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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…

One of the three living orders of the class Amphibia, encompassing the frogs and toads. Anura is the largest order of amphibians (class Amphibia), containing more than 7000 species of frogs (Fig. 1) and toads. Because members of the order Anura…

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 taxonomic order of insects comprising the cockroaches and termites. The order Blattodea, also referred to as Blattaria, contains at least 4500 species of cockroaches (Fig. 1) and 3000 species of termites. Previously, termites were assigned to a…

Terrestrial and aquatic animals that are capable of excavating holes in the ground for protection from adverse environmental conditions, as well as for storing food. Burrowing animals are uniquely adapted species, having the capacity to dig burrows…

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…

A member of the mammalian family Camelidae in the order Artiodactyla. Camels are even-toed, hoofed mammals (ungulates). There are three extant camel species: the Bactrian camel (Camelus bactrianus; Fig. 1), the wild Bactrian camel (C. ferus), 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.…

Any device connected internally or externally to a computer and used in the transfer of data to or from a computer. A personal computer (PC) processes information. Data (unprocessed information) must get into the computer, and the processed…

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 common name for three living species of mammals in the family Elephantidae (order Proboscidea). Elephants are extant members of the mammalian family Elephantidae, which is one of several families included in the order Proboscidea. The remaining…

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.…

A mammalian order containing the egg-laying platypus and echidnas. Members of the order Monotremata (monotremes) are egg-laying mammals belonging to the subclass Prototheria (in some alternative classifications, the monotremes have been assigned to…

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…

The scientific study of parasites and parasitism. Parasitology is the branch of biology dealing with parasitic organisms (see illustration), with particular focus being directed toward understanding the relationship between the parasite and the…

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 large crustacean superorder. Members of the superorder Peracarida, commonly termed peracarids, comprise a major category of eumalacostracan crustaceans (subclass Eumalacostraca, class Malacostraca, subphylum Crustacea, phylum Arthropoda). The most…

An order of placental mammals containing the largest and most powerful living terrestrial mammals—the elephants. Proboscideans once had a wide distribution, reaching every continent except Australia and Antarctica. Although early forms lacked both…

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,…

An electronic display that allows a user to interact with a computer system by touching an area or areas on the display. Touch-screen displays have been in use for many years in ATMs, checkout registers, and other applications, but gained mass…

A nocturnal, burrowing, insectivorous, piglike mammal (Orycteropus afer) in the order Tubulidentata. The mammalian order Tubulidentata contains a single family, Orycteropodidae, and a single extant aardvark species, Orycteropus afer (Fig. 1).…