Histology

What the microscope shows and why it matters: the cell, the four basic tissues and the histology of each organ, described in words, with clinical notes and a link to the same structure in Durgical's 3D anatomy.

84 pages

• Methods and stains
• The cell
• Epithelium
• Connective tissue, cartilage and bone
• Muscle tissue
• Nervous tissue
• Blood and hematopoiesis
• Cardiovascular system
• Skin and breast
• Respiratory system
• Digestive system
• Urinary system
• Endocrine system
• Reproductive system
• Special senses
• Lymphoid organs
• A to Z

Methods and stains

How tissue is prepared and stained, and what each stain shows.

• Microscopy and stains: H&E, PAS, trichrome, silver, immunohistochemistryTissue is almost transparent; stains give it colour so nuclei, cytoplasm, fibres, mucus and specific proteins can be told apart.Technique
• From biopsy to slide: fixation, sectioning, frozen sectionBefore tissue can be seen under a microscope it must be preserved, hardened, cut very thin and stained.Technique
• Electron microscopy: what it shows that light can'tIt uses electrons instead of light and shows structures far smaller than light can, such as membranes, ribosomes and the inside of cilia.Technique

The cell

Membrane, organelles, junctions, cytoskeleton, the cell cycle and cell injury.

• The cell membrane and transportThe membrane is the cell's border: a double layer of fat with proteins that lets some substances through and blocks others.Topic
• Organelles: nucleus, ER, Golgi, mitochondria, lysosomes, peroxisomesOrganelles are the cell's compartments; each has a job, and when one fails a specific disease appears.Topic
• Cell junctions: tight, adherens, desmosomes, gap, hemidesmosomesCell junctions seal, glue and connect neighbouring cells; when the immune system attacks them, the skin blisters.Topic
• Cytoskeleton: microfilaments, intermediate filaments, microtubulesThe cytoskeleton is the cell's inner scaffold: it gives shape, moves the cell and shares out the chromosomes when it divides.Topic
• Cilia, microvilli and stereociliaThese are projections of the cell surface: cilia sweep mucus, microvilli absorb, and stereocilia sense sound.Topic
• Cell cycle, mitosis and meiosisThe cell cycle is the sequence by which a cell grows, copies its DNA and divides; its brakes fail in cancer.Process
• Cell injury, apoptosis and necrosisA damaged cell can recover, die in an orderly way (apoptosis) or die in a messy, inflammatory way (necrosis).Process
• Cell adaptations: hypertrophy, hyperplasia, atrophy, metaplasiaUnder a sustained stimulus, cells grow, multiply, shrink or change type to survive.Process
• Stem cells and tissue renewal: labile, stable and permanent tissuesSome tissues renew themselves all the time, some only after injury, and some almost never. Stem cells make the difference.Process

Epithelium

Covering and glandular epithelia.

• Epithelium: classification (the table to know)Epithelium is the sheet of tightly packed cells that covers the body, lines its cavities and tubes, and forms glands.Tissue
• Simple, stratified, pseudostratified and transitional epitheliumHow the main covering epithelia look under the microscope, and why each organ has the lining it has.Tissue
• Glands: exocrine vs endocrine, secretion typesGlands are epithelial cells that make and release products, either into a duct (exocrine) or into the blood (endocrine).Tissue
• Basement membraneA thin sheet of proteins under every epithelium that anchors it to the tissue below and acts as a filter and a barrier.Tissue

Connective tissue, cartilage and bone

Fibres, cells, fat, cartilage and bone.

• Connective tissue proper: cells and fibresThe tissue that binds and supports the body: a few cells scattered in a matrix of fibres and gel.Tissue
• Adipose tissue: white and brown fatFat tissue stores energy, insulates and cushions the body, makes hormones, and in its brown form produces heat.Tissue
• Cartilage: hyaline, elastic, fibrocartilageCartilage is a firm, flexible tissue with no blood vessels: it lines joints, shapes the ear and airway, and cushions the spine.Tissue
• Bone tissue: cells, compact and spongy boneBone is a living, mineralized connective tissue that is constantly built and broken down by its own cells.Tissue
• Ossification, growth plate and bone remodelingHow bone forms in the embryo, how long bones grow in length at the growth plate, and how adult bone is renewed and repaired.Process
• Extracellular matrix: collagen types, elastin, ground substanceThe extracellular matrix is the material between cells: fibres that give strength and stretch, and a gel that holds water.Topic
• Synovial joints: capsule, synovial membrane and articular cartilageMovable joints are sealed bags of slippery fluid: a fibrous capsule, a thin lining that makes the fluid, and smooth cartilage on the bone ends.Tissue

Muscle tissue

Skeletal, cardiac and smooth muscle.

• Skeletal muscle: sarcomere and contractionSkeletal muscle is the striated, voluntary muscle that moves the skeleton; its repeating unit, the sarcomere, shortens to make force.Tissue
• Cardiac muscle: intercalated discsCardiac muscle is the striated, involuntary muscle of the heart; its branching cells are joined end to end so they beat as one.Tissue
• Smooth muscleSmooth muscle is the non-striated, involuntary muscle in the walls of vessels, airways, the gut, the bladder and the uterus.Tissue
• Muscle injury, repair and satellite cellsSkeletal muscle repairs itself with its own stem cells, the satellite cells; heart muscle mostly scars; smooth muscle regrows best.Process
• Neuromuscular junction: motor end plate and acetylcholine receptorsThe spot where a motor nerve tells a muscle fibre to contract, using acetylcholine. Several diseases and toxins act right here.Topic

Nervous tissue

Neurons, glia, peripheral nerves and the central nervous system.

• The neuron and synapseNeurons are the cells that carry electrical signals; they pass the message to the next cell at a synapse.Cell
• Glial cells: astrocytes, oligodendrocytes, microglia, Schwann cellsGlial cells support, insulate, feed and defend neurons; they outnumber neurons and, unlike them, can divide.Cell
• Peripheral nerve: endoneurium, perineurium, epineurium; myelinA peripheral nerve is a cable of many axons, each with its Schwann cells, bundled by three layers of connective tissue.Tissue
• Gray and white matter, cerebellar cortex, meningesHow the brain and spinal cord look under the microscope: gray matter with cell bodies, white matter with myelinated tracts, wrapped in three meninges.Tissue
• Nerve injury and regeneration (Wallerian degeneration)When a nerve is cut or crushed, the part beyond the injury breaks down and is cleared; peripheral axons can then slowly regrow, central ones mostly cannot.Process
• Blood–brain barrier, choroid plexus and cerebrospinal fluidThe brain guards what enters it from the blood, and it floats in a clear fluid made by tufts of vessels inside its cavities.Topic

Blood and hematopoiesis

The cells of the smear and where they are made.

• Blood cells on a smear: erythrocytes, leukocytes, plateletsA drop of blood spread on a slide shows red cells, five kinds of white cells and tiny platelets, each with a look that tells you what it is.Cell
• Hematopoiesis and bone marrowBlood cells live from hours to months, so the bone marrow keeps making new ones from a small pool of stem cells.Process

Cardiovascular system

Vessel layers, the heart wall and lymphatics.

• Blood vessels: artery, vein and capillary layersArteries and veins share three wall layers; arteries have a thick muscle layer for pressure, veins a thin wall and valves, capillaries only endothelium.Tissue
• The heart wall, valves and conduction tissueThe heart wall has three layers: a thin inner lining, a thick muscle and an outer coat. Valves are flaps of lining around a fibrous core.Organ
• Lymphatic vesselsLymphatic vessels collect the fluid that leaks out of capillaries and return it to the veins, passing it through lymph nodes on the way.Tissue

Skin and breast

Epidermis, dermis, appendages, wound healing and the breast.

• Skin: epidermis layers, dermis, hypodermisSkin has an outer epidermis of stacked cells, a strong dermis of collagen with vessels and nerves, and a fatty hypodermis underneath.Organ
• Keratinocytes, melanocytes, Langerhans and Merkel cellsThe epidermis is mostly keratin-making cells, helped by pigment cells, immune sentinel cells and touch cells.Cell
• Hair follicles, sebaceous and sweat glands, nailsHair, oil glands, sweat glands and nails grow down from the epidermis into the dermis and help with protection and temperature.Organ
• Wound healing: phases and scarA wound closes in four overlapping phases: stop the bleeding, clean up, rebuild with new tissue, then remodel it into a scar over months.Process
• Where blisters form: pemphigus vs pemphigoidIn pemphigus the skin cells come apart inside the epidermis; in pemphigoid the whole epidermis lifts off the dermis.Topic
• Skin receptors: Meissner, Pacinian, Merkel, Ruffini and free nerve endingsTiny nerve endings in the skin feel touch, vibration, stretch, heat, cold and pain, each type tuned to its own job.Cell

Respiratory system

From the airway to the alveolus, and the serous membranes.

• The airway: from nose to bronchiolesThe tubes that carry air are lined by a moist, ciliated layer that traps dust and germs; their support changes from cartilage to muscle as they narrow.Organ
• The alveolus: type I and II pneumocytes, surfactant, the blood–air barrierAlveoli are tiny air sacs with walls so thin that oxygen passes straight into the blood; a soap-like film keeps them from collapsing.Organ
• Serous membranes: pleura and peritoneumThe lungs and belly organs are wrapped in thin, slippery membranes that let them slide; fluid builds up between the layers when they are diseased.Tissue

Digestive system

The layers of the GI tract, each segment and its glands.

• The GI tract wall: mucosa, submucosa, muscularis, serosa; enteric plexusesFrom the esophagus to the anus the gut wall has the same four layers; what changes from organ to organ is the lining and its glands.Topic
• Oral cavity, tongue and teethThe mouth is lined by a tough layered skin-like tissue; the tongue is muscle covered with papillae, and teeth are built of enamel, dentin and pulp.Organ
• Esophagus and the Barrett changeThe esophagus is a muscular tube lined by layered flat cells; long-standing acid reflux can turn its lower lining into intestine-like tissue.Organ
• Stomach: gastric glands, parietal and chief cellsThe stomach lining is full of tiny pits and glands; some cells make acid, others make enzymes, mucus or hormones.Organ
• Small intestine: villi, crypts, enterocytes, Paneth cells; duodenum vs jejunum vs ileumThe small intestine is lined by finger-like villi covered with absorbing cells; each part has a feature that lets you tell it apart.Organ
• Large intestine, appendix and anal canalThe large intestine has no villi; its lining is straight glands full of mucus cells that absorb water and make stool slide.Organ
• Liver: lobule, acinus, hepatocytes, sinusoids, Kupffer cellsThe liver is built of cords of cells that blood flows past slowly; where a cell sits in that flow decides what it does and what can hurt it.Organ
• Gallbladder and bile ductsThe gallbladder is a small sac that stores and concentrates bile; its wall is thin and lacks some layers the gut has.Organ
• Pancreas: acini and isletsThe pancreas is two glands in one: grape-like clusters make digestive enzymes, and scattered pale islands make insulin and glucagon.Organ
• Salivary glands: serous and mucous aciniThree pairs of glands make saliva: one makes watery saliva, one mixed, and one thick mucus; their ducts adjust the salt before it reaches the mouth.Organ
• Enteroendocrine cells and GALTScattered in the gut lining are hormone-making cells and a large immune system that samples what we swallow.Cell

Urinary system

The nephron, the filtration barrier and the urinary tract.

• Kidney: cortex, medulla and the nephronThe nephron is the unit that filters blood and makes urine; each segment has its own epithelium, which explains its job.Organ
• Glomerulus and the filtration barrier: podocytes, mesangium, JGAThe glomerulus is a tuft of capillaries that filters blood; its three-layer barrier lets water and salts through but holds back proteins.Organ
• Ureter, bladder and urethra: urotheliumThe urinary tract is lined by urothelium, an epithelium that stretches without tearing and seals the wall off from urine.Organ

Endocrine system

Pituitary, thyroid, parathyroid, adrenal and islets.

• Pituitary: adenohypophysis and neurohypophysisThe pituitary is two glands in one: the front part makes hormones and the back part stores the ones the hypothalamus makes.Organ
• Thyroid follicles, C cells and parathyroidThe thyroid stores its hormone in colloid-filled follicles; the parathyroids, stuck behind it, control blood calcium.Organ
• Adrenal cortex zones and medullaThe adrenal gland has a three-layer cortex that makes steroids and a medulla that releases adrenaline.Organ
• Islets of Langerhans: α, β, δ cellsIslets are clusters of cells inside the pancreas that release insulin and glucagon into the blood to control sugar.Cell
• Pineal gland and the diffuse endocrine systemThe pineal makes melatonin, which sets the day-night rhythm; many other hormone cells are spread through other organs.Organ

Reproductive system

Testis, male ducts and glands, ovary, uterus, cervix and placenta.

• Breast: ducts, lobules and changes with life stageThe breast is a modified sweat gland: lobules make milk and ducts carry it to the nipple, inside fat and fibrous tissue that change through life.Organ
• Fertilisation, implantation and the deciduaHow a sperm and an egg meet in the uterine tube, and how the early embryo travels to the uterus and burrows into its lining.Process
• Testis: seminiferous tubules, Sertoli and Leydig cells, spermatogenesisThe testis makes sperm in coiled tubes and testosterone in the cells that sit between them.Organ
• Epididymis, vas deferens, seminal glands and prostateThe ducts mature and move sperm; the glands add the fluid that makes up most of semen.Organ
• Ovary: follicles, ovulation and corpus luteumThe ovary keeps immature eggs in follicles; each cycle one matures, is released and leaves a corpus luteum that makes progesterone.Organ
• Uterus and the endometrial cycleThe lining of the uterus grows, gets ready and is shed each month, following the hormones of the ovary.Organ
• Cervix and vagina: the transformation zoneTwo epithelia meet in the cervix; that zone of change is where HPV causes nearly all cervical cancers.Organ
• Uterine tubeThe uterine tube catches the egg and carries it to the uterus with cilia and contractions; fertilisation usually happens there.Organ
• PlacentaThe placenta links fetus and mother: its villi, bathed in maternal blood, swap oxygen, nutrients and waste.Organ

Special senses

Eye, retina, inner ear, smell and taste.

• Vestibular apparatus: maculae, cristae and otolithsThe balance organ of the inner ear: small patches of hair cells that feel gravity and tilting, and crests that feel the head turning.Organ
• Conjunctiva, eyelid glands and lacrimal glandThe eyelids, their oil glands and the tear gland keep the front of the eye wet, clean and protected.Organ
• The eye wall: cornea, sclera, uvea and lensThe eyeball has three layers: a tough outer coat that is clear at the front, a middle layer full of vessels and pigment, and the retina inside.Organ
• Retina: the ten layers, rods and conesThe retina is nerve tissue that turns light into signals. Under the microscope it shows ten layers, with the light sensors at the outer edge.Organ
• Inner ear: the cochlea and the organ of CortiThe cochlea is a fluid-filled spiral tube. Inside it, the organ of Corti has hair cells that turn sound vibration into nerve signals.Organ
• Olfactory epithelium and taste budsSmell relies on nerve cells in the roof of the nose, and taste on taste buds in the bumps of the tongue. Both kinds of cell are replaced over time.Organ

Lymphoid organs

Lymph node, spleen, thymus and tonsils under the microscope (their function is in Immunology).

• Lymph node and spleen under the microscopeThe lymph node filters lymph and the spleen filters blood. Under the microscope the node has a cortex and medulla; the spleen has white and red pulp.Organ
• Thymus and tonsils under the microscopeThe thymus is where T cells mature and it has no follicles. The tonsils are lymphoid nodules under the lining of the throat, with deep crypts.Organ

A to Z

• Adipose tissue: white and brown fatConnective tissue, cartilage and bone · Tissue
• Adrenal cortex zones and medullaEndocrine system · Organ
• Basement membraneEpithelium · Tissue
• Blood cells on a smear: erythrocytes, leukocytes, plateletsBlood and hematopoiesis · Cell
• Blood vessels: artery, vein and capillary layersCardiovascular system · Tissue
• Blood–brain barrier, choroid plexus and cerebrospinal fluidNervous tissue · Topic
• Bone tissue: cells, compact and spongy boneConnective tissue, cartilage and bone · Tissue
• Breast: ducts, lobules and changes with life stageReproductive system · Organ
• Cardiac muscle: intercalated discsMuscle tissue · Tissue
• Cartilage: hyaline, elastic, fibrocartilageConnective tissue, cartilage and bone · Tissue
• Cell adaptations: hypertrophy, hyperplasia, atrophy, metaplasiaThe cell · Process
• Cell cycle, mitosis and meiosisThe cell · Process
• Cell injury, apoptosis and necrosisThe cell · Process
• Cell junctions: tight, adherens, desmosomes, gap, hemidesmosomesThe cell · Topic
• Cervix and vagina: the transformation zoneReproductive system · Organ
• Cilia, microvilli and stereociliaThe cell · Topic
• Conjunctiva, eyelid glands and lacrimal glandSpecial senses · Organ
• Connective tissue proper: cells and fibresConnective tissue, cartilage and bone · Tissue
• Cytoskeleton: microfilaments, intermediate filaments, microtubulesThe cell · Topic
• Electron microscopy: what it shows that light can'tMethods and stains · Technique
• Enteroendocrine cells and GALTDigestive system · Cell
• Epididymis, vas deferens, seminal glands and prostateReproductive system · Organ
• Epithelium: classification (the table to know)Epithelium · Tissue
• Esophagus and the Barrett changeDigestive system · Organ
• Extracellular matrix: collagen types, elastin, ground substanceConnective tissue, cartilage and bone · Topic
• Fertilisation, implantation and the deciduaReproductive system · Process
• From biopsy to slide: fixation, sectioning, frozen sectionMethods and stains · Technique
• Gallbladder and bile ductsDigestive system · Organ
• Glands: exocrine vs endocrine, secretion typesEpithelium · Tissue
• Glial cells: astrocytes, oligodendrocytes, microglia, Schwann cellsNervous tissue · Cell
• Glomerulus and the filtration barrier: podocytes, mesangium, JGAUrinary system · Organ
• Gray and white matter, cerebellar cortex, meningesNervous tissue · Tissue
• Hair follicles, sebaceous and sweat glands, nailsSkin and breast · Organ
• Hematopoiesis and bone marrowBlood and hematopoiesis · Process
• Inner ear: the cochlea and the organ of CortiSpecial senses · Organ
• Islets of Langerhans: α, β, δ cellsEndocrine system · Cell
• Keratinocytes, melanocytes, Langerhans and Merkel cellsSkin and breast · Cell
• Kidney: cortex, medulla and the nephronUrinary system · Organ
• Large intestine, appendix and anal canalDigestive system · Organ
• Liver: lobule, acinus, hepatocytes, sinusoids, Kupffer cellsDigestive system · Organ
• Lymph node and spleen under the microscopeLymphoid organs · Organ
• Lymphatic vesselsCardiovascular system · Tissue
• Microscopy and stains: H&E, PAS, trichrome, silver, immunohistochemistryMethods and stains · Technique
• Muscle injury, repair and satellite cellsMuscle tissue · Process
• Nerve injury and regeneration (Wallerian degeneration)Nervous tissue · Process
• Neuromuscular junction: motor end plate and acetylcholine receptorsMuscle tissue · Topic
• Olfactory epithelium and taste budsSpecial senses · Organ
• Oral cavity, tongue and teethDigestive system · Organ
• Organelles: nucleus, ER, Golgi, mitochondria, lysosomes, peroxisomesThe cell · Topic
• Ossification, growth plate and bone remodelingConnective tissue, cartilage and bone · Process
• Ovary: follicles, ovulation and corpus luteumReproductive system · Organ
• Pancreas: acini and isletsDigestive system · Organ
• Peripheral nerve: endoneurium, perineurium, epineurium; myelinNervous tissue · Tissue
• Pineal gland and the diffuse endocrine systemEndocrine system · Organ
• Pituitary: adenohypophysis and neurohypophysisEndocrine system · Organ
• PlacentaReproductive system · Organ
• Retina: the ten layers, rods and conesSpecial senses · Organ
• Salivary glands: serous and mucous aciniDigestive system · Organ
• Serous membranes: pleura and peritoneumRespiratory system · Tissue
• Simple, stratified, pseudostratified and transitional epitheliumEpithelium · Tissue
• Skeletal muscle: sarcomere and contractionMuscle tissue · Tissue
• Skin receptors: Meissner, Pacinian, Merkel, Ruffini and free nerve endingsSkin and breast · Cell
• Skin: epidermis layers, dermis, hypodermisSkin and breast · Organ
• Small intestine: villi, crypts, enterocytes, Paneth cells; duodenum vs jejunum vs ileumDigestive system · Organ
• Smooth muscleMuscle tissue · Tissue
• Stem cells and tissue renewal: labile, stable and permanent tissuesThe cell · Process
• Stomach: gastric glands, parietal and chief cellsDigestive system · Organ
• Synovial joints: capsule, synovial membrane and articular cartilageConnective tissue, cartilage and bone · Tissue
• Testis: seminiferous tubules, Sertoli and Leydig cells, spermatogenesisReproductive system · Organ
• The airway: from nose to bronchiolesRespiratory system · Organ
• The alveolus: type I and II pneumocytes, surfactant, the blood–air barrierRespiratory system · Organ
• The cell membrane and transportThe cell · Topic
• The eye wall: cornea, sclera, uvea and lensSpecial senses · Organ
• The GI tract wall: mucosa, submucosa, muscularis, serosa; enteric plexusesDigestive system · Topic
• The heart wall, valves and conduction tissueCardiovascular system · Organ
• The neuron and synapseNervous tissue · Cell
• Thymus and tonsils under the microscopeLymphoid organs · Organ
• Thyroid follicles, C cells and parathyroidEndocrine system · Organ
• Ureter, bladder and urethra: urotheliumUrinary system · Organ
• Uterine tubeReproductive system · Organ
• Uterus and the endometrial cycleReproductive system · Organ
• Vestibular apparatus: maculae, cristae and otolithsSpecial senses · Organ
• Where blisters form: pemphigus vs pemphigoidSkin and breast · Topic
• Wound healing: phases and scarSkin and breast · Process

Educational content on normal microscopic structure and its clinical relevance; no substitute for studying slides or for a health professional's advice.

Representative model, for educational purposes only.