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A major plant hormone that plays an important role in plant growth and development. Abscisic acid (ABA), with a chemical formula of C15H20O4 (Fig. 1), is one of the most significant hormones found in plants. It is responsible for regulating various…

The formation of new animal individuals, which may occur by asexual or sexual means. All animals must reproduce asexually or sexually to create new individuals of their species. In asexual reproduction, a single parent can produce offspring that are…

An ongoing current event in which a large number of living species are threatened with extinction or are going extinct because of environmentally destructive human activities. The Earth is presently in the midst of a mass extinction event. In the…

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…

A neurodevelopmental condition that impairs the way that a person relates to and communicates with other people. Persons with autism have difficulties with social communication and interaction. They also can have unusual behaviors, such as…

A hormone, the only known secretory product of the parenchymal or C cells of the mammalian thyroid and of the ultimobranchial glands of lower animal forms. The hormone calcitonin (also referred to as thyrocalcitonin) has been isolated and…

The common name for a malignant neoplasm or tumor. Cancer is a serious medical condition that develops when the orderly relationship of cell division and cell differentiation becomes disordered, leading to the development of neoplasms. Neoplasms are…

Cells within a tumor that possess the capacity to self-renew and cause the heterogeneous lineages of cancerous cells that comprise the tumor. Cancer stem cells (CSCs) typically represent a small percentage of all cells in a tumor (rarely more than…

A firm, resilient connective tissue of vertebrates and some invertebrates. Cartilage (see illustration) is an important structural element found in the bodies of vertebrates and some invertebrates. It is a resistant, elastic connective tissue that…

The mechanism by which cells in a multicellular organism become specialized to perform specific functions in a variety of tissues and organs. Cell differentiation is the process through which cells are transformed into divergent types of cells…

The movement of cells, the movement of materials within cells, and changes in cell shape (including cell division). Cell motility is an important facet of cell biology, encompassing the movement of individual cells, the coordinated movement of many…

The organizing and compartmentation of eukaryotic cells. Eukaryotic cells are divided into several compartments, and each compartment has a characteristic structure, biochemical composition, and function (Fig. 1). These compartments are called…

The field concerning the interactions among cells and molecules of the immune system, and how such interactions contribute to the recognition and elimination of pathogens. Humans (and vertebrates in general) possess a range of nonspecific mechanical…

The collective processes by which metabolic materials are transported from one region of an organism to another region. The circulatory system, often termed the cardiovascular system, is responsible for the functions of circulation, allowing…

The major fibrous protein in animals. Collagen is present in all types of multicellular animals and is probably the most abundant animal protein in nature. It is estimated that collagen accounts for about 30% of the total human body protein.…

One of the four primary tissues of the body. Connective tissue (Fig. 1) provides the solid foundation required for underpinning, binding, and protecting tissues and organs of the body. Connective tissue differs from the other three tissues…

Any of a group of diseases involving connective tissue as the pathological target. Connective tissue comprises one of the primary tissues of the body and is distinguished by an abundance of fibrillar and nonfibrillar extracellular components, which…

A genome-engineering technique that allows precise targeting of specific stretches of genetic code and editing of DNA at designated locations. The CRISPR/Cas9 system has been used as a general tool to engineer the genomes of many different…

A member of a class of proteins containing hemes as prosthetic groups that catalyze oxidation–reduction reactions. Cytochromes are heme-containing proteins (see illustration) that play vital roles in electron transport within cells. It should be…

The branch of genetics that studies how genes control embryonic development. Advances in the field of developmental genetics have emphasized the degree of conservation throughout evolution of the genes (basic units in inheritance consisting of DNA)…

Various species of diving, air-breathing animals. Diving animals, which are found in every class of terrestrial vertebrates (amphibians, reptiles, birds, and mammals), are air-breathing species that have almost complete freedom in the aquatic…

The study of the glands of internal secretion—the endocrine glands—and the hormones that they synthesize and secrete. Endocrinology is the field of medicine and physiology concerned with the endocrine system, which regulates hormones (chemical…

A family of endogenous peptides with morphinelike action present within the central nervous system. The enkephalins were the first group of endorphins to be identified. They were discovered in 1975 and opened research into a wide number of centrally…

A hormone, also known as adrenaline, that is an important adrenal secretion. Epinephrine, or adrenaline, is an important secretion from the adrenal medulla (the inner part of the adrenal glands). In 1901, Jokichi Takamine isolated epinephrine in…

The common name for the steroid hormone 17β-estradiol. Estrogen (17β-estradiol; Fig. 1) is an important steroid hormone that aids mammalian sexual and reproductive development. In mammalian females, estrogen maintains secondary sex characters and…

Disorders of form and function that affect the eye. Eye disorders and diseases are pathological conditions of the eye. Some conditions can result in vision loss and even blindness. In contrast, others are less severe and temporary, being resolved by…

The intentional production of new genes and alteration of genomes by the deletion, substitution, or addition of new genetic material. Genetic engineering involves the artificial recombination of nucleic acid molecules in the test tube; their…

Plants that have altered genetic material (DNA) through recombinant DNA (rDNA) manipulation. Genetic engineering of plants allows for very specific genetic changes to be made to a plant's deoxyribonucleic acid (DNA), as opposed to the random…

Commercially grown plants that have altered genetic material through artificial recombinant manipulation. Genetic modification is the latest scientific tool for developing improved crop varieties. Such genetically modified (GM) crops can help to…

An organism that has had its genetic material manipulated or altered in a laboratory setting through the use of genetic or genomic engineering techniques. Genetically modified organisms (GMOs) are microorganisms, plants, or animals that have been…

Mature cells that have been reprogrammed genetically to an embryonic-like state that then can give rise to one of many different cell types. Induced pluripotent stem cells (iPS cells; Fig. 1) represent a major breakthrough in stem cell research.…

An emerging cancer therapy in which oncolytic viruses are used to target and destroy cancer cells without harming healthy cells. Oncolytic viruses (that is, viruses that destroy cancer cells, but not normal cells) are able to infect, replicate…

Any synthetic or semisynthetic organic chemical compound that resembles opium in its pharmacological action to reduce pain. An opioid is an organic chemical compound, created synthetically or semisynthetically, that duplicates the analgesic…

Any agent capable of causing disease. The term pathogen is usually restricted to living agents or microorganisms that produce disease. Pathogens are infectious and diverse, including bacteria (see illustration), viruses, rickettsia, fungi, yeasts,…

A theory expressing the idea that life on Earth passed through an era when biological processes were carried out predominantly by ribonucleic acids, without the involvement of deoxyribonucleic acids or proteins. The RNA World theory (or hypothesis)…

An electron microscope that builds up images of samples via the scanning of the sample's surface with a focused electron beam. The idea of the scanning electron microscope (SEM) dates back to the 1930s, with a great deal of the pioneering…

Cells that have the ability to self-replicate and to give rise to mature cells. The concept of stem cells (Fig. 1) was originally based on renewing tissues. Many adult tissues, including the skin, blood, and intestines, consist of mostly mature and…

An endocrine gland found in all vertebrates that produces, stores, and secretes the thyroid hormones. The primary function of the thyroid gland, in warm-blooded vertebrates at least, is to regulate the rate of metabolism. In humans, the gland is…

Any of the chemical messengers produced by the thyroid gland. For many years, it was generally believed that the thyroid gland produced only two closely related hormones, thyroxine and triiodothyronine. In 1961, though, a new hormonal principle,…

A major body division of the vertebrate trunk lying posterior to the thorax (chest) and, in mammals, bounded anteriorly by the diaphragm and extending to the pelvis. The abdomen (see illustration) is a major anatomical region of the body. The…

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…

An instrument that utilizes focused acoustic waves to produce images of surface and subsurface features in materials, and to measure elastic properties on a microscopic scale. It has been used to image and measure local elastic properties in metals,…

The discipline of acoustic phonetics can be narrowly defined as the study of the acoustic output of the vocal tract for speech, but ultimately it encompasses much more. Acoustic phonetics makes direct contact with, and in many cases is the…

A genus of Gram-negative, immotile and nonspore-forming, oval to rod-shaped, often pleomorphic bacteria that occur as parasites or pathogens in mammals (including humans), birds, and reptiles. Actinobacillus bacteria are facultatively aerobic,…

A nucleotide, composed of ribose, adenine, and a triphosphate group, that is an important energy compound in metabolism. Adenosine triphosphate (ATP) [Fig. 1] is a vital energy-rich chemical found in all living cells. It has a chemical formula of…

A type of connective tissue, especially in mammals, that is specialized for the storage of neutral fats (lipids). Adipose cells originate mainly from fibroblasts (connective tissue cells found in virtually all animal organs). In the mammalian…

A bacterial genus in the family Vibrionaceae comprising oxidase-positive, facultatively anaerobic, monotrichously flagellated Gram-negative rods. The mesophilic species of Aeromonas are A. hydrophila, A. caviae, and A. sobria; the psychrophilic one…

The branch of mathematics dealing with the solution of equations. These equations involve unknowns, or variables, along with fixed numbers from a specified system (Fig. 1). The origins of algebra were based on the need to develop equations that…

The study of zero sets of polynomial equations. Examples of zero sets of polynomial equations studied in algebraic geometry are the parabola y − x2 = 0, thought of as sitting in the (x, y)-plane, and the locus of all points (t, t2, t3, t4), which is…

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…