Dictionary

mega- Prefix meaning abnormally large, as in megalocephaly (an overly large head) and megacardia (an enlarged heart).

megacolon An abnormally enlarged colon. Megacolon can arise as a serious congenital problem in the newborn, caused by abnormal innervation of the colon and frequently requiring surgery.

megakaryocyte A giant cell in the bone marrow that is the ancestor of blood platelets, which are essential to normal blood clotting.

megavitamin therapy The use of massive doses of vitamins to treat disease. Because overuse of vitamins can cause disease, most physicians consider megavitamin therapy controversial. See also Appendix C, “Vitamins”; orthomolecular medicine; vitamin therapy.

meibomianitis Inflammation of the little glands in the tarsus of the eyelids. Chronic inflammation of these glands can lead to a lump in the eyelid known as a chalazion. Also known as meibomitis.

meibomitis See meibomianitis.

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…

meiotic Pertaining to meiosis.

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.

The prefix “melan-” derives from the Greek word “melas,” meaning black or dark. It appears in medical and biological terms to denote dark pigmentation or color, as in melanin (dark skin pigment) and melanoma (malignant tumor of melanocytes).​ Origin and Etymology This prefix stems from ancient Greek “μέλας” (mélas), signifying black or dark hue. In…

melancholia An old term for depression.

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,…