Cancer involves abnormal cells growing uncontrollably, invading nearby tissues, and spreading to other parts of the body through metastasis.
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Human skin is the body’s largest organ and acts as the main barrier against germs, injuries, and ultraviolet rays. It has three main layers: the epidermis, dermis, and subcutaneous tissue. The epidermis, which is the outer layer, is mostly made up of keratinocytes. In the basal layer of the epidermis, right where it meets the dermis, are special cells called melanocytes. These cells create skin pigment and are where melanoma begins.
Melanocytes come from neural crest cells and move to the skin before birth. Their main job is to make melanin, a pigment that absorbs ultraviolet light and protects the DNA in nearby skin cells. This process, called melanogenesis, turns the amino acid tyrosine into melanin, which is stored in small packets called melanosomes. The melanocytes use their long extensions to pass these melanosomes to nearby skin cells, forming a protective cap over the cell’s nucleus, often called a parasol. This shield helps determine skin color and protects the body from sun damage.
Melanoma happens when melanocytes become cancerous. Unlike other common skin cancers that start in keratinocytes, melanoma starts in the cells that make pigment. Although melanoma makes up only a small number of skin cancer cases, it causes most skin cancer deaths. This is because melanocytes are naturally able to move and share material with other cells. When they turn cancerous, they keep this ability, which lets them spread quickly through blood vessels and lymph nodes, even if the original tumor is small.
The pathogenesis of melanoma is a complex interplay between environmental exposure and genetic susceptibility. The central driver is DNA damage induced by ultraviolet (UV) radiation. UV radiation, specifically UVB, causes direct structural damage to DNA, forming cyclobutane pyrimidine dimers. If the cellular repair machinery fails to correct these lesions, mutations accumulate in critical genes that regulate cell growth and death. While the skin has robust repair mechanisms, the sheer volume of damage from intense sun exposure can overwhelm these systems.
When a melanocyte collects enough mutations, it starts to grow out of control. At first, it spreads sideways along the bottom layer of the epidermis. This stage is called the radial growth phase. At this point, the tumor can usually be cured with surgery because it hasn’t reached deeper tissues or blood vessels yet. It often looks like a flat, uneven spot that is getting bigger.
If not treated, the tumor changes and starts to grow downward into the deeper layer of the skin, called the dermis. This is known as the vertical growth phase and is very important for predicting outcomes. The deeper the cancer goes, the worse the outlook. Once the cancer reaches the dermis, it can spread to lymph nodes and distant organs like the lungs, liver, brain, and bones.
Melanoma is not just one disease but includes several different types, each with its own features and growth patterns. The most common type is Superficial Spreading Melanoma, which makes up most cases. This type usually grows across the skin for a long time before going deeper. It often appears on the trunk in men and on the legs in women, and is linked to occasional, intense sun exposure.
Nodular Melanoma is the second most common type and is known for being very aggressive. Unlike superficial spreading melanoma, it usually does not have a long sideways growth phase. Instead, it quickly grows downward into the deeper skin. It often shows up as a raised, firm bump that can be blue, black, or even pink if it has no pigment. Because it grows down instead of out, it is often found at a later stage, which makes the outlook worse.
Lentigo Maligna Melanoma usually appears on skin that has had a lot of sun over the years, often on the face and neck of older adults. It starts as a slow-growing, flat, dark patch called Lentigo Maligna, which can last for years before turning into melanoma. Acral Lentiginous Melanoma is the most common type in people with darker skin. It shows up on the palms, soles, or under the nails. This type is not caused by sun exposure but is linked to different genetic factors.
Most melanomas start in the skin, but melanocytes are also found in other parts of the body, so melanoma can develop elsewhere. The most common non-skin type is Ocular Melanoma, which starts in the eye’s uveal tract (the iris, ciliary body, and choroid). Uveal melanoma is different from skin melanoma, with its own genetic causes and a tendency to spread to the liver.
Mucosal Melanoma is a rare and aggressive type that starts in the moist linings of the body, like the nose, mouth, sinuses, anus, and vulva. These areas are hard to check, and symptoms like bleeding or blockage are often vague, so this melanoma is usually found late. The rich blood and lymph supply in these tissues also helps the cancer spread early.
Desmoplastic Melanoma is a rare cutaneous variant that warrants specific mention due to its deceptive appearance. It is composed of spindle-shaped cells in a fibrous matrix and often lacks pigment (amelanotic). It can resemble scar tissue or a benign cyst, leading to diagnostic delays. This subtype has a high tendency for neurotropism, meaning it spreads along nerve fibers, which complicates surgical clearance and local control.
Melanoma incidence has been rising steadily over the past several decades in fair-skinned populations worldwide. This increase is often attributed to changes in lifestyle, specifically the cultural valuation of tanning and increased recreational sun exposure, as well as an aging population. Australia and New Zealand historically report the highest rates of melanoma in the world, a consequence of a predominantly fair-skinned population living in a region with high ambient ultraviolet radiation levels.
People with darker skin have lower rates of melanoma because their skin contains more eumelanin, which blocks UV rays better than the lighter pigment found in people with red or blonde hair. However, when melanoma does happen in those with darker skin, it is often found later, partly because of less awareness and because it often appears in places that are not checked as often.
Even though more people are getting melanoma, death rates have stayed the same or even dropped in some places. This is thanks to public health efforts that help people catch melanoma early and to new treatments for advanced cases. Immunotherapy and targeted therapy have turned advanced melanoma from a deadly disease into one that can often be treated or managed long-term. Still, melanoma remains a serious health issue, so prevention is very important.
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A mole, or nevus, is a benign cluster of melanocytes. Moles are very common and usually stop growing once they reach a specific size. They have a regular shape and uniform color. Melanoma is a malignant tumor where melanocytes grow uncontrollably. Unlike a mole, melanoma often continues to grow, change shape, and invade surrounding tissues.
No. While most melanomas produce melanin and appear dark brown or black, some melanomas are “amelanotic,” meaning they do not produce pigment. These can appear pink, red, purple, or even skin-colored. Because they lack the classic dark color, amelanotic melanomas are often harder to recognize and may be mistaken for harmless bumps or scars.
Yes. While UV radiation is the primary cause of most skin melanomas, the disease can arise in areas that never see the sun. This includes the soles of the feet, the palms of the hands, the nail beds, the genitals, and the inside of the mouth or nose. Genetic factors play a larger role in these types of melanoma.
Melanin pigment inside skin cells arranges itself in a cap over the nucleus of the cell. This cap acts like a microscopic umbrella or parasol. When UV rays hit the cell, melanin absorbs the energy and scatters it, preventing the radiation from reaching the DNA in the nucleus and thus protecting the cell from mutations.
Melanoma is more dangerous because of its ability to spread. Basal cell and squamous cell carcinomas rarely spread to other parts of the body. Melanoma cells, however, are biologically programmed to move. Even a relatively small or thin melanoma can release cells into the bloodstream or the lymphatic system, allowing cancer to spread to distant organs like the lungs or brain.
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