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Medical Dictionary


megalourethra (meg′a-lo-u-re′thra)
Congenital dilation of the urethra.

Large. [G. megas (megal-)]

megamerozoite (meg′a-mer-o-zo′it)
SYN: macromerozoite.

meganucleus (meg-a-noo′kle-us)
SYN: macronucleus (1) .

megapoietin (meg′a-poy′e-tin)
SYN: thrombopoietin. [mega- + G. poietes, maker, + -in]

megaprosopia (meg′a-pro-so′pe-a)
SYN: macroprosopia. [mega- + G. prosopon, face]

megaprosopous (meg-a-pros′o-pus)
SYN: macroprosopous.

megarectum (meg-a-rek′tum)
Extreme dilation of the rectum.

megaseme (meg′a-sem)
Denoting an orbital aperture with an index above 89. [mega- + G. sema, sign]

megasigmoid (meg-a-sig′moyd)
SYN: macrosigmoid.

megasomia (meg-a-so′me-a)
SYN: macrosomia.

megaspore (meg′a-spor)
SYN: macrospore.

megathrombocyte (meg-a-throm′bo-sit)
A large blood platelet, especially a young one recently released from the bone marrow. [mega- + G. thrombos, clot, + kytos, cell]

megaureter (meg′a-u-re′ter)
SYN: megaloureter. primary m. independent ureteral dilation; may be nonobstructive or related to congenital distal ureteral obstruction. secondary m. hydroureter secondary to vesicoureteral reflux or distal obstruction.

megavolt (meg′a-volt)
One million volts.

megavoltage (meg′a-vol′tij)
In radiation therapy, a term for voltage above one million volts.

megestrol acetate (me-jes′trol)
A synthetic progestin with progestational effects similar to those of progesterone; current uses include palliation in breast cancer and as an appetite stimulant in advanced malignancy.

meglitinides (meg-lit′in-idz)
A class of oral glucose-lowering drugs that act by closing ATP-dependent potassium channels in pancreatic beta cells, thus causing calcium channel opening and subsequent insulin release.

meglumine (meg′loo-men)
USAN-approved contraction for N-methylglucamine. m. acetrizoate a radiographic contrast medium. See acetrizoate sodium. m. diatrizoate a water-soluble organic iodine compound formerly used for excretory urography, for contrast visualization of the cardiovascular system, and orally for opacification of the gastrointestinal tract. SYN: methylglucamine diatrizoate. m. iothalamate N-methylglucamine salt of iothalamic acid (60% solution); a diagnostic radiopaque medium for intravascular use in angiography and urography.

megohm (meg′om)
One million ohms.

megophthalmus (meg-of-thal′mus)
SYN: megalophthalmos.

megoxycyte (meg-oks′e-sit)
SYN: megoxyphil.

megoxyphil, megoxyphile (meg-oks′e-fil, fil)
An eosinophilic leukocyte containing coarse granules. SYN: megoxycyte. [mega- + G. oxys, acid, + phileo, to like]

megrim (me′grim)
Obsolete term for migraine.

Meibom, Meibomius
Hendrik (Heinrich), German anatomist, 1638–1700. See meibomian cyst, meibomian glands, under gland, meibomian sty.

meibomian (mi-bo′me-an)
Attributed to or described by Meibom.

meibomitis, meibomianitis (mi′bo-mi′tis, mi-bo′me-a-ni′tis)
Inflammation of the meibomian glands.

Georg, German serologist, *1875. See Porges-M. test.

Henri, French physician, 1866–1940. See M. disease.

Joe V., U.S. gynecologist, 1892–1963. See M. syndrome.

Ernst, German physician, 1878–1945. See M. test.

For words beginning thus and not found here, see mio-.

meiosis (mi-o′sis)
A special process of cell division comprising two nuclear divisions in rapid succession that result in four gametocytes, each containing half the number of chromosomes found in somatic cells. SYN: meiotic division. [G. m., a lessening]

meiotic (mi-ot′ik)
Pertaining to meiosis.

See Wachstein-M. stain for calcium-magnesium-ATPase.

Georg, German histologist, 1829–1905. See M. corpuscle, M. plexus.

1. SYN: honey. 2. Unit of pitch; a pitch of 1000 mels results from a simple tone of frequency 1000 Hz at 40 dB above the normal threshold of audibility.

mel-, melo-
1. Limb. [G. melos] 2. A cheek. [G. melon] 3. Honey, sugar. SEE ALSO: meli-. [L. mel, mellis, G. meli, melitos] 4. Sheep. [G. melon]

melagra (me-lag′ra)
Rheumatic or myalgic pains in the arms or legs. [G. melos, limb, + agra, seizure]

melalgia (me-lal′je-a)
Pain in a limb; specifically, burning pain in the feet extending up the leg and even to the thigh. [G. melos, a limb, + algos, pain]

melamine formaldehyde (mel′a-men)
SYN: melamine resin.

melan-, melano-
Black, extreme darkness of hue. [G. melas]

melancholia (mel-an-ko′le-a)
1. A severe form of depression marked by anhedonia, insomnia, psychomotor changes, and guilt. 2. A symptom occurring in other conditions, marked by depression of spirits and by a sluggish and painful process of thought. SYN: melancholy. [melan- + G. chole, bile. See humoral doctrine] hypochondriacal m. m. with many associated physical complaints, often with little basis in fact. involutional m. a depressive disorder of middle life, commonly associated with the climacteric.

melancholic (mel-an-kol′ik)
1. Relating to or characteristic of melancholia. 2. Formerly, denoting a temperament characterized by irritability and a pessimistic outlook. 3. A person who is exhibiting melancholia.

melancholy (mel′an-kol-e)
SYN: melancholia.

melanemia (mel-a-ne′me-a)
The presence of dark brown, almost black, or black granules of insoluble pigment (melanin) in the circulating blood. [melan- + G. haima, blood]

melaniferous (mel-a-nif′er-us)
Containing melanin or other black pigment. [melan- (melanin) + L. ferro, to carry]

melanin (mel′a-nin)
Any of the dark brown to black polymers of indole-5,6-quinone and/or 5,6-dihydroxyindole 2-carboxylic acid that normally occur in the skin, hair, pigmented coat of the retina, and inconstantly in the medulla and zona reticularis of the adrenal gland. M. may be formed in vitro or biologically by oxidation of l-tyrosine or l-tryptophan, the usual mechanism being the enzymatic oxidation of l-tyrosine to 3,4-dihydroxy-l-phenylalanine (dopa) and dopaquinone by monophenol monooxygenase, and the further oxidation (probably spontaneous) of this intermediate to m.. Cf.:eumelanin, pheomelanin. SYN: melanotic pigment. [G. melas (melan-), black] artificial m., factitious m. SYN: melanoid.

melanism (mel′a-nizm)
Unusually marked, diffuse, melanin pigmentation of body hair and skin (usually not affecting the iris). SEE ALSO: melanosis.

See melan-.

melanoacanthoma (mel′a-no-ak-an-tho′ma)
A seborrheic keratosis with melanin pigmentation associated with proliferation of intraepidermal melanocytes. [melano- + G. akantha, thorn, + suffix -oma, tumor]

melanoameloblastoma (mel′a-no-am′e-lo-blas-to′ma)
SYN: melanotic neuroectodermal tumor of infancy. [melano- + ameloblastoma]

melanoblast (mel′a-no-blast)
A cell derived from the neural crest; it migrates to various parts of the body early in embryonic life, and then becomes a mature melanocyte capable of forming melanin. [melano- + G. blastos, germ, sprout]

melanocyte (mel′a-no-sit)
A pigment-producing cell located in the basal layer of the epidermis with branching processes by means of which melanosomes are transferred to epidermal cells, resulting in pigmentation of the epidermis. SYN: melanodendrocyte, pigment cell of skin. [melano- + G. kytos, cell]

melanocytoma (mel′a-no-si-to′ma)
1. A pigmented tumor of the uveal stroma. 2. Usually benign melanoma of the optic disk, appearing in markedly pigmented individuals as a small deeply pigmented tumor at the edge of the disk, sometimes extending into the retina and choroid; malignant metaplasia is rare. [megalo- + cyto- + G. -oma; tumor]

melanodendrocyte (mel′a-no-den′dro-sit)
SYN: melanocyte. [melano- + G. dendron, tree, + kytos, a hollow (cell)]

melanoderma (mel′a-no-der′ma)
1. An abnormal darkening of the skin by deposition of excess melanin. 2. Hyperpigmentation of the skin by melanin or deposition of dark substances such as silver, iron, and drug derivatives. [melano- + G. derma, skin] m. cachecticorum m. of the cachectic, occurring in certain chronic diseases, such as malaria and tuberculosis. parasitic m. excoriations and m. caused by scratching the bites of the body louse, Pediculus corporis. SYN: vagabond's disease, vagrant's disease. racial m. the normally dark skin of members of some nonwhite races. senile m. cutaneous pigmentation occurring in the aged. SYN: melasma universale.

melanodermatitis (mel′a-no-der-ma-ti′tis)
Excessive deposit of melanin in an area of dermatitis.

melanogen (me-lan′o-jen, mel′a-no-jen)
A colorless substance that may be converted into melanin; e.g., some patients with widespread metastases of melanoma excrete m. in their urine, and melanin is formed when the urine is exposed to air ( i.e., oxidized) for a few hours. [melanin + G. -gen, producing]

melanogenemia (mel′a-no-je-ne′me-a)
The presence of melanin precursors in the blood; may occur in malignant melanoma with metastasis. [melanogen + G. haima, blood]

melanogenesis (mel′a-no-jen′e-sis)
Formation of melanin. [melanin + G. genesis, production]

melanoglossia (mel′a-no-glos′e-a)
SYN: black tongue. [melano- + G. glossa, tongue]

melanoid (mel′a-noyd)
A dark pigment, resembling melanin, formed from glucosamines in chitin. SYN: artificial melanin, factitious melanin.

melanokeratosis (mel′a-no-ker-a-to′sis)
Migration of conjunctival melanoblasts into the cornea. [melano- + kerato- + G. -osis, condition]

melanoleukoderma (mel′a-no-loo-ko-der′ma)
Marbled, or marmorated, skin. [melano- + G. leukos, white, + derma, skin] m. colli SYN: syphilitic leukoderma.

melanoliberin (mel′a-no-lib′er-in)
A hexapeptide similar to oxytocin; it stimulates the release of melanotropin. SYN: melanotropin-releasing factor, melanotropin-releasing hormone. [melanotropin + L. libero, to free, + -in]

melanoma (mel′a-no′ma)
A malignant neoplasm, derived from cells that are capable of forming melanin, arising most commonly in the skin of any part of the body, or in the eye, and, rarely, in the mucous membranes of the genitalia, anus, oral cavity, or other sites; occurs mostly in adults and may originate de novo or from a pigmented nevus or lentigo maligna. In the early phases, the cutaneous form is characterized by proliferation of cells at the dermal-epidermal junction which soon invade adjacent tissues. The cells vary in amount and pigmentation of cytoplasm; the nuclei are relatively large and frequently bizarre in shape, with prominent acidophilic nucleoli; and mitotic figures tend to be numerous. Prognosis correlates with the depth of skin invasion. Melanomas frequently metastasize widely; regional lymph nodes, skin, liver, lungs, and brain are likely to be involved. Intense, intermittent sun exposure, especially of fair-skinned children, increases the risk of m. later in life. SYN: malignant m.. [melano- + G. -oma, tumor] acral lentiginous m. a form of malignant lentigo m. that occurs in palms, soles, and subungual areas. amelanotic m. an anaplastic m. consisting of cells derived from melanocytes but not forming melanin. benign juvenile m. SYN: Spitz nevus. Cloudman m. a transplantable m. that arose spontaneously in a mouse of DBA strain, and which grows and metastasizes in mice of related strains. desmoplastic malignant m. (dez-mo-plas-mik) a m. with marked fibrosis surrounding atypical spindle-shaped melanocytes in the dermis, tending to invade widely around small nerves. Harding-Passey m. a melanin-forming tumor that arose spontaneously in a non-inbred mouse, and that is transplantable to mice of many strains but does not ordinarily metastasize. malignant m. SYN: m.. malignant lentigo m. a m. arising (in unusual cases) from a malignant lentigo. malignant m. in situ a m. limited to the epidermis and composed of nests of atypical melanocytes and scattered single cells extending into the upper epidermis; local excision is curative although the lesion, if untreated, may soon invade the dermis. Malignant lentigo may be considered a slowly progressive type of malignant m. in situ. minimal deviation m. a malignant m. showing less cytologic atypia than is usual in m. cells that nevertheless demonstrate asymmetric expansile invasion of the dermis, or metastasis. nodular m. primary cutaneous m. that presents as rapidly growing smoothly spheroid or ulcerated nodules in which tumor cells microscopically invade the dermis beneath all of the lateral epidermal margins of involvement. subungual m. a m. beginning in the skin at the border of or beneath the nail, usually of acral lentiginous type (q.v.). superficial spreading m. primary cutaneous m. characterized by intraepidermal growth extending laterally beyond the site of dermal invasion.

melanomatosis (mel′a-no-ma-to′sis)
A condition characterized by numerous, widespread lesions of melanoma. [melanoma + G. -osis, condition]

melanonychia (mel′a-no-nik′e-a)
Black pigmentation of the nails. [melano- + G. onyx (onych-), nail]


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