Biology

Hexapoda — six-legged arthropods and their major groups Hexapoda Hexapoda is the arthropod subphylum defined by three pairs of legs. It includes the true insects and several small, wingless lineages; this entry covers anatomy, major groups, taxo…

Hibernation: seasonal dormancy and reduced metabolism in animals Hibernation Hibernation is a prolonged state of reduced activity and metabolism used by many animals to survive periods of scarce food or adverse temperatures.

Hindbrain (Rhombencephalon) Hindbrain The hindbrain, or rhombencephalon, is the posterior portion of the brain in vertebrates. It contains the medulla, pons and cerebellum and manages balance, motor control and vital a…

Hinge teeth (bivalve shell dentition) Hinge teeth Hinge teeth are the interlocking projections on a bivalve's hinge line that align the two shell valves. Their form and arrangement are important for shell mechanics, classification…

Histology: The Microscopic Study of Cells and Tissues Histology Histology examines the microscopic structure of plant and animal tissues. This article outlines methods, history, applications in research and medicine, and how histology differs f…

Histone (chromatin protein) Histone Histones are basic nuclear proteins that package eukaryotic DNA into nucleosomes, regulate chromatin structure and gene activity, and are central to epigenetic control and genome m…

Holozoa: the clade containing animals and their closest single-celled relatives Holozoa Holozoa is the evolutionary group that includes animals (Metazoa) and several groups of unicellular relatives. It is central to studies of animal origins and the evolution of multi…

Homeostasis: biological self-regulation and internal stability Homeostasis Homeostasis is the physiological process by which living organisms maintain a stable internal environment despite external changes, using coordinated feedback mechanisms across mul…

Homologous chromosomes Homologous chromosome Paired chromosomes in diploid organisms that carry the same genes in the same order; they pair in meiosis, exchange genetic material, and are central to inheritance and genetic div…

Homology (biology): traits shared through common ancestry Homology Homology describes similarities of structures, genes or behaviors inherited from a common ancestor. It is distinguished from analogy and underpins comparative biology, phylogenetic…

Horizontal gene transfer: mechanisms, evolution, and significance Horizontal gene transfer Horizontal gene transfer is the non‑inheritance movement of genetic material between organisms. This article explains mechanisms, examples, evolutionary roles, detection methods, a…

Hormone: chemical messengers in animals and plants Hormone Hormones are chemical messengers produced by glands and other cells that coordinate growth, metabolism, reproduction and stress responses by binding target receptors locally or thr…

Horn (anatomy): structure, function and cultural uses Horn (anatomy) A horn is a permanent keratin-covered projection on the skull of certain mammals. This article explains horn anatomy, variation, uses in animal behavior and human culture, and diff…

Host (biology): organism that supports parasites and symbionts Host (biology) An organism that harbors another organism (parasite, mutualist, or commensal), providing habitat, nutrients, or a site for reproduction. Includes definitive, intermediate, reservoi…

Host defence peptide (antimicrobial peptide) Host defence peptide Host defence peptides are short, evolutionarily conserved peptides of the innate immune system that kill microbes and modulate immunity; they are explored as alternatives or comple…

Hox gene family: master regulators of body patterning Hox gene Hox genes are conserved developmental regulators that specify the anterior–posterior layout of animals. They act as transcription factors and their arrangement and expression influ…

Human biology: form, function, variation and evolution of Homo sapiens Human biology Survey of human biology: scope, major subfields, methods, history, applications (public health, anthropology, medicine) and distinctions from related disciplines.

Human body: structure, function, development and health Human body A concise overview of the human body's organization, major systems, development, functions, and health considerations, with distinctions and notable facts.

Human Cloning: Methods, History, and Ethical Considerations Human cloning Human cloning is producing genetically similar human cells, tissues, or whole individuals by laboratory methods such as somatic cell nuclear transfer; it involves scientific, ethic…

Human genome Comprehensive overview of the human genome: structure, major sequencing efforts, functional elements, variation, medical applications and ongoing research into noncoding regions an…

Human microbiome: community, function, and health Human microbiome The human microbiome is the collective community of microorganisms that live on and in people. This article explains composition, roles, research history, health effects and clinic…

Human reproductive system: structure, function, and health Human reproductive system Overview of the human reproductive system, its male and female components, development and cycles, biological role, health issues, and variation among mammals and other animals.

Human skeleton: structure, development, functions, and variation Human skeleton Overview of the human skeleton covering its major parts, functions, development from birth to adulthood, sexual differences, bone biology, common disorders, and forensic and clinic…

Humerus (upper arm bone) Humerus The humerus is the long bone of the upper arm in tetrapods. It connects the shoulder and elbow, provides sites for muscle attachment, and is important in movement, clinical diagnos…

Humoral immunity Humoral immunity is the branch of the immune response mediated by soluble molecules — notably antibodies, complement, and antimicrobial peptides — that neutralize and clear pathoge…

Hybrid vigour (heterosis) Hybrid vigour Hybrid vigour, or heterosis, is the phenomenon where offspring from genetically diverse parents show greater fitness, growth, or fertility; important in breeding, agriculture, and…

Hydrogenosome A hydrogenosome is an organelle in some anaerobic eukaryotes that generates ATP without oxygen, producing hydrogen as a by-product and illustrating mitochondrial evolution and meta…

Hydrogenothermaceae: thermophilic, hydrogen‑oxidizing bacteria of geothermal habitats Hydrogenothermaceae Hydrogenothermaceae are a family of thermophilic, chemolithoautotrophic bacteria found in hot springs and hydrothermal vents; they oxidize hydrogen or reduced sulfur and contribute…

Hymenium — the spore-bearing layer of fungi Hymenium The hymenium is the fertile tissue on a fungal fruiting body where asci or basidia form and release spores. Its position and microscopic details are key for identifying and classif…

Hyperthermophile: organisms adapted to very high temperatures Hyperthermophile Hyperthermophiles are microbes that grow optimally at very high temperatures. This article explains their traits, habitats, evolutionary significance, biotechnological uses, and no…

Hypertrophy: cellular enlargement of tissues and organs Hypertrophy Hypertrophy is an increase in the size of cells leading to enlarged tissues or organs. It occurs physiologically (e.g., muscle growth) and pathologically (e.g., organ enlargement i…

Hypothalamus: brain center for homeostasis and neuroendocrine regulation Hypothalamus The hypothalamus is a small but essential brain region that links the nervous and endocrine systems to control temperature, hunger, thirst, sleep cycles and hormone release.

IC50: definition, measurement, interpretation, and applications IC50 IC50 is the concentration of a substance that produces 50% inhibition of a defined biological function. This article explains its meaning, measurement, limitations, and common uses…

Ice algae: biology, ecology and roles in snow and sea ice environments Ice algae Photosynthetic microorganisms that grow on snow and ice; this article summarizes their traits, habitats, ecological roles, adaptations and relevance to polar environments and clima…

Ichneumon (Ichneumonoidea): parasitoid wasps and their biology Ichneumon Ichneumon refers to the superfamily Ichneumonoidea: diverse, mostly solitary parasitoid wasps (families Ichneumonidae and Braconidae) that attack other insects and are important in…

Ichthyology: The Scientific Study of Fishes Ichthyology Ichthyology is the branch of zoology concerned with fishes: their classification, anatomy, ecology, evolution, and importance to people and ecosystems.

Identical twins (monozygotic twins) Identical twins Monozygotic or identical twins develop from a single fertilized egg that splits early in development, producing two individuals with nearly the same genetic makeup and unique prena…

Idiopathic pulmonary fibrosis: overview, features, diagnosis and management Idiopathic pulmonary fibrosis Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive scarring lung disease of unknown cause that primarily affects older adults; this article explains its features, diagno…

Ileum: structure, function, and clinical importance Ileum The ileum is the distal portion of the small intestine in humans, typically 2–4 meters long. This article covers its anatomy, histology, role in absorption and immunity, and common…

Imago (adult stage of an insect) Imago Imago is the final, sexually mature stage in an insect's life cycle. It follows the last moult or emergence from the pupa and is the only stage with functional reproductive organs…

Immortality: concepts, biology, culture, and technological approaches Immortality Overview of immortality: definitions, natural examples, cultural and religious beliefs, scientific and technological efforts, ethical issues, and symbolic uses across traditions.

Immune system: structure, function, and clinical significance Immune system The immune system is the network of cells, tissues and molecules that defends organisms from infection, distinguishes self from non‑self, and underlies vaccination, allergy and aut…

Immune system (medical): types, components, functions and clinical significance Immunity (medical) Comprehensive overview of medical immunity: innate and adaptive systems, key cells and molecules, clinical applications such as vaccination and immunotherapy, and common dysfunctio…

Immunology: the Study of the Immune System Immunology Immunology examines the body's defenses against infection and disease, describing cells, organs, molecules, memory, disorders, and clinical applications such as vaccination and imm…

Inbreeding: mating of closely related organisms Inbreeding Inbreeding is reproduction between close relatives. This article explains definitions, genetic effects, common forms, uses in breeding and research, conservation and health consequ…

Incisor: structure, function, and variation in mammals Incisor Front teeth adapted for cutting and nipping. Overview of incisor anatomy, roles across species, development, common variations and clinical relevance.

Infant mortality: definition, measurement, causes, and public health significance Infant mortality Infant mortality measures deaths of children under one year per 1,000 live births. This article explains how it is calculated, major causes, historical trends, public health import…

Infertility: Causes, Diagnosis, and Treatment Options Infertility Infertility is the inability to produce offspring. This article outlines causes, diagnosis, treatments such as IVF and insemination, and social options including adoption, across h…