Biology

Sipuncula (peanut worms) — marine unsegmented worms Sipuncula Sipuncula, or peanut worms, are marine, bilaterally symmetrical worms. This article explains their anatomy, life habits, taxonomy and ecological role, and highlights key distinguis…

Sirenomelia (Mermaid Syndrome): overview, causes, diagnosis, and management Sirenomelia Sirenomelia, or mermaid syndrome, is a rare congenital malformation marked by fused lower limbs and severe caudal organ abnormalities. This article covers features, causes, diagnos…

Sister group (sister taxon) — closest relatives in phylogenetics Sister group A sister group (or sister taxon) is the immediate closest relative of a taxon on a phylogenetic tree. This entry explains the concept, identification, history, uses, examples, and…

Skeletal muscle: structure, function, and clinical relevance Skeletal muscle Overview of skeletal muscle anatomy, contraction, metabolism, development, fiber types, adaptations and common clinical issues, with links to related topics.

Skeleton: structure, types, functions and the human skeletal system Skeleton A skeleton is the supporting and protective framework of an organism. This article explains types (endo-, exo-, hydrostatic), structure and development, main functions, and the hum…

Skene's glands (paraurethral glands) Skene's gland Small paraurethral glands in females located near the urethra; homologous to the male prostate. Involved in lubrication, sometimes linked to female ejaculation and certain infectio…

Skin: structure, functions, variation, and human significance Skin Comprehensive overview of skin as an organ and animal covering: its structure, functions, variation among species, health and care, evolutionary background, and cultural uses.

Skull (cranium): structure, function, and diversity Skull A skull is the bony or cartilaginous framework of the head in vertebrates that protects the brain, supports sensory organs, and provides attachment for facial structures and muscle…

Sleep: biology, stages, functions, patterns and health Sleep Sleep is a recurring biological state of reduced consciousness found across many animals. This article explains sleep stages, physiological roles, patterns, common problems and pra…

Sleeping Beauty transposon system A reconstructed Tc1/mariner DNA transposon platform used for non‑viral stable gene integration and long‑term transgene expression in research and therapeutic development.

Slime moulds: biology, life cycle, ecology, and scientific importance Slime mould Overview of slime moulds: diverse eukaryotic organisms with amoeboid cells and multicellular stages, their life cycles, habitats, ecological roles, taxonomy, and use in research an…

Small interfering RNA (siRNA) Small interfering RNA Short double-stranded RNA molecules that direct sequence-specific gene silencing through RNA interference; important in biology, research, and therapeutic development.

Small intestine The small intestine is the long, narrow organ between the stomach and large intestine where most digestion and nutrient absorption occur; it comprises the duodenum, jejunum and ile…

Smooth muscle: structure, function, locations and clinical relevance Smooth muscle Smooth muscle is non‑striated, involuntary muscle found in organs and vessels. This article explains its structure, types, physiology, locations and clinical significance in health…

Snakefly (Raphidioptera): appearance, life cycle, and natural history Snakefly Snakeflies are a small order of predatory endopterygote insects with an elongated 'neck' (prothorax). About 210 species occur mainly in temperate parts of the Northern Hemisphere.

Sneeze: reflex, causes, health implications and cultural notes Sneeze A sneeze is an involuntary reflex that expels air, mucus and particles from the nose and mouth; this article explains the mechanism, common triggers, risks, etiquette and cultural…

Small nuclear ribonucleoproteins (snRNPs) snRNP snRNPs are complexes of small nuclear RNA and proteins that assemble into spliceosomes to remove introns and regulate alternative splicing in eukaryotic gene expression.

Somatic Cell: Definition, Characteristics, Development and Significance Somatic cell Somatic cells are the non-reproductive cells forming an organism's tissues and organs. This entry explains their chromosome content, mitotic division, developmental origin, excepti…

Somatic nervous system — structure, function, and clinical significance Somatic nervous system The somatic nervous system is the peripheral network that conveys sensory information and controls voluntary skeletal muscles, mediates many reflexes, and differs from the autonomi…

Somatosensory system: senses of touch, temperature, pain and body position Somatosensory system Overview of the somatosensory system: receptors, pathways, cortical areas, functions, clinical relevance and distinctions between touch, proprioception, interoception and pain.

Soricomorpha: the proposed shrew‑form grouping of small mammals Soricomorpha A formerly proposed taxonomic grouping for shrews, moles and solenodons. Molecular studies revealed older insectivore groupings were not natural, leading to the adoption of Eulipot…

Spawn (biology) Spawn are the eggs and reproductive output of many aquatic animals. This article explains spawning methods, development of young, ecological roles, and human relevance in fisheries…

Speciation: How new species arise and the mechanisms involved Speciation Speciation is the evolutionary process by which populations evolve to become distinct species. This article covers modes, history, genetic evidence, examples and why isolation and…

Sperm: the male reproductive cell and its role in fertilization Sperm Sperm are male gametes that deliver paternal DNA to the egg. Found across animals and many plants, they vary in form, motility and delivery, and are central to natural reproduction…

Spermatophore: Packaged Sperm and Its Role in Animal Reproduction Spermatophore A spermatophore is a discrete packet of sperm produced by males and transferred to females in many invertebrates and some vertebrates; it plays roles in fertilization, mating strat…

Spider silk: biology, properties, and human applications Spider silk An encyclopedic overview of spider silk: how spiders make it, its structure and mechanical properties, natural functions, varieties, and ongoing efforts to replicate it for medical…

Spike protein (viral surface glycoprotein) Spike protein A viral spike protein is a surface glycoprotein that mediates attachment and entry into host cells; it determines host range, tissue tropism and is a major target for vaccines and…

Spine (vertebral column): structure, function, evolution and health Spine The spine, or vertebral column, is the central bony axis of vertebrate bodies. This article describes its parts, roles, evolutionary origin, and common conditions affecting it.

Spiracle: Respiratory Openings in Arthropods and Fish Spiracle Spiracles are external openings used for breathing in many arthropods and some fishes; they connect to internal air or gill passages and help control gas exchange, water balance, a…

Spirillum — spiral-shaped bacteria: morphology, taxonomy and significance Spirillum Spirillum denotes spiral-shaped bacteria and a historical genus name. Overview of morphology, taxonomy, ecology, medical relevance and how spirilla differ from other bacterial shap…

Spirochaetaceae Spirochaetaceae is a family of helical, motile bacteria (spirochetes) that includes free-living species and medically important pathogens such as Treponema pallidum, the agent of s…

Spirochaete (Spirochaetae) — helical, motile bacteria and their significance Spirochaete Spirochaetes are a phylum of distinctive spiral-shaped, double-membrane bacteria with internal flagella. They include free-living species and important pathogens such as Treponema,…

Spliceosome: cellular machinery for pre-mRNA splicing Spliceosome The spliceosome is a dynamic nuclear ribonucleoprotein complex that removes introns from pre-mRNA, carries out two-step transesterification chemistry, and enables alternative splic…

Sporangium: enclosure for spores and its role in plant and fungal life cycles Sporangium A sporangium is a single-celled or multicellular structure where spores form. Found in plants, fungi and other groups, sporangia function in both asexual and sexual reproduction an…

Spore (reproductive and survival cell) Spore A spore is a microscopic reproductive or survival cell produced by fungi, non-seed plants, some bacteria and protozoa. This article explains types, life cycles, roles, and how spor…

Squamosal bone The squamosal is a cranial bone forming the cheek region in many vertebrates; in mammals it contributes to the temporal bone and played a central role in the evolutionary shift in…

Staining (Microscopy): Principles, Methods, and Applications Staining Staining enhances contrast in microscopy by using dyes and labeling methods to reveal structures in cells and tissues for research, diagnostics, and teaching.

Stalking: animal behavior, human harassment, law, and prevention Stalking Comprehensive overview of stalking as a hunting tactic in animals and as harmful, repetitive surveillance and pursuit of people — traits, history, impacts, cyberstalking, and ways…

Staphylococcaceae: family of Gram‑positive cocci Staphylococcaceae Overview of the bacterial family Staphylococcaceae, its traits, common genera, clinical importance, laboratory identification, ecology, and historical classification.

Staphylococcus — genus of gram-positive cocci bacteria Staphylococcus Staphylococcus is a genus of spherical, gram-positive bacteria that form grape-like clusters. Most species are harmless skin and mucous membrane commensals; some, notably S. aureus…

Staphylococcus aureus A common Gram-positive bacterium that colonizes humans and causes a range of diseases from skin infections to life‑threatening invasive illness; notable for antibiotic resistance a…

Starvation: causes, physiology, signs, history, and recovery Starvation Starvation is the extreme lack of adequate food intake. This article explains how it develops, physiological effects, common signs, historical uses, treatment challenges and preven…

Steganinae — a subfamily of Drosophilidae Steganinae Steganinae is the smaller of two subfamilies within the fruit fly family Drosophilidae, containing a modest number of genera and species distinguished by morphological and molecula…

Stem Overview of “stem” across disciplines: plant anatomy and roles, linguistic stems and stemming, STEM education, music and audio stems, computing uses, nautical and idiomatic senses.

Stem cell: biology, types, and medical significance Stem cell Overview of stem cells: their defining properties, categories (embryonic, adult, induced), historical development, laboratory culture, clinical uses, and ethical considerations.

Sterile Sterile: describes environments or items free of living microorganisms, or organisms unable to reproduce; used in medicine, microbiology and reproductive health, distinct from asep…

Sterilisation: permanent prevention of reproduction Sterilisation Sterilisation is a medical or veterinary procedure that permanently prevents animals or humans from reproducing. This article covers methods, history, uses, legal and ethical issue…

Sterilization in Microbiology: Methods, Uses, and Distinctions Sterilization (microbiology) Sterilization removes or destroys all viable microorganisms from a surface, fluid, or instrument. This article explains major methods, applications, validation, and differences fro…