Histology

01 Overview & Significance

Histology is the study of microscopic anatomy—the structure and organization of cells and tissues that compose every organ in the human body. Understanding tissue architecture is foundational to pathology, since nearly every disease process produces identifiable changes at the cellular or tissue level. The ability to recognize normal histologic patterns is the prerequisite for identifying pathologic deviations in biopsy specimens, surgical resections, and autopsy material.

The four fundamental tissue types—epithelial, connective, muscle, and nervous—combine in characteristic patterns to form every organ. Each tissue type has a distinct embryologic origin, structural organization, and functional role. Organs are defined by their unique combination of these tissue types arranged in reproducible architectural patterns.

The Four Fundamental Tissue Types

02 Tissue Processing & Preparation

Proper tissue processing is essential for producing high-quality histologic sections. The standard workflow involves fixation, dehydration, clearing, embedding, sectioning, and staining. Each step introduces potential artifacts that the histologist must recognize and distinguish from genuine pathology.

Fixation

10% neutral buffered formalin (NBF) is the universal fixative in clinical pathology. It cross-links proteins (primarily through lysine residues), preserving tissue architecture and preventing autolysis. Fixation should begin within minutes of excision; delayed fixation causes autolytic artifacts—loss of nuclear detail, cytoplasmic pallor, and detachment of epithelial lining.

Embedding & Sectioning

After dehydration through graded alcohols and clearing in xylene, tissue is infiltrated with paraffin wax (melting point 56–58°C). A microtome cuts 4–6 μm sections that are floated onto glass slides. For rapid intraoperative diagnosis, frozen sections bypass embedding: tissue is snap-frozen in OCT compound and cut on a cryostat at −20°C, yielding results within 15–20 minutes but with inferior morphologic detail.

Common Processing Artifacts

03 Staining Techniques

Stains transform colorless tissue sections into interpretable images by selectively binding to cellular and extracellular components. The hematoxylin and eosin (H&E) stain is the cornerstone of histologic diagnosis, but hundreds of special stains and immunohistochemical markers exist for specific diagnostic questions.

H&E Stain

Hematoxylin is a basic (cationic) dye that binds acidic structures—primarily DNA and RNA—staining nuclei blue-purple (basophilic). Eosin is an acidic (anionic) dye that binds basic structures—most cytoplasmic proteins—staining them pink-red (eosinophilic/acidophilic). The combination reveals nuclear-to-cytoplasmic ratio, chromatin pattern, cell boundaries, and tissue architecture.

Key Special Stains

Immunohistochemistry (IHC)

IHC uses antibodies conjugated to chromogens (typically DAB, producing brown staining) to detect specific antigens in tissue sections. It is indispensable for tumor classification, prognostic markers, and identifying cell lineage. Key principles include antigen retrieval (heat-induced or enzymatic) to unmask formalin-crosslinked epitopes, and the use of appropriate positive and negative controls.

04 Simple Epithelia

Epithelial tissue lines body surfaces, cavities, and tubes, and forms glands. All epithelia rest on a basement membrane (basal lamina + reticular lamina), are avascular (nourished by diffusion from underlying connective tissue), and exhibit polarity (apical, lateral, and basal domains). Classification is based on the number of cell layers (simple vs. stratified) and cell shape (squamous, cuboidal, columnar).

Simple Squamous Epithelium

A single layer of flat cells with disc-shaped nuclei. Optimized for diffusion and filtration due to minimal cytoplasmic thickness. Special names: endothelium (lines blood vessels), mesothelium (lines serous cavities—pleura, pericardium, peritoneum). Found also in Bowman capsule, thin loop of Henle, and alveolar type I cells.

Simple Cuboidal Epithelium

A single layer of cube-shaped cells with round, central nuclei. Specialized for secretion and absorption. Found in kidney tubules (proximal and distal convoluted tubules), thyroid follicles, small collecting ducts, and covering the ovary (germinal epithelium). Proximal tubule cells have a prominent brush border (microvilli) that dramatically increases absorptive surface area.

Simple Columnar Epithelium

A single layer of tall cells with oval, basally located nuclei. The primary lining of the GI tract from the stomach to the rectum. May be ciliated (fallopian tubes, small bronchi) or non-ciliated with microvilli (intestinal absorptive cells). Goblet cells are interspersed throughout, secreting mucus for lubrication and protection.

Pseudostratified Columnar Epithelium

Appears stratified but all cells contact the basement membrane—not all reach the apical surface. Nuclei at varying heights create the false impression of multiple layers. The ciliated form lines the respiratory tract (trachea, bronchi) and contains goblet cells; together they form the mucociliary escalator. The non-ciliated form with stereocilia (long microvilli) lines the epididymis and vas deferens.

Lateral Domain Specializations

The lateral domain of epithelial cells mediates cell-cell adhesion and communication. In addition to the junctional complex (tight junctions, adherens junctions, desmosomes), the lateral membrane contains lateral intercellular spaces that widen during active fluid transport (as seen in gallbladder epithelium absorbing water). Cadherins are calcium-dependent transmembrane adhesion molecules: E-cadherin predominates in epithelia, N-cadherin in neural and mesenchymal cells, P-cadherin in placenta. Loss of E-cadherin expression is a hallmark of lobular carcinoma of the breast and diffuse gastric carcinoma, and is used diagnostically on IHC.

Basal Domain Specializations

The basal domain anchors epithelial cells to the underlying basement membrane through hemidesmosomes (integrin α6β4 + BP antigens connecting to keratin intermediate filaments) and focal adhesions (integrin-mediated connections to the actin cytoskeleton). Basal infoldings are deep membrane invaginations at the basal surface that increase surface area for ion transport; they are packed with mitochondria and prominent in the distal convoluted tubule, striated ducts of salivary glands, and choroid plexus epithelium. These appear as basal striations on light microscopy.

05 Stratified & Transitional Epithelia

Stratified epithelia have multiple cell layers, providing protection against mechanical abrasion, chemical insult, and desiccation. Classification is based on the shape of the surface (apical) cells, regardless of the shape of deeper layers.

Stratified Squamous Epithelium

The most widespread protective epithelium. The keratinized form covers the skin (epidermis)—surface cells are dead, anucleate, and filled with keratin. The non-keratinized form lines the oral cavity, esophagus, vagina, and anal canal—surface cells retain their nuclei and remain moist. Basal cells are mitotically active and continuously replace cells lost from the surface.

Stratified Cuboidal & Columnar

Rare epithelia with limited distribution. Stratified cuboidal (2 layers): lines sweat gland ducts and large excretory ducts of salivary glands. Stratified columnar (2+ layers): found in the male urethra, large excretory ducts, and conjunctiva of the eye. Neither type is commonly tested but may appear as distractors.

Transitional Epithelium (Urothelium)

Unique to the urinary tract (renal pelvis, ureters, bladder, proximal urethra). Surface cells are large, dome-shaped "umbrella cells" with specialized plaques of uroplakin proteins on the apical surface. When the bladder is relaxed, epithelium appears 5–6 layers thick with rounded surface cells; when distended, it thins to 2–3 layers with flattened surface cells. This is the only epithelium that changes appearance with mechanical stretch.

Epithelial Cell Junctions

Basement Membrane

The basement membrane underlies all epithelia and surrounds muscle cells, Schwann cells, and adipocytes. It consists of the basal lamina (lamina lucida + lamina densa, primarily type IV collagen, laminin, entactin/nidogen, and heparan sulfate proteoglycans) and the reticular lamina (type III collagen fibers from underlying connective tissue). It acts as a structural scaffold, molecular filter, and barrier to cell migration—cancer invasion through the basement membrane defines the transition from carcinoma in situ to invasive carcinoma.

06 Glandular Epithelium

Glands are epithelial structures specialized for secretion. They arise embryologically as epithelial invaginations into underlying connective tissue. Glands are classified as exocrine (secrete via ducts to a surface) or endocrine (secrete hormones directly into blood; ductless).

Exocrine Gland Classification

Modes of Secretion

Endocrine Glands

Endocrine glands lack ducts and release hormones into fenestrated capillaries. They are organized as cords and clusters (adrenal cortex, anterior pituitary, parathyroid) or follicles (thyroid—the only endocrine gland that stores its product extracellularly as colloid). Endocrine cells may also be scattered individually within other epithelia as the diffuse neuroendocrine system (DNES/APUD cells): enteroendocrine cells of the gut, pulmonary neuroendocrine cells, and C cells of the thyroid.

Myoepithelial Cells

Myoepithelial cells are contractile epithelial cells located between the secretory/ductal epithelium and the basement membrane of many exocrine glands (salivary glands, mammary gland, sweat glands, lacrimal gland). They have features of both epithelial (cytokeratin+) and smooth muscle (SMA+, calponin+) cells. Their contraction assists in expelling secretory products into ducts. Diagnostically, the presence of myoepithelial cells on IHC (p63+, SMA+, calponin+) distinguishes benign breast lesions and carcinoma in situ from invasive carcinoma, where the myoepithelial layer is lost.

Salivary Gland Summary

07 Connective Tissue Proper

Connective tissue is the most abundant and widely distributed tissue type. Unlike epithelium, its cells are separated by abundant extracellular matrix (ECM) consisting of ground substance and fibers. All connective tissue derives from embryonic mesenchyme (mesoderm), except in the head region where neural crest contributes.

Connective Tissue Cells

Connective Tissue Fibers

Collagen Types (High-Yield)

Types of Connective Tissue Proper

Types of Connective Tissue Proper

Ground Substance

The amorphous, gel-like component of ECM composed of glycosaminoglycans (GAGs), proteoglycans, and glycoproteins. GAGs are long, unbranched polysaccharide chains (hyaluronic acid, chondroitin sulfate, heparan sulfate, keratan sulfate, dermatan sulfate) that attract water through their negative charges, creating a hydrated gel that resists compression. Proteoglycans consist of a core protein with covalently attached GAG side chains; aggrecan (the major proteoglycan of cartilage) contains ~100 chondroitin sulfate and keratan sulfate chains. Adhesive glycoproteins (fibronectin, laminin) mediate cell-matrix interactions through integrin receptors.

08 Cartilage

Cartilage is an avascular, aneural connective tissue with a firm but flexible ECM. Chondrocytes reside in lacunae and receive nutrition by diffusion through the matrix. Cartilage grows by interstitial growth (mitosis of existing chondrocytes) and appositional growth (new cartilage deposited from perichondrium).

Types of Cartilage

The perichondrium surrounds hyaline and elastic cartilage (but not fibrocartilage or articular cartilage). Its outer fibrous layer contains fibroblasts; the inner chondrogenic layer contains chondroblasts responsible for appositional growth.

Cartilage Matrix Composition

Hyaline cartilage matrix contains type II collagen (providing tensile strength) and aggrecan (a large proteoglycan with chondroitin sulfate and keratan sulfate GAG chains that forms massive aggregates with hyaluronic acid). These aggrecan aggregates trap water, giving cartilage its resilience to compression—a property essential for weight-bearing at articular surfaces. The territorial matrix (pericellular, around lacunae) is rich in proteoglycans and stains darker with basic dyes than the interterritorial matrix between cell clusters. Cartilage matrix is PAS+ and stains with Alcian blue due to its high GAG content.

09 Bone & Ossification

Bone is a mineralized connective tissue providing structural support, mineral homeostasis (calcium/phosphate reservoir), and hematopoiesis. Its matrix is ~35% organic (type I collagen + osteocalcin, osteopontin) and ~65% inorganic (hydroxyapatite crystals, Ca₁₀(PO₄)₆(OH)₂).

Bone Cells

Compact (Cortical) Bone

The osteon (Haversian system) is the structural unit: concentric lamellae of mineralized matrix surrounding a central Haversian canal containing blood vessels and nerves. Osteocytes in lacunae communicate through canaliculi. Volkmann canals connect adjacent Haversian canals and the periosteal/endosteal surfaces. Interstitial lamellae are remnants of remodeled osteons.

Types of Ossification

In endochondral ossification, the epiphyseal growth plate is organized into zones (from epiphysis to diaphysis): reserve (resting chondrocytes) → proliferative (mitotic columns) → hypertrophic (enlarged chondrocytes, type X collagen, VEGF secretion) → calcification (matrix mineralizes, chondrocytes undergo apoptosis) → ossification (osteoblasts deposit bone on calcified cartilage scaffolding).

Bone Remodeling & Metabolic Bone Disease

Bone is continuously remodeled by coordinated osteoclast resorption and osteoblast formation. The RANK/RANKL/OPG axis is the master regulator: osteoblasts express RANKL, which binds RANK on osteoclast precursors to promote differentiation and activation. Osteoprotegerin (OPG) is a decoy receptor that inhibits RANKL. PTH increases RANKL expression (stimulates resorption); estrogen increases OPG (inhibits resorption).

10 Blood & Hematopoiesis

Blood is a specialized liquid connective tissue with a fluid ECM called plasma. Formed elements include erythrocytes (RBCs), leukocytes (WBCs), and thrombocytes (platelets). Blood smears stained with Wright-Giemsa (Romanowsky-type) stain are the standard for peripheral blood evaluation.

Formed Elements

Hematopoiesis

All blood cells derive from hematopoietic stem cells (HSCs) in the bone marrow. HSCs give rise to common myeloid progenitors (erythrocytes, granulocytes, monocytes, megakaryocytes) and common lymphoid progenitors (B cells, T cells, NK cells). Sites of hematopoiesis shift during development: yolk sac (weeks 3–8) → liver/spleen (weeks 6–30) → bone marrow (week 18 onward, exclusive site after birth).

11 Skeletal Muscle

Skeletal muscle constitutes ~40% of body mass and is responsible for voluntary movement. Fibers are multinucleated syncytia formed by myoblast fusion during development, with peripheral nuclei pushed against the sarcolemma. Fibers are arranged in fascicles, each surrounded by connective tissue sheaths.

Connective Tissue Organization

Sarcomere Ultrastructure

The sarcomere (Z-line to Z-line) is the functional contractile unit. Key bands and zones:

Titin (the largest known protein) spans from Z-disc to M-line, providing passive elasticity and centering thick filaments. Nebulin runs along thin filaments, regulating their length. The triad consists of one T-tubule flanked by two terminal cisternae of sarcoplasmic reticulum, coupling electrical excitation to calcium release.

Fiber Types

Neuromuscular Junction

The motor end plate is the specialized synapse between a motor neuron axon terminal and the skeletal muscle fiber. The presynaptic terminal contains acetylcholine (ACh) vesicles. The synaptic cleft contains acetylcholinesterase (AChE) anchored in the basal lamina. The postsynaptic membrane has junctional folds that concentrate nicotinic ACh receptors (nAChRs) at their crests. Depolarization spreads via T-tubules to trigger calcium release from the sarcoplasmic reticulum.

Satellite Cells

Satellite cells are resident muscle stem cells located between the sarcolemma and the basal lamina of skeletal muscle fibers. They are normally quiescent but can be activated by injury to proliferate, differentiate into myoblasts, and fuse with damaged fibers or form new fibers. This is the basis of skeletal muscle regeneration. However, regenerative capacity is limited, and extensive damage leads to fibrosis and fat replacement rather than full restoration. Satellite cells are identified by PAX7 expression.

12 Cardiac Muscle

Cardiac muscle shares the striated pattern of skeletal muscle but has critical structural and functional differences. Cardiomyocytes are mononucleated (occasionally binucleated) with centrally located nuclei, branching fibers, and intercalated discs at cell junctions.

Intercalated Discs

These specialized junctions between cardiomyocytes contain three components:

The sarcoplasmic reticulum in cardiac muscle is less extensive than in skeletal muscle. T-tubules are wider and located at Z-lines (not A-I junctions as in skeletal muscle), forming dyads (one T-tubule + one SR cisterna) rather than triads. Approximately 20% of calcium for contraction enters from extracellular space through L-type Ca²⁺ channels (calcium-induced calcium release, CICR), making cardiac contraction dependent on extracellular calcium concentration.

Atrial cardiomyocytes contain atrial natriuretic peptide (ANP) granules visible by electron microscopy. Purkinje fibers are modified cardiomyocytes of the conduction system: they are larger, paler (glycogen-rich), have fewer myofibrils, and conduct impulses rapidly through the ventricular myocardium.

Cardiac Histopathology Timeline

13 Smooth Muscle

Smooth muscle is the involuntary, non-striated muscle of visceral organs, blood vessels, and the iris. Cells are fusiform (spindle-shaped) with a single central, cigar-shaped nucleus that may appear corkscrew-shaped in contracted cells on longitudinal section.

Key Features

Smooth muscle lacks sarcomeres, troponin, and T-tubules. Contraction is regulated by calmodulin/MLCK (myosin light chain kinase) rather than troponin. Calcium binds calmodulin → activates MLCK → phosphorylates myosin light chain → cross-bridge cycling. Relaxation requires myosin light chain phosphatase (MLCP). Dense bodies (analogous to Z-discs) anchor actin filaments to the cytoskeleton and cell membrane. Caveolae serve as rudimentary T-tubule equivalents.

Comparison of Muscle Types

14 Neurons & Synapses

Neurons are electrically excitable cells that transmit information through electrical impulses and chemical synapses. They are post-mitotic in the adult (with limited exceptions in the hippocampus and olfactory bulb). Each neuron has a cell body (soma/perikaryon), dendrites (receive input), and an axon (conducts output).

Neuronal Classification

Cellular Features

The neuronal soma contains a large, pale, euchromatic nucleus with a prominent nucleolus. Nissl substance (rough ER + polyribosomes) appears as basophilic clumps in the soma and dendrites but is absent from the axon hillock and axon. Neurofilaments (intermediate filaments specific to neurons) provide structural support and are stained by silver impregnation. Lipofuscin pigment accumulates with age as "wear-and-tear" pigment from lysosomal activity.

Synapses

Chemical synapses contain a presynaptic terminal (bouton) with neurotransmitter-filled synaptic vesicles, a synaptic cleft (~20 nm), and a postsynaptic membrane with receptor proteins. Axodendritic synapses are most common; axosomatic and axoaxonic types also occur. Electrical synapses (gap junctions) allow direct ion flow between neurons and are found in cardiac and smooth muscle for synchronization.

Axonal Transport

Axonal transport is essential for moving organelles, vesicles, and proteins between the soma and axon terminals. Anterograde transport (soma → terminal) uses kinesin motors on microtubules; fast anterograde (~400 mm/day) carries membrane-bound organelles and vesicles, while slow anterograde (~1–5 mm/day) carries cytoskeletal components and soluble enzymes. Retrograde transport (terminal → soma) uses dynein motors (~200 mm/day); carries endosomes, growth factors (e.g., NGF), and also transports viruses (rabies, herpes) and toxins (tetanus) to the cell body.

Neurotransmitter Types & Vesicle Morphology

15 Glial Cells & Myelination

Glial cells (neuroglia) outnumber neurons and provide structural support, metabolic maintenance, and insulation. Unlike neurons, most glial cells retain mitotic capacity, making them the most common source of primary CNS tumors.

CNS Glial Cells

PNS Glial Cells

Myelination

Myelin is a lipid-rich membrane that wraps around axons, increasing conduction velocity through saltatory conduction (action potentials jump between nodes of Ranvier). In the CNS, each oligodendrocyte myelinates segments of up to 30–60 axons. In the PNS, each Schwann cell myelinates a single internode (~1 mm) of one axon. Unmyelinated axons in the PNS are enclosed in Schwann cell cytoplasm (Remak bundles) without wrapping.

Blood-Brain Barrier

The blood-brain barrier (BBB) is formed by three components: (1) continuous capillary endothelium with tight junctions (claudin-5, occludin), (2) pericytes embedded in the capillary basement membrane, and (3) astrocyte foot processes (perivascular end-feet) covering >99% of the capillary surface. The BBB restricts paracellular diffusion and requires specific transporters for glucose (GLUT1), amino acids, and other essential molecules. Circumventricular organs (area postrema, median eminence, posterior pituitary, pineal gland) lack a BBB, featuring fenestrated capillaries that allow direct hormonal sensing and secretion.

CNS Reactions to Injury

16 Peripheral Nerve & Ganglia

Peripheral nerves have a connective tissue organization analogous to skeletal muscle:

Ganglia

Sensory (dorsal root / cranial) ganglia: contain pseudounipolar neurons with large, round, pale cell bodies surrounded by satellite cells. No synapses occur here. Autonomic ganglia: contain multipolar neurons; synapses occur between pre- and postganglionic neurons. Sympathetic chain ganglia and parasympathetic terminal ganglia (e.g., myenteric plexus, submucosal plexus) are clinically important.

17 Skin & Appendages

The skin is the largest organ, composed of epidermis (keratinized stratified squamous epithelium) and dermis (connective tissue), overlying the hypodermis (subcutaneous fat, not technically part of the skin).

Epidermal Layers (Thick Skin, Deep to Superficial)

Epidermal Cell Types

Skin Appendages

Hair follicles: dermal papilla at base provides inductive signals; matrix cells proliferate and keratinize. Sebaceous glands: holocrine secretion of sebum; associated with hair follicles. Eccrine sweat glands: merocrine secretion; coiled gland in dermis with duct to skin surface; thermoregulation; most of body. Apocrine sweat glands: axilla, groin; open into hair follicle; odorless secretion metabolized by bacteria to produce odor.

Dermis

The dermis has two layers. The papillary dermis is superficial loose connective tissue that interdigitates with the epidermis via dermal papillae, containing capillary loops and sensory receptors (Meissner corpuscles for fine touch). The reticular dermis is deep dense irregular connective tissue (type I collagen and elastic fibers) that provides tensile strength. Langer lines (cleavage lines) reflect the predominant collagen fiber orientation; surgical incisions along Langer lines produce less scarring.

Sensory Receptors in Skin

18 Gastrointestinal Tract

The GI tract follows a consistent four-layer plan from esophagus to anal canal: mucosa (epithelium + lamina propria + muscularis mucosae), submucosa, muscularis externa (inner circular + outer longitudinal smooth muscle), and serosa/adventitia.

Regional Epithelial Variation

Gastric Gland Cells

Small Intestine Specializations

Duodenum: Brunner glands in submucosa (alkaline mucus, bicarbonate). Jejunum: tallest plicae circulares, longest villi. Ileum: Peyer patches (aggregated lymphoid follicles) in submucosa; M cells overlie follicles for antigen sampling. Paneth cells at crypt bases secrete defensins and lysozyme (innate immunity); they contain large eosinophilic apical granules.

Appendix

The appendix shares the four-layer GI wall plan but is notable for abundant lymphoid tissue in the lamina propria and submucosa, often with prominent germinal centers. The lumen is small and irregular. Obliteration of the lumen by lymphoid tissue or a fecalith predisposes to acute appendicitis. Histologically, transmural neutrophilic infiltration and necrosis are the hallmarks of acute appendicitis. Carcinoid tumors are the most common appendiceal neoplasm.

Anal Canal Transition

The dentate (pectinate) line marks the transition from simple columnar epithelium (above, endoderm-derived) to stratified squamous epithelium (below, ectoderm-derived). Above the line: internal hemorrhoidal plexus (portal drainage), visceral innervation (insensitive to pain). Below the line: external hemorrhoidal plexus (caval drainage), somatic innervation (sensitive to pain). This is a high-yield anatomic-histologic correlation.

GI Endocrine Cells

19 Liver, Gallbladder & Pancreas

Liver

The liver is organized into hepatic lobules: hexagonal units centered on a central vein with portal triads (hepatic artery, portal vein, bile duct) at the periphery. Hepatocytes are arranged in anastomosing plates separated by sinusoids (fenestrated capillaries lined by endothelial cells lacking a continuous basement membrane). The space of Disse lies between hepatocytes and sinusoidal endothelium, containing hepatic stellate cells (Ito cells) that store vitamin A and, when activated, produce collagen (cirrhosis).

The portal lobule (centered on portal triad, draining bile) and hepatic acinus (of Rappaport; functional unit based on blood flow zones 1–3) are alternative organizational models. Zone 1 (periportal) receives blood first (most oxygenated); zone 3 (pericentral) is most susceptible to ischemic injury and is the site of CYP450 drug metabolism.

Hepatic Zonation & Pathology

Gallbladder

Lined by simple columnar epithelium with prominent mucosal folds (rugae). It concentrates bile by absorbing water and ions. Key features: no submucosa, no muscularis mucosae; the wall has mucosa, muscularis (smooth muscle), and serosa/adventitia. Rokitansky-Aschoff sinuses are mucosal herniations into the muscular wall, prominent in chronic cholecystitis.

Pancreas

The exocrine pancreas (~98% of mass) consists of serous acini with centroacinar cells (unique feature; pale cells within the acinar lumen that begin the duct system and secrete bicarbonate). The islets of Langerhans (endocrine, ~2%) are pale clusters of cells with fenestrated capillaries: alpha cells (glucagon, peripheral), beta cells (insulin, central, ~70%), delta cells (somatostatin), and PP cells (pancreatic polypeptide).

Bile Canaliculi & Bile Flow

Bile is secreted into bile canaliculi—tiny channels formed by grooves in adjacent hepatocyte membranes sealed by tight junctions. Bile flows in the opposite direction to blood: from zone 3 (pericentral) toward zone 1 (periportal), draining into canals of Hering (lined partly by hepatocytes, partly by cholangiocytes), then interlobular bile ducts in the portal triads, and ultimately the common bile duct. Cholangiocytes (bile duct epithelial cells) modify bile composition by secreting bicarbonate and absorbing water. They are the target of autoimmune destruction in primary biliary cholangitis (PBC; anti-mitochondrial antibodies) and primary sclerosing cholangitis (PSC; concentric "onion-skin" periductal fibrosis).

Kupffer Cells & Sinusoidal Endothelium

Kupffer cells are liver-resident macrophages within sinusoidal lumens that filter portal blood, removing bacteria, endotoxin, and senescent red blood cells. They are CD68+ and PAS-diastase+. The sinusoidal endothelium is fenestrated without a conventional basement membrane, allowing direct plasma contact with hepatocytes through the space of Disse. This unique configuration facilitates the liver's metabolic, synthetic, and detoxification functions.

20 Kidney & Urinary Tract

Each kidney contains ~1 million nephrons, the functional units responsible for filtration, reabsorption, secretion, and concentration. The nephron comprises the renal corpuscle (glomerulus + Bowman capsule) and a tubular system.

Nephron Segments

Juxtaglomerular Apparatus

The JGA comprises juxtaglomerular (JG) cells (modified smooth muscle in afferent arteriole; secrete renin), macula densa (specialized cells in thick ascending limb/early DCT; sense NaCl concentration), and extraglomerular mesangial cells (lacis cells). This apparatus regulates glomerular filtration rate and blood pressure via the renin-angiotensin-aldosterone system.

Renal Interstitium & Vasculature

The renal interstitium in the medulla contains interstitial cells that produce prostaglandins and erythropoietin. The vasa recta are thin-walled vessels that descend into the medulla alongside the loops of Henle, functioning as countercurrent exchangers to maintain the medullary osmotic gradient. The peritubular capillaries in the cortex arise from efferent arterioles and surround the tubules, enabling tubular reabsorption and secretion.

Ureter & Bladder

The ureter is lined by transitional epithelium (urothelium) and has two smooth muscle layers (inner longitudinal, outer circular) that reverse in the distal third to three layers (inner longitudinal, middle circular, outer longitudinal). The bladder has urothelium with umbrella cells, a thick lamina propria, and three smooth muscle layers (detrusor muscle) arranged in an interlacing pattern. Staging of bladder cancer depends on depth of invasion through these layers: CIS (urothelium only) → T1 (lamina propria) → T2 (muscularis propria/detrusor) → T3 (perivesical fat) → T4 (adjacent organs).

21 Respiratory System

The respiratory system transitions from the conducting zone (nasal cavity to terminal bronchioles) to the respiratory zone (respiratory bronchioles, alveolar ducts, alveolar sacs, alveoli) where gas exchange occurs.

Airway Epithelial Changes

Alveolar Cell Types

Type I pneumocytes: extremely thin squamous cells covering ~95% of alveolar surface; form the gas exchange barrier. Type II pneumocytes: cuboidal cells with lamellar bodies that produce surfactant (dipalmitoylphosphatidylcholine, DPPC); serve as progenitor cells for both type I and type II cells. Alveolar macrophages (dust cells): phagocytose inhaled particles and pathogens.

The blood-air barrier consists of (thin to thick): surfactant → type I pneumocyte → fused basal laminae → capillary endothelium. Total thickness: ~0.5 μm. This is the thinnest diffusion barrier in the body.

Club (Clara) Cells

Club cells (formerly Clara cells) are non-ciliated secretory cells found in terminal and respiratory bronchioles. They secrete club cell secretory protein (CC16), a component of the bronchiolar surfactant, and also function as progenitor cells for bronchiolar epithelial regeneration. Club cells contain abundant smooth ER and participate in xenobiotic metabolism (cytochrome P450 enzymes). They have a dome-shaped apical surface that protrudes into the airway lumen.

Pulmonary Vasculature

The lung has a dual blood supply: pulmonary arteries (deoxygenated blood from the right ventricle; travel alongside bronchi in bronchovascular bundles) and bronchial arteries (oxygenated blood from aorta; supply airway walls, visceral pleura). Pulmonary arteries are thin-walled elastic arteries that become muscular arteries at the level of bronchioles. Chronic hypoxia causes pulmonary arterial hypertension through medial hypertrophy and intimal fibrosis of pulmonary arterioles—histologically visible as thickened vascular walls and, in severe cases, plexiform lesions.

22 Lymphoid Organs

Lymphoid organs are classified as primary (where lymphocytes mature: bone marrow for B cells, thymus for T cells) or secondary (where immune responses are initiated: lymph nodes, spleen, MALT).

Lymph Node

Bean-shaped; surrounded by a capsule. Cortex: lymphoid follicles—primary (inactive, dense, small lymphocytes) and secondary (active, with germinal centers containing centroblasts, centrocytes, follicular dendritic cells, and tingible body macrophages). Paracortex: T-cell zone; high endothelial venules (HEVs) for lymphocyte entry. Medulla: medullary cords (plasma cells, macrophages) and medullary sinuses.

Spleen

The largest lymphoid organ. White pulp: periarterial lymphoid sheath (PALS, T cells around central arterioles) + lymphoid follicles (B cells). Marginal zone: between white and red pulp; APCs, marginal zone B cells. Red pulp: splenic cords (of Billroth) and sinusoids; functions as a blood filter, removing old/damaged RBCs (culling) and inclusions (pitting, e.g., Howell-Jolly bodies).

Thymus

Bilobed organ with lobules containing darker cortex (densely packed immature T cells/thymocytes, cortical epithelial cells, macrophages) and paler medulla (mature T cells, medullary epithelial cells, Hassall corpuscles—whorled keratinized epithelial cell remnants unique to thymus). The blood-thymus barrier in the cortex prevents premature antigen exposure. The thymus involutes with age (replaced by adipose tissue).

Mucosa-Associated Lymphoid Tissue (MALT)

MALT includes unencapsulated lymphoid aggregates in mucosal sites: Peyer patches (ileum), tonsils (palatine, pharyngeal/adenoid, lingual), appendix, and bronchus-associated lymphoid tissue (BALT). These sites sample antigens from mucosal surfaces via M cells (microfold cells) in the follicle-associated epithelium. M cells lack microvilli and a glycocalyx, facilitating antigen uptake by transcytosis to underlying dendritic cells and lymphocytes. MALT is the site of origin for MALT lymphoma (extranodal marginal zone B-cell lymphoma), which is associated with chronic H. pylori infection in the stomach.

23 Endocrine Glands

Pituitary Gland (Hypophysis)

The anterior pituitary (adenohypophysis) is organized as cords and clusters of cells. Three histologic types based on H&E staining: acidophils (GH, prolactin), basophils (FSH, LH, TSH, ACTH), and chromophobes (degranulated or inactive cells). The posterior pituitary (neurohypophysis) contains unmyelinated axons from hypothalamic neurons (supraoptic → ADH; paraventricular → oxytocin), Herring bodies (axonal swellings containing neurosecretory granules), and pituicytes (modified glial cells).

Thyroid Gland

The thyroid is the only endocrine gland that stores its product extracellularly. Follicles lined by simple cuboidal epithelium surround colloid (thyroglobulin). Active follicles have tall columnar cells with scalloped colloid; inactive follicles are flat with dense colloid. Parafollicular (C) cells are found between follicles or within the follicular epithelium; they secrete calcitonin and are the cells of origin for medullary thyroid carcinoma.

Adrenal Gland

The adrenal cortex (mesoderm origin) has three zones: zona glomerulosa (mineralocorticoids, aldosterone), zona fasciculata (glucocorticoids, cortisol; "spongiocytes" with lipid droplets), and zona reticularis (androgens, DHEA). Mnemonic: GFR = Salt, Sugar, Sex. The adrenal medulla (neural crest origin) contains chromaffin cells that secrete epinephrine and norepinephrine; stains brown with chromium salts (chromaffin reaction).

Parathyroid Glands

Composed of chief (principal) cells (produce PTH; small cells with central round nuclei) and oxyphil cells (larger, eosinophilic, abundant mitochondria; function uncertain; increase with age). Adipose tissue increases with age, comprising up to 50–70% of the gland in elderly individuals.

Pineal Gland

The pineal gland contains pinealocytes (produce melatonin from serotonin; regulate circadian rhythm) and glial cells (astrocyte-like interstitial cells). Corpora arenacea ("brain sand") are calcified concretions that increase with age and are visible on skull X-rays; they serve as a useful midline marker in radiology. Pineal tumors can cause Parinaud syndrome (paralysis of upward gaze) by compressing the superior colliculus.

Endocrine Pancreas — Additional Detail

24 Reproductive Organs

Male Reproductive System

The seminiferous tubule is lined by Sertoli cells (supporting, forming the blood-testis barrier with tight junctions) and spermatogenic cells in various stages: spermatogonia (basal) → primary spermatocytes → secondary spermatocytes → spermatids → spermatozoa (luminal). Leydig (interstitial) cells lie between tubules; they are large, eosinophilic cells with lipid droplets and Reinke crystals; they produce testosterone under LH stimulation.

The epididymis is lined by pseudostratified columnar epithelium with stereocilia (for sperm maturation and concentration). The vas deferens has thick muscular walls (inner longitudinal, middle circular, outer longitudinal) for peristaltic propulsion of sperm. The prostate gland is a tubuloalveolar gland surrounding the prostatic urethra; it produces prostatic fluid rich in PSA, citric acid, and zinc. Corpora amylacea (prostatic concretions) increase with age.

Female Reproductive System

The ovary has a cortex containing follicles at various stages of development (primordial → primary → secondary → Graafian/antral → corpus luteum → corpus albicans) and a vascular medulla. The uterus has three layers: endometrium (functional + basal layers), myometrium (thick smooth muscle), and serosa/adventitia. The endometrium undergoes cyclic changes: proliferative phase (estrogen; straight glands, mitotic activity), secretory phase (progesterone; coiled glands, glycogen-rich secretions, spiral arteries), and menstrual phase (spiral artery constriction, ischemic necrosis, shedding).

The cervix has an ectocervix lined by stratified squamous epithelium and an endocervix lined by simple columnar mucus-secreting epithelium. The squamocolumnar junction (transformation zone) is the site of cervical dysplasia and squamous cell carcinoma; it is the target of the Pap smear.

Placenta

The mature placenta is organized into chorionic villi, which are the functional exchange units. Each villus contains a core of fetal mesenchyme with fetal blood vessels, covered by an outer layer of syncytiotrophoblast (continuous multinucleated layer in direct contact with maternal blood; produces hCG, hPL, progesterone) and an inner layer of cytotrophoblast (discrete cells; mitotically active; serve as stem cells for syncytiotrophoblast). In early pregnancy, both layers are prominent; by term, cytotrophoblast cells become sparse and the syncytiotrophoblast thins, forming the vasculosyncytial membrane for efficient gas exchange.

Breast

The mammary gland is a modified apocrine sweat gland. The functional unit is the terminal duct lobular unit (TDLU), consisting of extralobular and intralobular ducts opening into acini (lobules). During lactation, acinar cells enlarge with lipid droplets (apocrine secretion of fat) and protein-rich secretions (merocrine). The resting breast is primarily interlobular connective tissue and fat, with small inactive TDLUs. Most breast carcinomas (both ductal and lobular) originate in the TDLU. Myoepithelial cells surround ducts and acini; their loss on IHC (p63, SMA, calponin negative) confirms invasive carcinoma.

25 Eye & Ear

Eye

The eye has three tunics: fibrous (sclera + cornea), vascular (uvea) (choroid, ciliary body, iris), and neural (retina). The cornea is avascular and has 5 layers: corneal epithelium (stratified squamous, non-keratinized), Bowman layer (acellular collagen), stroma (90% of thickness; type I collagen lamellae with keratocytes), Descemet membrane (thick basement membrane of endothelium), and corneal endothelium (simple squamous; maintains corneal clarity via Na⁺/K⁺-ATPase pump).

The retina has 10 layers. Key cell types: rods (dim-light vision; rhodopsin) and cones (color/acuity; iodopsin) are photoreceptors whose outer segments are phagocytosed daily by the retinal pigment epithelium (RPE). The fovea centralis has only cones with a 1:1 cone-to-ganglion-cell ratio for maximum acuity. The optic disc (blind spot) is where ganglion cell axons exit; no photoreceptors.

Ear

The organ of Corti in the cochlea is the receptor organ for hearing. It sits on the basilar membrane and contains inner hair cells (primary sensory transducers; ~3,500) and outer hair cells (amplifiers; ~12,000). Hair cell stereocilia are deflected by endolymph movement, opening mechanotransduction channels. Tip links connecting stereocilia are critical for channel gating.

The vestibular system includes the maculae of utricle (horizontal acceleration) and saccule (vertical acceleration), which contain hair cells with stereocilia embedded in a gelatinous otolithic membrane with calcium carbonate crystals (otoconia). The cristae ampullares in semicircular canals detect angular acceleration; hair cells project into a gelatinous cupula.

Cochlear Histology Details

The cochlea is divided into three chambers: scala vestibuli (perilymph), scala media / cochlear duct (endolymph, high K⁺), and scala tympani (perilymph). The stria vascularis on the lateral wall of the cochlear duct is the only vascularized epithelium in the body; it produces endolymph and maintains the endocochlear potential (+80 mV) essential for hair cell transduction. The Reissner membrane separates scala vestibuli from scala media. The basilar membrane (narrow and stiff at base → wide and floppy at apex) supports the organ of Corti and provides the tonotopic gradient (base = high frequency, apex = low frequency).

Retinal Layers (High-Yield)

The 10 layers of the retina from outermost (closest to choroid) to innermost (closest to vitreous): (1) Retinal pigment epithelium (RPE), (2) Photoreceptor layer (rods and cones outer/inner segments), (3) External limiting membrane, (4) Outer nuclear layer (photoreceptor nuclei), (5) Outer plexiform layer (synapses), (6) Inner nuclear layer (bipolar, amacrine, horizontal, Muller cell nuclei), (7) Inner plexiform layer (synapses), (8) Ganglion cell layer, (9) Nerve fiber layer (ganglion cell axons → optic nerve), (10) Internal limiting membrane (Muller cell end-feet). Light must pass through all inner layers to reach photoreceptors. The RPE phagocytoses shed photoreceptor outer segment discs daily.

Lens

The lens is a transparent, biconvex, avascular structure suspended by zonular fibers (zonules of Zinn) from the ciliary body. It is enclosed in a thick elastic lens capsule (the thickest basement membrane in the body, type IV collagen). The anterior surface is lined by a single layer of lens epithelial cells that proliferate at the equator and differentiate into lens fibers—elongated, anucleate cells packed with crystallin proteins. Cataracts result from opacification of crystallins due to age, UV exposure, diabetes (sorbitol accumulation via aldose reductase), or corticosteroids.

Blood Vessel Histology

Blood vessels have three tunics: tunica intima (endothelium + subendothelial CT + internal elastic lamina), tunica media (smooth muscle +/− elastic lamellae), and tunica adventitia (CT with vasa vasorum and nervi vasorum). Large arteries (aorta, pulmonary) are elastic arteries with abundant elastic lamellae in the media. Medium arteries (coronary, renal, femoral) are muscular arteries with prominent smooth muscle media and well-defined internal and external elastic laminae. Arterioles (1–2 layers of smooth muscle) are the primary regulators of vascular resistance and blood pressure.

26 Pathologic Histology

Understanding normal tissue architecture enables recognition of pathologic patterns. Key categories include cellular adaptations, cell injury/death, inflammation, neoplasia, and deposition diseases.

Cellular Adaptations

Cell Death

Necrosis (pathologic): coagulative (most organs; ischemia), liquefactive (brain, abscess), caseous (TB), fat necrosis (pancreas), fibrinoid (vessels). Nuclear changes: pyknosis (condensation) → karyorrhexis (fragmentation) → karyolysis (dissolution). Apoptosis (programmed): cell shrinkage, chromatin condensation, membrane blebbing, apoptotic bodies; no inflammatory reaction; mediated by caspases (intrinsic/mitochondrial and extrinsic/death receptor pathways).

Inflammation Patterns

Neoplastic Patterns

Fibrosis & Wound Healing

Tissue repair proceeds through overlapping phases: hemostasis (platelet plug, fibrin clot) → inflammation (neutrophils, then macrophages) → proliferation (granulation tissue: new capillaries + fibroblasts + loose ECM) → remodeling (collagen maturation, type III replaced by type I, wound contraction by myofibroblasts). Granulation tissue is the hallmark of healing: it appears as beefy red, granular tissue rich in thin-walled capillaries and plump fibroblasts.

Deposition Diseases

Dysplasia vs. Neoplasia

Dysplasia is a precancerous condition characterized by disordered cellular maturation with architectural and cytologic abnormalities (nuclear enlargement, hyperchromasia, increased mitoses, loss of polarity) but without invasion through the basement membrane. Dysplasia can be classified as low-grade or high-grade. Carcinoma in situ (CIS) represents full-thickness dysplasia (high-grade) that has not breached the basement membrane. Once tumor cells penetrate the basement membrane into underlying stroma, the lesion is classified as invasive carcinoma. This transition is the critical event that enables metastatic potential.

27 Clinical Correlates & Diagnostic Stains

Histologic findings directly guide clinical decision-making. This section summarizes high-yield diagnostic patterns and the stains used to identify them.

Diagnostic Stain Applications by Organ System

Immunofluorescence Patterns in Renal Disease

Electron Microscopy in Diagnosis

EM remains essential for specific diagnoses where light microscopy and IHC are insufficient:

CK7/CK20 Pattern for Tumor Site of Origin

Molecular Techniques in Histopathology

28 High-Yield Review & Reference Tables

Pathognomonic Histologic Findings

IHC Tumor Panels Quick Reference

Tissue Type Identification Quick Reference

Metaplasia Examples (High-Yield)

Organ Identification Quick Reference

Commonly Tested Histology Comparisons