leptin A hormone produced mainly by adipocytes (fat cells) that is involved in the regulation of body fat. Leptin interacts with areas of the brain that control hunger and behavior and signals that the body has had enough to eat. A small number of people have genetic mutations in the leptin gene, leading to a…
leprosy An infectious disease of the skin, nervous system, and mucous membranes that is caused by the bacteria Mycobacterium leprae. Leprosy is transmitted via person-to-person contact. For thousands of years leprosy was one of the world’s most feared communicable diseases because the nerve and skin damage often led to terrible disfigurement and disability. Today leprosy…
lens, ocular In a microscope, the lens closest to the eye. Also known as eyepiece. Most light microscopes are binocular, with one ocular lens for each eye.
lens, objective In a microscope, the lens nearest to the object being examined. Most light microscopes now have a turret that bears a selection of objective lenses.
lens The transparent structure inside the eye that focuses light rays onto the retina.
Lennox-Gastaut syndrome A severe form of epilepsy that usually begins in early childhood. It is characterized by frequent seizures of multiple types, mental impairment, and a slow spike-and-wave pattern seen on an EEG. The seizures are notoriously hard to treat and may lead to falls and injuries. Treatment involves anti-epileptic medications.
Lennox syndrome See Lennox-Gastaut syndrome.
Lemierre’s disease A rare, potentially lethal complication of tonsillitis caused by anaerobic (not requiring oxygen) bacteria including Fusobacteria. The disease usually develops after a bacterial infection has created an abscess near the tonsils. Deep in the abscess, the anaerobic bacteria can flourish. The bacteria penetrate from the abscess into the neighboring jugular vein in the…
leishmaniasis A parasitic disease that is spread by the bite of sand flies infected with the protozoa Leishmania. There are several forms of leishmaniasis, the most common being cutaneous and visceral leishmaniasis (known as kala-azar). The cutaneous form of the disease causes skin sores and is usually named for a geographic place (for example, Jericho…
Leishmania A group of parasites that cause several human diseases. See also leishmaniasis.
leiomyosarcoma A malignant tumor that originates in smooth muscle, the major structural component of most hollow internal organs and the walls of blood vessels. Leiomyosarcoma can occur almost anywhere in the body but is most frequently found in the uterus and gastrointestinal tract. Complete surgical excision, if possible, is the treatment of choice.
Leigh’s disease A rare, inherited disorder characterized by degeneration of the central nervous system. Leigh’s disease can be caused by mutations in mitochondrial DNA or by deficiencies of an enzyme called pyruvate dehydrogenase. Symptoms usually begin between the ages of 3 months and 2 years and progress rapidly. Early symptoms may include poor sucking ability…