous capsule. They may push up against nearby tissues but they do not invade/intermingle with other tissues. Cancer cells, in contrast, don’t respect boundaries and invade tissues. This results in the fingerlike projections that are often noted on radiologic scans of cancerous tumors. The word cancer, in fact, comes from the latin word for crab used to describe the crablike invasion of cancers into nearby tissues.

Figure: Normal & Cancer cell differences