Labile Cells Body cells that have a limited lifespan and are capable of routine division and replacement.  The squamous epithelium of skin, mouth, vagina and cervix, columnar epithelium of intestinal tract, transitional epithelium of urinary tract, and hematopoietic stem cells of the bone marrow are examples of labile cells. Latency Period The time between induction of a lesion and its clinical appearance.  An example is cancer which develops gradually (subclinically) over a period of several years before it becomes manifest to the individual or clinician.  This period is referred to as the "latency period". Lethal Injury Damage to a cell or the body so severe that death results. Leukemia Cancer of the hematopoietic system, the blood-forming organs. Lipids Essential substances of all cells and a major energy reserve for the body.  Lipids may be stored as fatty acids or as triglycerides. Lou Gehrig's Disease A disease in which the myelin around nerves is lost causing paralysis and loss of sensory and motor function.  Same as Amyotrophic Lateral Sclerosis. Lymphatic System An organ system that returns tissue fluid to blood and defends against foreign organisms.  Organs include the spleen, lymph nodes, thymus, and the lymphatic vessels. Lysosomes Organelles that consist of vesicles that contain strong digestive enzymes.  Lysosomes are responsible for the intracellular removal of damaged organelles or pathogens.