Medical Dictionary Definitions A-Z List
Medical Dictionary Definitions A - Z - «M»:
Meiotic nondisjunction: Failure of two members of a chromosome
pair to separate (disjoin) during meiosis so that both go to one daughter cell and none to
the other. This mechanism is responsible for the extra chromosome 21 in trisomy 21 (Down
syndrome) and for extra and missing chromosomes causing o...
Melan- (prefix): Prefix meaning dark or
black. It comes from the Greek "melas", black. Examples of terms
containing melan- include melancholia, melanin, melanocytes, melanoma
Melancholia: Old term for depression. ...
Melanin: A skin pigment (substance that gives the
skin its color). Dark-skinned people have more melanin than light-
skinned people. Melanin also acts as a sunscreen and protects the
skin from ultraviolet light. Melanin is produced by cells called
melanocytes. It provides some protection again sk...
Melanocyte: A pigment-producing cell in the skin, hair and eye that determines their color. The pigment that melanocytes make is called melanin. The major determinant of color is not the number but rather the activity of the melanocytes. Melanin production takes place in unique organelles (tiny stru...
Melanoderma: Increased skin pigmentation, darkening of the skin. This is directly due to an increased amount of melanin pigment in the skin when there is increased production of melanin by the melanocytes (the cells that make melanin) or if there is a rise in the number of melanocytes.
Melanoma: The most dangerous form of skin cancer, a malignancy of the melanocyte, the cell that produces pigment in the skin. Melanoma is most common in people with fair skin, but can occur in people with all skin colors. Most melanomas present as a dark,
mole-like spot that spreads and, unlike a m...
Melanoma of the eye
Melanoma of the eye: See: Intraocular melanoma....
Melanoma thickness: A method for determining the prognosis (outlook) with melanoma. The thickness of a melanoma is related to the 5-year survival rate after surgical removal of the tumor. Also called Breslow thickness.
Named for the physician Alexander Breslow who in 1975 observed that as the thic...
Melanoma vaccine: A cancer vaccine prepared from human melanoma cancer cells. It may be used alone or with other therapy in treating melanoma....
Melanoma, juvenile: A benign raised pink or red scaly area typically seen on a preadolescent child's skin, usually on the cheek.
Also called a benign compound nevus or a spindle and
epithelioid cell nevus....
Melanoma, acral-lentiginous: One of the four clinical types of malignant melanoma, uncommon in whites but the most common type in nonwhites. It starts as an irregular enlarging black flat spot (macule), most often on the palm and sole, less often on a mucosal surface, such as the vulva or vagina....
Melanoma, amelanotic: A form of melanoma in which the malignant cells (melanocytes) do not make the pigment melanin. Amelanotic melanomas may be pink, red, or have light brown, tan, or gray at the edges and are usually detectable only on close examination of the skin....
Melanoma, benign: A tumor of the melanocytes that
is not cancerous....
Melanoma, choroidal: Malignant melanoma that arises in the eye. See ocular melanoma....