The net movement (transport) of water and solutes from outside an organism to its interior. The unidirectional flow of materials into an animal from the environment generally takes place across the alimentary tract, the lungs, or the skin. In each location, a specific cell layer called an epithelium (Fig. 1) regulates the passage of materials. See also: Cell (biology); Epithelium; Respiration

An age-contingent reduction in physiological function and activity, with a concomitant increase in mortality rate and a diminution in reproductive rate. Aging is the process of becoming older, which is influenced by genetic and environmental factors. However, definitions of aging differ between biologists and behavioral scientists. Biologists regard aging as reflecting the sum of multiple and typical biological decrements occurring after sexual maturation; in contrast, behavioral scientists view it as reflecting regular and expected changes occurring in genetically representative organisms advancing through the life cycle under normal environmental conditions. It is difficult to define normal aging because many changes observed in older adults and perceived as concomitants of normal aging are effects of disease in later life. The behavioral science view allows for incremental as well as decremental changes with aging. Senescence is not always equated with aging; it is viewed as the increasing vulnerability or decreasing capacity of an organism to maintain homeostasis as it progresses through its life span, leading to death (Fig. 1). Gerontology refers to the study of aging. Geriatrics refers to the clinical science that is concerned with health and illness in the elderly. See also: Cell (biology); Cell senescence; Death; Genetic influences on aging; Genetics; Human genetics

A regulatory enzyme whose activity is modified by the binding of an activator molecule to an alternate (allosteric) site that is different from the active (catalytic) site. Allosteric enzymes (Fig. 1) are special enzymes involved in regulatory functions. These enzymes utilize end-product inhibition. In this control mechanism, the end product of a biosynthetic pathway can react with the first enzyme of the pathway and prevent its activity. Importantly, this end-product inhibition is a device through which a cell conserves its economy by shutting off the synthesis of building blocks when too many are present. See also: Cell (biology); Enzyme; Enzyme inhibition

Programmed cell death triggered by extracellular signals or genetically determined events and carried out by physiological processes within the cell. Apoptosis is one of the major forms of cell death in multicellular organisms (necrosis is the other form). Also referred to as programmed cell death, apoptosis is defined by specific changes in cell morphology, followed by the engulfment of the cell corpse by phagocytes (Fig. 1). Often, the death of certain cells through apoptosis enhances the overall health of an individual. The term apoptosis originates from a Greek word that means "to fall," as happens to the foliage of tree leaves in autumn, which helps the tree's survival. Defects in apoptosis are associated with a large number of diseases in humans. The most notable of these diseases are cancers. See also: Cancer; Cell (biology); Death; Oncology

A group of diverse microscopic prokaryotic organisms that constitute one of the three domains of life. Taxonomists recognize three domains of cellular life: Archaea, Bacteria, and Eukaryota [(also termed Eukarya or Eucarya), Fig. 1]. The members of the Archaea and Bacteria comprise the prokaryotes and look identical under a microscope; however, they differ in their chemical composition and metabolic processes. Archaea microorganisms are often found in some of the most extreme environments on Earth, including anaerobic, hypersaline, extremely cold, and extremely high temperature environments. For example, they comprise about one-third of the prokaryotic biomass in coastal Antarctic waters, providing food for many marine animals. In addition, some are symbionts in the digestive system of animals. Notably, the members of the Archaea are not known to cause disease in humans or animals. See also: Bacteria; Eukaryota; Microbiology; Prokaryote; Symbiosis

Any treatment or procedure that includes the in-vitro handling of oocytes (immature ova or egg cells) and sperm or embryos for the purpose of establishing a pregnancy. Assisted reproductive technology (ART) is undertaken as a treatment of infertility, with a goal of achieving human reproduction. ART procedures include (but are not limited to) in-vitro fertilization (IVF) and embryo transfer (ET) [Fig. 1], gamete intrafallopian transfer, gamete and embryo cryopreservation (freezing, or vitrification, and storage of cells or tissues), oocyte and embryo donation, and surrogacy. Note that intracervical or intrauterine insemination with either partner or donor spermatozoa is not included under the umbrella of ART. See also: Animal reproduction; Biotechnology; Fertilization (animal); Infertility; Ovum; Pregnancy; Sperm cell

An abundant type of glial cell of the central nervous system. Astrocytes (Fig. 1) are specialized glial cells (nonneuronal cells that surround neurons) of the central nervous system (CNS) that exert numerous essential functions in both health and disease. Astrocytes (also known as astroglia) interact closely with neurons throughout the CNS and are critical for establishing and maintaining neural circuits that implement neurological functions. Astrocyte dysfunctions are increasingly recognized as playing important roles in a variety of neurological disorders. See also: Central nervous system; Glial cell; Nervous system (vertebrate); Nervous system disorders; Neurobiology; Neuron

A form of microscopy in which a sharp tip is scanned over the surface of a sample while sensing the interaction force between the tip and the sample. Due to its ability to observe and manipulate surfaces in aqueous solution, atomic force microscopy (AFM) has revolutionized the way in which microscopists explore biological structures, from single molecules to living cells.

The occurrence in an organism of an immune response to one of its own tissues, that is, a response to a self constituent. Autoimmunity is an immune state in which antibodies are formed against the person's own body tissues. Foreign substances to which an organism makes a protective immune response are called nonself. Two leading mechanisms of autoimmunity are molecular mimicry and failed immune tolerance. Molecular mimicry happens when the immune system responds to a substance similar to a molecule present in the body and consequently mounts an autoimmune response. Immune tolerance operates efficient discrimination between self and nonself—the basis of normal immune function—which means the body will not react to substances that could provoke an immune response but are harmless and should provoke none. Failure of immune tolerance to self constituents results in an autoimmune response that is often, although not invariably, associated with autoimmune disease (Fig. 1). Autoimmune disease occurs when the autoimmune response to self constituents has damaging effects of a structural or functional character. See also: Acquired immunological tolerance; Antibody; Immunology

Destruction of the components of a cell by its own enzymes. Autophagy is a process of all eukaryotic cells. Various types of autophagy occur during normal development or as a response to various types of stress, including starvation. Defects in autophagy are correlated with a number of diseases in humans. Advances in the study of autophagy in baker's yeast (Saccharomyces cerevisiae) have provided insight into the molecular basis of this process in more complex eukaryotes, including animals and humans. See also: Cell (biology); Enzyme; Eukaryotae; Yeast