Cell Explorer
The unit of life, alive on screen.
The cell is the smallest unit of life, and every organ is built from just four basic tissues — epithelial, connective, muscle and nervous. This section covers eukaryotic cell structure, membrane transport, energy metabolism, the flow of genetic information, the cell cycle and cell death, and the histology of the basic tissues.
Organelles 13
Eukaryotic cell (overview)
The basic structural and functional unit of the body: a membrane-bounded, compartmentalised unit containing a nucleus, cytoplasmic organelles and a cytoskeleton.
OrganelleCytoplasm and cytosol
Everything between the plasma membrane and the nuclear envelope; its fluid part, the cytosol, is a crowded protein-rich gel where glycolysis, free-ribosome protein synthesis and many signalling reactions take place.
OrganelleRibosome
A non-membranous ribonucleoprotein machine (≈25–30 nm) that translates mRNA into protein; the eukaryotic 80S ribosome is built from a 60S and a 40S subunit.
OrganelleRough endoplasmic reticulum
A network of flattened membrane sacs (cisternae) studded with ribosomes, continuous with the outer nuclear membrane, where secretory, lysosomal and membrane proteins are made, folded and N-glycosylated.
OrganelleSmooth endoplasmic reticulum
A network of ribosome-free, anastomosing membrane tubules, continuous with the RER, specialised for lipid and steroid synthesis, drug detoxification, glycogen metabolism and Ca²⁺ storage.
OrganelleGolgi apparatus
A polarised stack of flattened cisternae (cis → medial → trans) that receives proteins from the ER, modifies their sugars, and sorts and packages them for secretion, the plasma membrane or lysosomes.
OrganelleSecretory and transport vesicles
Small membrane-bounded carriers that shuttle cargo between compartments and to the cell surface, where they fuse with the plasma membrane to release their contents (exocytosis).
OrganelleEndosome
A system of acidic membrane compartments that receives endocytosed material, sorts receptors for recycling and delivers cargo for degradation to lysosomes.
OrganelleLysosome
The cell's digestive organelle: a single-membrane vesicle with an acidic interior (pH ≈4.5–5) containing ≈40–60 hydrolytic enzymes that degrade macromolecules from endocytosis, phagocytosis and autophagy.
OrganellePeroxisome
A small single-membrane organelle that uses oxidases to break down very-long-chain and branched fatty acids, producing hydrogen peroxide that its catalase then destroys; it also makes plasmalogens for myelin.
OrganelleProteasome
A barrel-shaped, ATP-dependent protease complex (26S) in the cytosol and nucleus that destroys proteins tagged with chains of ubiquitin.
OrganelleMitochondrion
A double-membraned organelle with its own DNA that produces most of the cell's ATP by oxidative phosphorylation and also controls apoptosis, Ca²⁺ buffering and many metabolic pathways.
OrganelleCytoplasmic inclusions
Stored or accumulated materials in the cytoplasm — glycogen, lipid droplets, pigments and crystals — that are not metabolically active organelles.
Membrane & transport 4
Plasma membrane
The ≈8 nm lipid bilayer with embedded proteins that encloses the cell, controls what enters and leaves, and carries receptors, transporters and adhesion molecules.
Physiological processMembrane transport
The ways substances cross the plasma membrane: passively down their gradients (simple and facilitated diffusion, osmosis), actively against them using ATP or ion gradients, and in bulk by endocytosis and exocytosis.
Physiological processResting membrane potential
The voltage across the plasma membrane of a resting cell (inside negative, typically −70 to −90 mV), generated mainly by K⁺ leaking out down the concentration gradient built by the Na⁺/K⁺-ATPase.
Physiological processCell signalling and receptors
How cells detect chemical signals with receptors and convert them into intracellular responses through second messengers, protein kinases and changes in gene expression.
Nucleus & genetics 9
Nucleus
The largest organelle (≈5–10 µm), enclosed by a double-membraned nuclear envelope, that houses the genome as chromatin and is the site of DNA replication, transcription, RNA processing and ribosome assembly.
OrganelleNuclear envelope and nuclear pores
Two concentric lipid bilayers around the nucleus, separated by the perinuclear space and fused at thousands of nuclear pore complexes that control traffic between nucleus and cytoplasm.
OrganelleNucleolus
A dense, non-membranous nuclear body that forms around the ribosomal RNA genes and is the site of rRNA transcription, processing and ribosome subunit assembly.
MoleculeChromatin and chromosomes
The complex of DNA with histone and non-histone proteins that packages ≈2 m of DNA into the nucleus; it condenses into 46 visible chromosomes during cell division.
Cell processDNA replication
The semiconservative copying of the entire genome once per cell cycle during S phase, from tens of thousands of origins, with an error rate of only ≈1 in 10⁹–10¹⁰ nucleotides.
Cell processDNA repair
A set of pathways that continually detect and correct DNA damage and replication errors; inherited defects in them cause cancer-predisposition syndromes.
Cell processTranscription and RNA processing
Synthesis of RNA from a DNA template by RNA polymerase, followed in eukaryotes by capping, splicing and polyadenylation of pre-mRNA in the nucleus before export to the cytoplasm.
Cell processTranslation and the genetic code
Decoding of mRNA into a polypeptide by ribosomes, using transfer RNAs that pair their anticodons with the triplet codons of the genetic code.
Cell processRegulation of gene expression
The control of when, where and how much each gene is expressed — at the level of chromatin, transcription, RNA processing, translation and protein stability — which makes cells with identical DNA different.
Cytoskeleton & cell surface 6
Cytoskeleton
A dynamic network of three protein filament systems — actin microfilaments (7 nm), intermediate filaments (10 nm) and microtubules (25 nm) — that shapes the cell, resists mechanical stress and, with motor proteins, moves organelles, chromosomes and the whole cell.
OrganelleCentrosome and centrioles
The main microtubule-organising centre of animal cells: a pair of perpendicular centrioles (each nine microtubule triplets) embedded in pericentriolar material that nucleates microtubules and organises the mitotic spindle.
OrganelleCilia
Hair-like, microtubule-based projections of the cell surface: motile cilia (9+2 axoneme) beat in coordinated waves to move fluid, whereas single non-motile primary cilia (9+0) act as sensory antennae.
OrganelleFlagellum
A single long whip-like motile projection with a 9+2 axoneme; in humans found only as the tail of the spermatozoon.
OrganelleMicrovilli
Finger-like, actin-supported projections of the apical plasma membrane (≈1 µm tall) that multiply the absorptive surface of epithelial cells, forming the brush border.
OrganelleCell junctions
Specialised protein complexes that seal epithelia (tight junctions), anchor cells to each other and to the basement membrane (adherens junctions, desmosomes, hemidesmosomes) and couple neighbouring cells chemically and electrically (gap junctions).
Cell processes 5
Cellular respiration
The stepwise oxidation of glucose (and fats and amino acids) to CO₂ and water that captures energy as ATP: glycolysis in the cytosol, then the link reaction, citric-acid cycle and oxidative phosphorylation in mitochondria — ≈30–32 ATP per glucose.
Cell processApoptosis
Programmed, energy-dependent cell death executed by caspases, in which the cell shrinks and fragments into membrane-bound apoptotic bodies that are phagocytosed without inflammation.
Cell processNecrosis
Death of cells in living tissue after severe injury, characterised by swelling, loss of membrane integrity, leakage of cell contents and inflammation.
Cell processAutophagy
"Self-eating": the delivery of cytoplasmic components — damaged organelles, protein aggregates, microbes — to lysosomes for degradation and recycling, increased during starvation and stress.
Cell processCell differentiation
The process by which unspecialised cells become specialised cell types by switching on cell-type-specific sets of genes, while keeping the same genome.
Cell cycle & division 3
Cell cycle
The ordered sequence of growth, DNA replication and division (G1 → S → G2 → M) by which a cell produces two genetically identical daughters, driven by cyclin–CDK complexes and guarded by checkpoints involving p53 and Rb.
Cell processMitosis and cytokinesis
Division of the replicated nucleus into two genetically identical nuclei in five phases — prophase, prometaphase, metaphase, anaphase and telophase — followed by cytokinesis, the division of the cytoplasm.
Cell processMeiosis
Two successive divisions after a single round of DNA replication that halve the chromosome number (2n → n) and shuffle genetic material, producing haploid gametes.
Tissue overview 2
The four basic tissues
All organs are built from four basic tissues — epithelial, connective, muscle and nervous — each defined by its cells, its extracellular matrix and its function.
MembraneBody membranes
Sheets of tissue that cover or line body surfaces and cavities: mucous membranes (open to the exterior), serous membranes (closed body cavities), synovial membranes (joint cavities) and the cutaneous membrane (skin).
Epithelial tissue 11
Epithelial tissue
Sheets of closely packed, polarised cells with little extracellular matrix that cover body surfaces, line cavities and ducts, and form glands; avascular and resting on a basement membrane.
TissueSimple squamous epithelium
A single layer of thin, flattened, polygonal cells with bulging central nuclei, specialised for rapid diffusion, filtration and smooth, slippery surfaces.
TissueSimple cuboidal epithelium
A single layer of cube-shaped cells with central round nuclei, found in glandular ducts, kidney tubules and the thyroid follicles, where it secretes and absorbs.
TissueSimple columnar epithelium
A single layer of tall cells with elongated, basally located nuclei, lining most of the digestive tract (non-ciliated, often with microvilli and goblet cells) and parts of the female reproductive tract (ciliated).
TissuePseudostratified columnar epithelium
A single layer of cells of different heights, all resting on the basement membrane but not all reaching the surface, so their nuclei lie at different levels and the tissue looks stratified; typically ciliated with goblet cells ("respiratory epithelium").
TissueStratified squamous epithelium
A multilayered epithelium in which basal cuboidal cells divide and move upward, flattening toward the surface; it protects against abrasion and, when keratinised (epidermis), against water loss.
TissueStratified cuboidal epithelium
An uncommon epithelium of usually two layers of cuboidal cells, lining the ducts of sweat glands and larger ducts of salivary, mammary and pancreatic glands.
TissueStratified columnar epithelium
A rare epithelium with columnar surface cells over one or more layers of smaller basal cells, found at transitions between other epithelia and in large ducts.
TissueTransitional epithelium (urothelium)
A stratified epithelium unique to the urinary tract whose large dome-shaped surface "umbrella" cells flatten as the organ fills, forming a stretchable, urine-tight barrier.
TissueGlandular epithelium
Epithelial cells specialised for secretion, forming exocrine glands (secreting via ducts onto surfaces) and endocrine glands (ductless, secreting hormones into the blood).
MembraneBasement membrane
A thin, sheet-like layer of specialised extracellular matrix — basal lamina plus reticular lamina — that underlies all epithelia and surrounds muscle cells, adipocytes and Schwann cells.
Connective tissue 8
Connective tissue
The most abundant and diverse tissue: relatively few cells scattered in an abundant extracellular matrix of fibres and ground substance, which binds, supports, protects, stores energy and transports.
TissueAreolar (loose) connective tissue
The most widespread connective tissue: a loose, gel-rich meshwork of all three fibre types and many cell types that packages organs, underlies epithelia and surrounds vessels and nerves.
TissueAdipose tissue
Loose connective tissue dominated by fat-storing adipocytes: white fat stores energy, insulates, cushions and secretes hormones; brown fat burns fat to generate heat.
TissueReticular connective tissue
A delicate three-dimensional network of thin reticular (type III collagen) fibres and reticular cells that forms the supporting stroma of lymphoid organs, bone marrow and the liver.
TissueDense regular connective tissue
Tightly packed, parallel bundles of type I collagen with rows of flattened fibroblasts, giving great tensile strength in one direction; forms tendons, most ligaments and aponeuroses.
TissueDense irregular connective tissue
Thick type I collagen bundles interwoven in many directions, resisting tension from all sides; forms the reticular dermis, organ capsules, periosteum and perichondrium.
TissueElastic connective tissue
Dense connective tissue rich in elastic fibres or sheets that stretch and recoil, found in the walls of large arteries, the ligamenta flava and the airways.
MoleculeExtracellular matrix
The non-cellular material secreted by cells — collagen, elastic and reticular fibres embedded in a hydrated ground substance of glycosaminoglycans, proteoglycans and adhesive glycoproteins — that gives tissues their mechanical properties and signals to cells.
Cartilage 3
Hyaline cartilage
The most common cartilage: a glassy, avascular matrix of type II collagen and water-binding aggrecan, with chondrocytes in lacunae, that provides smooth, resilient support and the template for most of the fetal skeleton.
TissueElastic cartilage
Hyaline-like cartilage with a dense network of branching elastic fibres, giving flexibility and shape recovery; found in the auricle, external acoustic meatus, auditory tube and epiglottis.
TissueFibrocartilage
A tough blend of dense regular connective tissue and cartilage — rows of chondrocytes between thick bundles of type I collagen — that resists compression and shear; it has no perichondrium.
Muscle & nervous tissue (overview) 2
Muscle tissue (overview)
Tissue of elongated, excitable cells (fibres) packed with actin and myosin that contract to produce movement; three types — skeletal, cardiac and smooth — differ in structure, control and regenerative capacity.
TissueNervous tissue (overview)
Tissue of neurons, which generate and conduct electrical signals and communicate at synapses, and the more numerous supporting neuroglia; it forms the brain, spinal cord, ganglia and nerves.
Cell types 7
Cell types of the body (catalogue)
A guide to the major specialised cell types of the human body, each a variation on the common eukaryotic plan adapted to a particular function.
CellFibroblast
The principal resident cell of connective tissue, which synthesises and remodels the extracellular matrix — collagen, elastic and reticular fibres and ground substance.
CellAdipocyte
A cell specialised for storing triacylglycerol and releasing fatty acids on demand, and an endocrine cell secreting leptin and adiponectin; white adipocytes are unilocular, brown adipocytes multilocular.
CellChondrocyte
The cell of cartilage: chondroblasts secrete cartilage matrix and become chondrocytes trapped in lacunae, where they maintain the type II collagen and aggrecan of the matrix.
CellStem cells
Unspecialised cells that can both renew themselves and give rise to differentiated cell types; they are graded by potency from totipotent (zygote) to pluripotent, multipotent and unipotent.
CellGoblet cell
A goblet-shaped, mucus-secreting unicellular exocrine gland scattered among the columnar cells of intestinal and respiratory epithelia.
CellMesenchymal cell
An unspecialised, stellate embryonic connective-tissue cell (and its adult descendant, the mesenchymal stem/stromal cell) that gives rise to fibroblasts, chondroblasts, osteoblasts, adipocytes and smooth muscle.
Specialised cells in other systems 17
Neuron
The excitable cell that is the structural and functional unit of the nervous system. It receives input on its dendrites and soma, integrates it at the axon hillock, and transmits action potentials along its axon to synapses on other cells.
CellAstrocyte
The most abundant glial cell of the CNS. It is star-shaped, with processes whose end-feet ensheathe capillaries and synapses. Astrocytes maintain the blood–brain barrier and the ionic and neurotransmitter environment, and they form glial scars after injury.
CellErythrocyte (red blood cell)
Erythrocytes (red blood cells) are biconcave cells about 7.5 µm across, without a nucleus and packed with haemoglobin. They carry oxygen from the lungs to the tissues, help return CO₂ to the lungs and circulate for about 120 days.
CellPlatelets (thrombocytes)
Platelets are small cell fragments without a nucleus, about 2–4 µm across, shed from megakaryocytes in the bone marrow. They stick to damaged vessel walls, clump together into a plug and provide the surface on which the clotting cascade runs.
CellNeutrophil
The most abundant leukocyte (≈ 40–75%): a short-lived, highly motile phagocyte with a 3–5-lobed nucleus that is first to arrive at sites of acute bacterial infection and kills microbes with oxidants, granule enzymes and extracellular traps.
CellMacrophage
Large, long-lived tissue phagocytes that engulf microbes, dead cells and debris, present antigen to T cells, orchestrate inflammation with cytokines and direct tissue repair; they take tissue-specific forms such as Kupffer cells and alveolar macrophages.
CellB lymphocyte
Lymphocytes that mature in the bone marrow, recognise intact antigen through a membrane-bound antibody (the B-cell receptor) and, when activated, become antibody-secreting plasma cells and memory B cells; they also present antigen to helper T cells.
CellT lymphocyte
Lymphocytes that mature in the thymus and recognise peptide antigens only when displayed on MHC molecules; they orchestrate (CD4 helper), execute (CD8 cytotoxic) and restrain (regulatory) cell-mediated adaptive immunity.
CellMast cell
Long-lived, tissue-resident sentinel cells packed with histamine- and tryptase-containing granules, located near blood vessels and nerves beneath the skin and mucosae; IgE-bound mast cells trigger immediate allergic reactions and anaphylaxis.
CellOsteoblast
Bone-forming cell that secretes osteoid (type I collagen and non-collagenous proteins) and initiates its mineralisation. Osteoblasts also control osteoclast formation through RANKL and osteoprotegerin.
CellOsteocyte
A mature bone cell — a former osteoblast entombed within mineralised matrix in a lacuna — that maintains the matrix and acts as the mechanosensor and endocrine regulator of bone.
CellOsteoclast
A large, multinucleated bone-resorbing cell derived from the monocyte–macrophage lineage. It dissolves mineral with acid and digests collagen with proteases in a sealed resorption compartment.
CellSkeletal muscle fibre
A long, cylindrical, multinucleated syncytium formed by the fusion of myoblasts. It is packed with myofibrils and specialised membranes for rapid, synchronous contraction.
CellKeratinocyte
Keratinocytes are the dominant (≈90%) cells of the epidermis; they proliferate in the basal layer, fill with keratin as they move outward and die to form the protective stratum corneum.
CellMelanocyte
Melanocytes are neural crest-derived dendritic cells in the stratum basale that synthesise melanin in melanosomes and transfer it to surrounding keratinocytes, determining skin and hair colour and protecting against UV.
CellSpermatozoon
The motile male gamete (≈55–60 µm long): a haploid head capped by the acrosome, a mitochondria-packed midpiece and a long flagellum, specialised to reach and fertilise the oocyte.
CellOocyte
The female gamete — one of the largest human cells (≈100–120 µm) — ovulated as a secondary oocyte arrested in metaphase II, enclosed in the zona pellucida and corona radiata, and viable for only ≈12–24 hours.