Melanoma cells are a specific type of cancer cell that originate from melanocytes, the pigment-producing cells in the body. They are the cause of melanoma, a serious form of skin cancer. Melanoma cells have unique characteristics that differentiate them from normal cells, particularly in their growth patterns and ability to spread. Understanding these cellular properties is important for recognizing the disease and developing effective treatments.
Origin and Formation
Melanoma cells develop when melanocytes undergo genetic mutations. Melanocytes are found primarily in the skin, where they produce melanin, a pigment that gives skin its color and protects against ultraviolet (UV) radiation. Mutations in a cell’s DNA can disrupt the controls on cell division and growth, leading to uncontrolled proliferation.
About 90% of melanomas are attributed to UV radiation exposure from the sun or tanning devices. UV rays can directly damage the DNA within skin cells, causing specific types of DNA lesions. While DNA repair mechanisms typically address this damage, persistent exposure can lead to unrepaired mutations in genes that regulate cell growth and division, such as tumor suppressor genes or oncogenes. About half of all melanomas show a mutation in the BRAF oncogene.
The transformation from a melanocyte to a melanoma cell involves genetic alterations. Initially, these mutated cells may grow in a disordered fashion, but they eventually acquire additional mutations that allow them to grow uncontrollably and invade surrounding tissues. This process can take many years, with melanoma sometimes appearing long after initial UV exposure, especially in individuals with intense sun exposure during childhood or young adulthood.
Distinctive Characteristics and Behavior
Melanoma cells exhibit unique features that distinguish them from healthy cells. They often display irregular shapes and sizes, known as pleomorphism, and can have darker pigmentation due to unregulated melanin production. Their nuclei may also appear atypical, with irregular shapes and prominent nucleoli.
Melanoma cells rapidly and uncontrollably proliferate. Unlike normal cells, melanoma cells divide without restraint, forming a tumor. This unchecked division contributes to the rapid growth of melanoma lesions.
Beyond their irregular appearance and rapid growth, melanoma cells are invasive. They can extend beyond their original location in the epidermis, growing into deeper layers of the dermis and subcutaneous tissue. This invasive nature allows them to infiltrate surrounding healthy tissues. Melanoma cells also have metastatic potential, meaning they can detach from the primary tumor, enter the bloodstream or lymphatic system, and travel to distant organs to form new tumors. This ability to spread throughout the body makes melanoma dangerous if not detected and treated early.
How Melanoma Cells Are Identified
Identifying melanoma cells begins with a visual examination of suspicious skin lesions, often guided by the ABCDEs of melanoma. While visual assessment can indicate a potential issue, definitive identification requires a biopsy and pathological examination. During a biopsy, a tissue sample from the suspicious lesion is removed and sent to a lab for analysis.
A pathologist or dermatopathologist will analyze the biopsy sample under a microscope. They look for specific cellular characteristics that confirm the presence of melanoma cells. At the cellular level, pathologists identify features such as cellular pleomorphism, nuclear atypia (abnormal nuclei), and increased mitotic activity (cells actively dividing). The depth of invasion, measured as Breslow thickness, is also assessed, indicating how deep the melanoma has grown into the skin.
Specialized immunohistochemical stains, such as S-100 protein, HMB-45, and Melan-A, are used to highlight specific proteins expressed by melanocytes, aiding in differentiation from benign growths. The presence of lymphovascular or perineural invasion (melanoma cells in blood vessels, lymphatic vessels, or around nerves) is also noted, as it can indicate a higher likelihood of spread. In some cases, molecular testing may be performed on biopsy samples to identify specific gene mutations, such as in the BRAF gene, which can influence treatment options.
Differentiating Melanoma from Benign Growths
Distinguishing melanoma cells from benign moles (nevi) is a task for pathologists. While both originate from melanocytes, their cellular characteristics and growth patterns differ significantly. Benign moles consist of melanocytes that appear uniform in size and shape, arranged in an orderly fashion, often in clusters or “nests”. These cells show minimal proliferative activity and mature as they extend deeper into the skin.
In contrast, melanoma cells exhibit marked pleomorphism (varying widely in size and shape) and display dysplastic features (abnormal development). Their growth patterns are chaotic and disorganized, often with irregular nests of cells and individual melanocytes scattered throughout the epidermis, a phenomenon known as pagetoid spread. Unlike benign cells, melanoma cells maintain their atypical appearance even in deeper parts of the lesion and show a higher mitotic rate, reflecting their uncontrolled division.
Immunohistochemical stains can further aid in this differentiation. For example, HMB-45 staining in benign nevi is limited to melanocytes in the epidermis and upper dermis, showing a maturation pattern where staining lessens with depth. In melanoma, HMB-45 staining is consistently present throughout the lesion, including in deeper melanocytes, reflecting a lack of maturation. Pathologists also look for atypical cells at the basal layer of the epidermis and irregular junctional activity, which are suggestive of malignancy.