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Nausea
An unpleasant sensation in the stomach usually accompanied by the urge to vomit. Common causes are early pregnancy, sea and motion sickness, emotional stress, intense pain, food poisoning, and various stomach infections.

Necrosis
The sum of the morphological changes indicative of cell death and caused by the progressive degradative action of enzymes, it may affect groups of cells or part of a structure or an organ.

Needle Biopsy
Needle biopsy is a procedure using a needle to take a sample of tissue or fluid. The procedure is also called fine needle aspiration.

Neoplasia
New and abnormal growth of tissue (neoplasm), which may be benign or cancerous, that serves no purpose.

Neoplasm
Also called tumor. Any new and abnormal growth; specifically a new growth of tissue in which the growth is uncontrolled and progressive. Malignant neoplasms are distinguished from benign in that the former show a greater degree of anaplasia and have the properties of invasion and metastasis.

Nephron
The nephron is the functional unit of the kidney, responsible for the actual purification and filtration of the blood. About one million nephrons are in the cortex of each kidney, and each one consists of a renal corpuscle and a renal tubule which carry out the functions of the nephron.

Neurilemma
The plasma membrane surrounding a Schwann cell of a myelinated nerve fiber and separating layers of myelin.

Neuroblastoma
A childhood tumor that arises in the adrenal gland or in tissue in the nervous system that is related to the adrenal gland.

Neuroendocrine
Descriptive of cells that release a hormone into the circulating blood in response to a neural stimulus. Such cells may comprise a peripheral endocrine gland (e.g., the insulin-secreting beta cells of the islets of Langerhans in the pancreas and the adrenaline-secreting chromaffin cells of the adrenal medulla); others are neurons in the brain (e.g., the neurons of the supraoptic nucleus that release antidiuretic hormone from their axon terminals in the posterior lobe of the hypophysis).

Neurofibromatosis
A disorder inherited as an autosomal dominant and characterized especially by brown spots on the skin, neurofibromas of peripheral nerves, and deformities of subcutaneous tissue and bone.

Neuroglia
Supporting tissue intermingled with the essential elements of nervous tissue especially in the brain, spinal cord, and ganglia.

Neuron
A grayish or reddish granular cell with specialized processes that is the fundamental functional unit of the nervous tissue.

Neutron
A neutron is a subatomic particle found in the nucleus of every atom except that of simple hydrogen. The particle derives its name from the fact that it has no electrical charge; it is neutral.

NK cells (Natural Killer Cells)
Natural killer cells are large lymphocytes. They are the first line of defense against viruses and other invaders because they do not need to wait for an antibody response to identify foreign cells and invaders.

Notochord
An axial mesodermal tissue found in embryonic stages of all chordates and protochordates, often regressing as maturity is approached. Typically a rod shaped mass of vacuolated cells. It lies immediately below the nerve cord and may provide mechanical strength to the embryo.

Nuclear Medicine
Nuclear medicine is a medical specialty that uses very small amounts of radioactive materials or radiopharmaceuticals to diagnose and and treat disease. It is safe, painless, and cost-effective.