melanoma, lentigo maligna One of the four clinical types of malignant melanoma and the slowest growing type. Lentigo maligna melanoma typically begins as a patch of mottled pigmentation that is dark brown, tan, or black on sun-exposed skin, such as on the face.
melanoma, choroidal See melanoma, ocular.
melanoma, amelanotic A colorless melanoma that is detectable only on close examination of the skin.
melanoma, acral-lentiginous A type of malignant melanoma that is uncommon in white people but the most common type in nonwhite people. Acral-lentiginous melanoma starts as an irregular enlarging black flat spot (macule), most often on the palm of the hand and the sole of the foot, less often on a mucosal surface, such as the…
melanoma The most dangerous form of skin cancer, a malignancy of melanocytes, the cells that produce pigment in the skin. Melanoma is most common in people with fair skin, but it can occur in people with all skin colors. Most melanomas present as dark, mole-like spots that spread and, unlike moles, have irregular borders. The…
melanocyte A cell in the skin that produces and contains the pigment melanin.
melanin The pigment that gives human skin, hair, and eyes their color. Dark-skinned people have more melanin in their skin than light-skinned people have. Melanin is produced by cells called melanocytes. It provides some protection again skin damage from the sun, and the melanocytes increase their production of melanin in response to sun exposure. Freckles,…
melancholia An old term for depression.
melan- Prefix meaning dark or black, as in melancholia (a dark and gloomy mood) and melanin (a dark pigment).
meiotic nondisjunction Failure of two members of a chromosome pair to separate from one another during meiosis, causing both chromosomes to go to a single daughter cell. Meiotic nondisjunction is responsible for the extra chromosome 21 in trisomy 21 (Down syndrome) and for extra and missing chromosomes that cause other birth defects and many miscarriages.
meiotic Pertaining to meiosis.
meiosis The process chromosomes undertake during germ-cell formation to halve the chromosome number from 46 to 23. In meiosis, the 46 chromosomes in the cell divide to make two new cells with 23 chromosomes each. Before meiosis is complete, however, chromosomes pair with their corresponding chromosomes and exchange bits of genetic material. In women, X…