Veterinary Histology - Cumulative Exam Prep (Under 1 Week)

Core Textbooks to Use

• Junqueira's Basic Histology (17th ed.) - the gold standard for histology exams
• Histology: A Text and Atlas (Ross & Pawlina) - great for visual/microscopy questions

Core Topics - Ranked by Exam Priority

TIER 1 - Study First (Highest Yield)

• H&E staining: what stains acidophilic (pink) vs basophilic (blue/purple)
• Special stains: PAS (glycogen/mucus), Masson's trichrome (collagen), Weigert (elastic fibers), silver stains
• Light microscopy vs electron microscopy (TEM vs SEM) - what each reveals
• Fixation, embedding, sectioning artifacts

• Classification: simple vs stratified vs pseudostratified
• Types: squamous, cuboidal, columnar - where each is found
• Keratinized vs non-keratinized stratified squamous (skin vs oral mucosa vs esophagus vs rumen)
• Transitional epithelium (urothelium) - bladder
• Cell junctions: tight junctions, desmosomes, gap junctions, hemidesmosomes
• Microvilli (brush border), cilia, stereocilia - where found, function
• Glands: exocrine (serous vs mucous vs mixed), endocrine; secretion types (merocrine, apocrine, holocrine)

• Components: cells (fibroblasts, mast cells, macrophages, plasma cells, adipocytes) + fibers (collagen, reticular, elastic) + ground substance
• Types: loose (areolar) vs dense (regular vs irregular)
• Collagen types I-IV - where each is found (I = skin/tendon/bone, II = cartilage, III = reticular, IV = basement membranes)
• Mast cell granules: heparin, histamine

• 3 cartilage types: hyaline, fibrocartilage, elastic - locations + what distinguishes them
• Bone: compact vs cancellous, osteon/Haversian system, osteoblasts vs osteoclasts vs osteocytes
• Endochondral vs intramembranous ossification
• Growth plate zones (resting, proliferating, hypertrophic, calcifying)

• Leukocyte identification: neutrophil, eosinophil, basophil, monocyte, lymphocyte - lobes, granule colors
• Vet-specific: differences in species blood smears (cow, horse, dog, cat - especially feline Barr bodies, equine Rouleaux)
• Erythropoiesis and granulopoiesis stages
• Bone marrow: red vs yellow marrow

TIER 2 - Study Second (High Yield)

• Skeletal muscle: sarcomere structure (A, I, H, Z bands, M line), sliding filament theory
• Cardiac muscle: intercalated discs, gap junctions, single nucleus, branching
• Smooth muscle: no striations, dense bodies, single nucleus
• Vet note: muscle fiber types differ between species (especially in fast vs slow animals)

• Neurons: multipolar, bipolar, unipseudounipolar - classification + examples
• Glial cells: astrocytes, oligodendrocytes, Schwann cells, microglia - function
• Myelination: CNS (oligodendrocytes) vs PNS (Schwann cells)
• Nodes of Ranvier, saltatory conduction
• Peripheral nerve coverings: endoneurium, perineurium, epineurium

• Layers of blood vessels: tunica intima, media, adventitia
• Elastic (large) vs muscular (medium) vs capillary arteries
• Continuous vs fenestrated vs sinusoidal capillaries - where each occurs
• Cardiac histology: myocardium, Purkinje fibers (large, pale cells, glycogen-rich)

• 4 layers: mucosa (epithelium + lamina propria + muscularis mucosae), submucosa, muscularis externa (inner circular + outer longitudinal), serosa/adventitia
• Each organ's unique features:
  • Esophagus: stratified squamous, submucosal glands
  • Stomach: gastric pits, chief cells (pepsinogen), parietal cells (HCl), mucous neck cells
  • Small intestine: villi + crypts of Lieberkühn, Goblet cells, Paneth cells, enterocytes with microvilli
  • Large intestine: no villi, abundant goblet cells
• Vet-specific: ruminant forestomachs (rumen, reticulum, omasum) - stratified squamous, no gastric glands

• Diffuse lymphoid tissue vs lymphoid nodules (primary vs secondary/germinal centers)
• Thymus: cortex (dense lymphocytes) vs medulla (Hassall's corpuscles)
• Lymph node: cortex (B cells), paracortex (T cells), medulla (plasma cells + macrophages), sinuses
• Spleen: red pulp (sinusoids + cords of Billroth) vs white pulp (PALS + lymphoid nodules)

TIER 3 - Study Third (Moderate Yield)

• Liver: hepatocytes, portal triad (portal vein branch + hepatic artery branch + bile duct), central vein, liver lobule vs portal lobule vs liver acinus (Rappaport)
• Pancreas: exocrine (acini with zymogen granules) vs endocrine (islets of Langerhans - alpha/glucagon, beta/insulin, delta/somatostatin)
• Gallbladder: simple columnar epithelium, no muscularis mucosae, Rokitansky-Aschoff sinuses

• Conducting zone: pseudostratified ciliated columnar with goblet cells (trachea, bronchi), becomes simple columnar then simple cuboidal (bronchioles)
• Respiratory zone: respiratory bronchioles, alveolar ducts, alveoli
• Alveolar cells: Type I pneumocytes (gas exchange, flat) vs Type II (surfactant production, cuboidal)
• Vet-specific: horse lungs - no well-defined lobulation; cat - minimal smooth muscle in bronchioles

• Kidney: cortex vs medulla, nephron components (glomerulus, Bowman's capsule, PCT, loop of Henle, DCT, collecting duct)
• PCT vs DCT distinctions on slides: PCT has brush border + more eosinophilic cytoplasm
• Juxtaglomerular apparatus: macula densa + JG cells (renin)
• Transitional epithelium in ureter and bladder

• Pituitary: adenohypophysis (acidophils = GH/prolactin, basophils = FSH/LH/TSH/ACTH) vs neurohypophysis
• Thyroid: follicles with colloid (thyroglobulin), parafollicular C cells (calcitonin)
• Adrenal: cortex (zona glomerulosa/aldosterone, zona fasciculata/cortisol, zona reticularis/androgens) + medulla (chromaffin cells/epinephrine)
• Parathyroid: chief cells (PTH), oxyphil cells

• Epidermis layers: stratum basale, spinosum, granulosum, lucidum (thick skin), corneum
• Dermis: papillary vs reticular
• Adnexa: hair follicles, sebaceous glands, sweat glands (eccrine vs apocrine)
• Vet-specific: species differences - dog/cat vs horse vs ruminant skin; hoof/claw structure; wool follicles in sheep

• Testis: seminiferous tubules, Sertoli cells, Leydig cells, spermatogenesis stages
• Ovary: follicle stages (primordial, primary, secondary, Graafian/antral), corpus luteum, corpus albicans
• Uterus: endometrium (functionalis + basalis), myometrium; estrous cycle changes
• Vet-specific: placental types (diffuse/epitheliochorial in horse/pig, zonary in dog/cat, cotyledonary in ruminants, discoid in rodents)

6-Day Study Plan (Starting Today - Monday July 13)

Veterinary-Specific Things Examiners Love to Test

1. Ruminant forestomachs - stratified squamous epithelium throughout (rumen, reticulum, omasum); only the abomasum has gastric glands
2. Placental classification by maternal-fetal tissue contact layers - horse/pig (6 layers), dog/cat (3 layers - endotheliochorial), ruminants (cotyledons/caruncles), primates (hemochorial)
3. Species blood smear differences - feline neutrophils have rod-shaped nuclei; equine erythrocytes form Rouleaux; bovine lymphocytes have nuclear pockets (Barr bodies)
4. Skin adnexa - compound follicles in sheep/goat; hoof histology (laminar dermis)
5. Avian tissues (if your course covers birds) - nucleated red blood cells, bursa of Fabricius, gizzard epithelium
6. Sweat glands - most domestic animals use apocrine glands (in hair follicles), unlike humans who rely on eccrine

Exam Technique Tips

• Slide identification questions: Always note: epithelium type first, then lumen shape, then special features (villi, follicles, etc.)
• "What organ is this?" - Go layer by layer: mucosa type → presence of glands → muscle layer type → serosa vs adventitia
• Staining questions: Remember the mnemonic - "B" is Blue = Basophilic (nuclei, RER, ribosomes) / "A" is Acidophilic = pink/red (cytoplasm, collagen, mitochondria)
• Review actual histology slides/images - many exams have "identify this tissue" photo questions

TODAY'S TOPICS - Full Exam Notes

Monday: Histology Methods & Microscopy | Epithelial Tissue | Connective Tissue

TOPIC 1: HISTOLOGY - METHODS AND MICROSCOPY

Definition of Histology

Tissue Preparation - The Step-by-Step Process

Step 1: Fixation

• To terminate cell metabolism (stopping all enzymatic activity)
• To prevent autolysis (self-digestion by lysosomal enzymes)
• To kill pathogenic microorganisms
• To harden tissue by denaturing or cross-linking proteins

Step 2: Dehydration and Embedding

Step 3: Sectioning

Step 4: Staining

Hematoxylin and Eosin (H&E) Staining

• Cell nuclei (DNA and RNA are negatively charged due to phosphate groups)
• Rough endoplasmic reticulum (ribosomal RNA)
• Ribosomes
• Cartilage matrix (due to sulfated proteoglycans)

• Cytoplasm (most cytoplasmic proteins are positively charged at physiological pH)
• Collagen fibers
• Mitochondria
• Red blood cells (hemoglobin)
• Muscle cytoplasm

Special Staining Methods

Types of Microscopy

Light Microscopy (LM)

Electron Microscopy (EM)

• Transmission Electron Microscopy (TEM): Electrons pass THROUGH ultra-thin (50-100 nm) sections. Reveals internal ultrastructure of cells - organelles, membranes, filaments. Tissue stained with heavy metals (osmium tetroxide, uranyl acetate, lead citrate). Produces a 2D image.
• Scanning Electron Microscopy (SEM): Electrons bounce OFF the surface of a specimen coated with a thin metal layer. Produces a dramatic 3D image of surface topography (e.g., the surface of villi, cilia, red blood cells).

Autoradiography

Histochemistry and Cytochemistry

• Enzyme histochemistry: Uses substrates that produce a colored insoluble product at the site of enzyme activity (e.g., alkaline phosphatase, ATPase in muscle fibers)
• Immunohistochemistry (IHC): Antibodies labeled with peroxidase (enzyme-linked) or fluorescent dyes bind to specific target proteins. Allows identification of specific cell types, pathogens, and tumor markers. Extremely important in veterinary diagnostic pathology.
• In situ hybridization: Uses labeled complementary nucleic acid probes to detect specific RNA or DNA sequences within intact tissue sections.

TOPIC 2: EPITHELIAL TISSUE

Definition and General Characteristics

1. Cellularity - cells are closely packed with very little extracellular matrix between them
2. Polarity - epithelial cells have a distinct apical (free) surface, lateral surfaces (facing adjacent cells), and a basal surface (facing the basement membrane)
3. Supported by a basement membrane - all epithelia rest on a basement membrane (basal lamina) which anchors them to the underlying connective tissue
4. Avascular - epithelium has no blood vessels; it receives nutrients by diffusion from the underlying connective tissue (lamina propria)
5. High capacity for renewal - constantly replaced by mitosis from stem cells in the basal layer

Classification of Epithelium

• Simple = one cell layer thick (all cells touch the basement membrane)
• Stratified = two or more cell layers thick (only the basal layer touches the basement membrane)
• Pseudostratified = appears stratified but is actually simple (all cells contact the basement membrane, but not all reach the free surface; nuclei appear at different heights, creating the illusion of stratification)

• Squamous = cells are flattened; width is much greater than height (like floor tiles)
• Cuboidal = cells are approximately equal in width, height, and depth (like dice)
• Columnar = cells are taller than they are wide (like columns)

Types of Epithelium - Detailed Description

1. Simple Squamous Epithelium

• Where found: Lining of blood and lymphatic vessels (called endothelium); lining of body cavities - pleural, pericardial, peritoneal (called mesothelium); Bowman's capsule (parietal layer) in the kidney; alveoli of the lungs (Type I pneumocytes); loop of Henle (thin segment)
• Function: Passive diffusion and filtration; reduces friction in body cavities
• On a slide: Cells appear as very thin, flat profiles. The nucleus bulges slightly into the lumen. Only the nucleus is clearly visible in cross-section.

2. Simple Cuboidal Epithelium

• Where found: Kidney tubules (proximal and distal convoluted tubules); small gland ducts; thyroid follicles; surface of the ovary (germinal epithelium); lens of the eye
• Function: Secretion and absorption
• On a slide: Cells look like boxes; nucleus is round and sits in the center of the cell.

3. Simple Columnar Epithelium

• Simple columnar non-ciliated: Lines most of the gastrointestinal tract (small intestine, large intestine); has microvilli forming a brush border (striated border) on the apical surface to increase absorptive area. Goblet cells (mucus-secreting unicellular glands) are interspersed between columnar cells.
• Simple columnar ciliated: Lines the fallopian tubes (uterine tubes/oviducts), uterus, small bronchioles
• Where found in veterinary species: The abomasum of ruminants (true stomach) is lined by simple columnar epithelium with gastric glands - this distinguishes it from the forestomachs (rumen, reticulum, omasum) which have stratified squamous epithelium.

4. Pseudostratified Columnar Epithelium

• Where found: Lining the trachea and major bronchi (with cilia and goblet cells) - also called respiratory epithelium; epididymis (with very long stereocilia); male urethra
• In veterinary species: The nasal cavity and trachea of all domestic mammals are lined by pseudostratified ciliated columnar epithelium with goblet cells. This is important for the mucociliary escalator (defense against inhaled particles).
• Function: Mucus production and cilia-driven movement of mucus toward the pharynx

5. Stratified Squamous Epithelium

• Found in: Skin (epidermis) of all vertebrates; hard palate; hoof wall; claw; horns
• Layers from base to surface: stratum basale → stratum spinosum → stratum granulosum → stratum lucidum (in thick skin) → stratum corneum

• Found in: Oral mucosa (mouth), esophagus, vagina, cornea of the eye
• Veterinary note: The rumen, reticulum, and omasum of ruminants are lined by stratified squamous non-keratinized epithelium (partially keratinized in some areas) because these organs function as fermentation vats and require a physically tough but absorptive lining. No digestive glands are present. This is a classic exam question in veterinary histology.

6. Stratified Cuboidal and Stratified Columnar Epithelium

7. Transitional Epithelium (Urothelium)

• Relaxed state: 5-7 cell layers thick; surface cells (umbrella cells or facet cells) are large, dome-shaped, and may be binucleate
• Distended state: cells flatten, appears 2-3 layers thick; umbrella cells become wide and flat
• The umbrella cells have a specialized apical membrane containing rigid plaques of proteins called uroplakins that prevent urine from leaking into the tissue.

The Basement Membrane

1. Basal lamina (produced by epithelial cells): Contains Type IV collagen, laminin, fibronectin, and perlecan (heparan sulfate proteoglycan)
2. Reticular lamina (produced by fibroblasts in underlying connective tissue): Contains Type III collagen (reticular fibers)

• Physical support and anchorage of the epithelium
• Filter for molecules passing between epithelium and connective tissue
• Regulates cell differentiation, migration, and polarity during tissue repair and embryogenesis
• Acts as a barrier that carcinoma cells must breach to invade (important in cancer)

Cell Junctions

1. Tight Junctions (Zonula Occludens)

• Located at the very apex (top) of the lateral membrane, forming a belt around the entire circumference of the cell
• Formed by transmembrane proteins: claudins and occludins, which interdigitate with corresponding proteins on adjacent cells
• Function: Creates a selective seal that prevents paracellular passage of molecules and ions between cells. Maintains cell polarity by preventing lateral diffusion of apical membrane proteins to the basal membrane.
• Example: Tight junctions of intestinal epithelium control what passes from gut lumen to blood.

2. Adherens Junctions (Zonula Adherens)

• Located just below the tight junction
• Formed by E-cadherin (calcium-dependent adhesion molecule) on the extracellular side; linked to actin filaments inside the cell via catenins
• Function: Cell-to-cell adhesion; maintains tissue integrity. Also plays a role in cell signaling.

3. Desmosomes (Macula Adherens)

• Spot-like, disc-shaped junctions scattered along the lateral cell membranes
• Formed by cadherins (desmogleins and desmocollins); linked to intermediate filaments (cytokeratins) inside the cell via desmoplakin and plakophilin
• Function: Provides strong mechanical attachment between cells, especially important in tissues subjected to mechanical stress (skin, cardiac muscle)
• Clinical relevance: Pemphigus vulgaris (an autoimmune disease) involves autoantibodies against desmogleins, causing blisters due to loss of cell adhesion.

4. Gap Junctions (Nexus)

• Small channels formed by connexin proteins; six connexins form a connexon (hemichannel); two connexons align between adjacent cells to form a channel
• Function: Direct cell-to-cell communication. Allow passage of small molecules (< 1,000 Da), ions, and second messengers (cAMP, Ca²⁺) between cells.
• Important in: cardiac muscle (electrical coupling), smooth muscle (coordinated contraction), hepatocytes

5. Hemidesmosomes

• Resemble half a desmosome
• Located on the basal surface of epithelial cells
• Formed by integrins linked to intermediate filaments inside the cell
• Function: Attaches the cell to the underlying basement membrane (specifically to laminin in the basal lamina)

Apical Surface Specializations

1. Microvilli

• Finger-like projections of the apical plasma membrane, each containing a core of actin filaments cross-linked by villin and fimbrin
• Length: ~1-3 micrometers; diameter ~0.1 micrometers
• When packed densely and regularly, they form a brush border (also called a striated border in intestinal cells)
• Where found: Proximal tubule of kidney (very prominent brush border - distinguishes PCT from DCT on slides), small intestinal enterocytes, hepatocytes (bile canaliculi)
• Function: Increase the absorptive surface area up to 20-40 times

2. Stereocilia (Stereovilli)

• Unusually long microvilli (not true cilia - they do not move)
• Contain an actin core, like microvilli
• Where found: Epididymis (longest stereocilia in the body - function in sperm maturation and resorption of fluid); inner ear (sensory hair cells - stereocilia respond to sound vibrations)
• Function: Absorption in the epididymis; mechanosensation in the ear

3. Cilia

• Motile hair-like extensions of the apical membrane
• Each cilium contains an axoneme - a core of 9 doublets of microtubules + 2 central single microtubules (9+2 arrangement)
• The axoneme grows from a basal body (modified centriole) at the cell surface
• Movement is powered by dynein ATPase - dynein arms on the outer doublets generate the sliding force
• Where found: Pseudostratified respiratory epithelium (trachea, bronchi); oviducts/uterine tubes; efferent ductules of testis; ependymal cells lining brain ventricles
• Function: In the respiratory tract, cilia beat rhythmically to sweep the mucus blanket (with trapped particles and pathogens) toward the pharynx - this is the mucociliary escalator, a primary innate defense mechanism
• Primary (non-motile) cilia: Have a 9+0 arrangement (no central pair); found on almost every cell; function as sensory antennae (chemosensors, mechanosensors)

Glands

Exocrine Glands

• Tubular glands: Secretory units are tube-shaped (e.g., intestinal crypts of Lieberkühn)
• Acinar (alveolar) glands: Secretory units are round/flask-shaped (e.g., sebaceous glands, pancreatic acini)
• Tubuloalveolar (tubuloacinar) glands: Mix of both (e.g., submandibular salivary gland, mammary gland)

• Simple glands: Unbranched duct
• Compound glands: Branched ducts (e.g., parotid gland, pancreas, mammary gland)

• Serous glands: Produce a watery, protein-rich (enzymatic) secretion. Cells are pyramidal with a round basal nucleus and eosinophilic zymogen granules apically. Examples: parotid salivary gland, pancreatic acini.
• Mucous glands: Produce thick, glycoprotein-rich mucus. Cells are columnar with a flat basal nucleus and pale/clear cytoplasm (mucin is washed out in routine processing). Examples: sublingual salivary gland, goblet cells.
• Mixed glands: Both serous and mucous cells. Example: submandibular (submaxillary) salivary gland, which has mucous acini capped by serous demilunes.

• Merocrine (eccrine) secretion: Products are released by exocytosis; the cell membrane is not lost. This is the most common type. Example: pancreatic acinar cells, parotid gland.
• Apocrine secretion: The apical portion of the cell cytoplasm is pinched off along with the secretory product. Some membrane and cytoplasm is lost. Example: apocrine sweat glands (in most domestic animals - dogs, cats, horses); mammary glands (lipid component of milk is released this way).
• Holocrine secretion: The entire cell disintegrates to release its secretory contents. The cell dies with each secretory cycle. Example: sebaceous glands (oil glands associated with hair follicles).

Endocrine Glands

TOPIC 3: CONNECTIVE TISSUE

Definition and General Characteristics

• Structural support and mechanical strength (tendons, bone, cartilage)
• Binding and anchoring of organs and tissues
• Transport of nutrients, oxygen, and waste products (blood is a specialized CT)
• Defense against infection (immune cells reside in CT)
• Fat storage and energy metabolism (adipose tissue)
• Wound repair and tissue regeneration (fibroblasts)

Components of Connective Tissue

1. Cells

2. Protein Fibers

3. Ground Substance

Connective Tissue Cells

Fibroblasts (and Fibrocytes)

• The most numerous and characteristic cells of connective tissue
• Fibroblasts are the active, metabolically busy form: large, spindle-shaped cells with an oval, pale-staining nucleus with prominent nucleoli. The cytoplasm is abundant and weakly basophilic.
• Function: Synthesize and secrete all components of the ECM - collagen, elastin, reticular fibers, and ground substance (proteoglycans, glycosaminoglycans, and glycoproteins)
• Fibrocytes are the inactive form: smaller, with a darker, more elongated nucleus and scant cytoplasm.
• When tissue is injured, fibrocytes can revert to active fibroblasts to rebuild the ECM.
• Myofibroblasts are fibroblasts that have acquired smooth-muscle-like contractile properties (contain α-smooth muscle actin). They are critical in wound contraction during healing.

Macrophages (Histiocytes)

• Derived from monocytes in the blood, which enter connective tissue and differentiate into macrophages
• Large cells with an irregular (kidney-shaped or indented) nucleus and abundant, often foamy cytoplasm containing lysosomes
• Function: Phagocytosis of pathogens, dead cells, and debris (professional phagocytes). Antigen presentation to lymphocytes. Secrete cytokines (IL-1, TNF-α) to initiate inflammation.
• Fixed macrophages: reside permanently in specific tissues (Kupffer cells in liver, alveolar macrophages in lung, microglia in CNS, osteoclasts in bone)
• Free macrophages: migrate into inflamed tissue in response to infection

Mast Cells

• Derived from bone marrow precursors; large, oval cells filled with prominent metachromatic cytoplasmic granules
• The granules contain: heparin (anticoagulant), histamine (vasodilator, increases vascular permeability), tryptase, chymase, and chemotactic factors for eosinophils and neutrophils
• Distribution: Abundant in loose connective tissue, especially around blood vessels; present in large numbers in skin, respiratory tract mucosa, and gastrointestinal tract
• Function: Primary effector cells of immediate (Type I) hypersensitivity reactions (allergic responses, anaphylaxis). Upon binding of IgE antibody + antigen to their Fc receptors → degranulation → release of preformed mediators → vasodilation, bronchoconstriction, increased vascular permeability.
• Staining: Granules stain metachromatic purple-red with toluidine blue or crystal violet; positive with PAS stain.

Plasma Cells

• Terminal differentiation product of B lymphocytes after antigen stimulation
• Distinctive appearance: round cells with an eccentric nucleus showing a "clock-face" or "cartwheel" chromatin pattern (alternating clumps of heterochromatin and pale euchromatin). Cytoplasm is deeply basophilic (due to extensive RER for antibody synthesis), often with a pale "Golgi hof" (a pale area adjacent to the nucleus where the Golgi apparatus is located).
• Function: Synthesize and secrete large quantities of antibodies (immunoglobulins)
• Found in: loose connective tissue of mucous membranes (lamina propria), lymph nodes, spleen, bone marrow

Adipocytes (Fat Cells)

• Large cells specialized for lipid storage
• In routine H&E sections, the lipid is dissolved out during processing, leaving a large empty vacuole with the nucleus pushed to the periphery ("signet ring" appearance)
• Part of connective tissue proper when scattered individually; form adipose tissue when aggregated in large numbers (discussed separately)

Lymphocytes and Other Leukocytes

• Lymphocytes (small round cells with a dark round nucleus) are normally present in small numbers in loose connective tissue. They increase dramatically during inflammation and immune responses.
• Eosinophils, neutrophils, and basophils can migrate from blood vessels into connective tissue during inflammation. Eosinophils are especially prominent in parasitic infections (important in veterinary medicine).

Connective Tissue Fibers

1. Collagen Fibers - The Most Abundant Fiber Type

• Appear as long, slightly wavy, eosinophilic (pink) bundles in H&E sections
• Stain blue/green with Masson's Trichrome; black with silver stains
• Extremely high tensile strength - can resist pulling forces better than steel wire of the same diameter
• Collagen fibers are inextensible (they do not stretch significantly)

2. Reticular Fibers

• Composed of Type III collagen, but the individual fibrils are much thinner and coated with glycoproteins
• Form delicate, branching networks (reticulum) rather than thick bundles
• Not visible on H&E - require silver impregnation stains (argyrophilic - they reduce silver salts to metallic silver, staining black)
• Where found: Supporting the soft parenchyma of organs - liver (form the framework between hepatocytes), spleen, lymph nodes, bone marrow, kidney, skeletal muscle (endomysium), peripheral nerves (endoneurium)
• Key concept: Reticular fibers form the scaffolding of lymphoid and hemopoietic organs. Loss of this scaffolding is seen in cirrhosis of the liver.

3. Elastic Fibers

• Composed of the protein elastin (an amorphous core) surrounded by a sheath of fibrillin microfibrils
• Properties: Can be stretched up to 1.5x their resting length and recoil to original length when tension is released - this is the key difference from collagen fibers
• Appear as thin, refractile, slightly yellow-green fibers in H&E; branch and anastomose (unlike collagen which runs in parallel bundles)
• Special stains: Stain black with Verhoeff's hematoxylin or Weigert's resorcin-fuchsin; also stain with orcein (brown)
• Where found: Elastic arteries (aorta, pulmonary artery) - where elasticity is needed to smooth out pulsatile blood flow; lung parenchyma (allows expansion and recoil); ligamentum nuchae (nuchal ligament of neck in large animals like cattle and horses - almost entirely elastic, supports the weight of the head); vocal folds; elastic cartilage (external ear, epiglottis)
• Veterinary note: The ligamentum nuchae in cattle and horses is a major source of elastic tissue. It is sometimes called "yellow ligament" because of its yellowish color in fresh specimens.

Ground Substance

• Long, unbranched polysaccharide chains made of repeating disaccharide units. Most carry many negative charges (sulfate and carboxylate groups), making them highly hydrophilic.
• Examples: hyaluronic acid (the only non-sulfated GAG), chondroitin sulfate, dermatan sulfate, keratan sulfate, heparan sulfate, heparin
• GAGs attract and hold water, creating a hydrated gel that resists compression. This gives cartilage its ability to withstand compressive loads.

• GAG chains covalently attached to a core protein. Can be thought of as "bottle brushes."
• Examples: aggrecan (major proteoglycan of cartilage), decorin, versican
• Aggrecan molecules attach to hyaluronic acid chains, forming huge macromolecular aggregates that trap enormous amounts of water.

• Proteins that link cells to ECM components and different ECM components to each other
• Examples: fibronectin (links collagen, fibrin, and cell-surface integrins; important in wound healing and cell migration), laminin (major adhesive glycoprotein of basal laminae), entactin/nidogen (links laminin to Type IV collagen in basal lamina)

Classification of Connective Tissue Proper

1. Embryonic Connective Tissue

2. Loose (Areolar) Connective Tissue

• Characterized by: relatively few fibers (collagen, reticular, elastic all present), abundant ground substance, and a diverse population of cells (fibroblasts, macrophages, mast cells, plasma cells, lymphocytes, adipocytes)
• The ECM is loosely organized - the collagen fibers form an open meshwork
• Where found: Directly beneath all epithelia (lamina propria of mucous membranes), surrounding and within most organs, adventitia of blood vessels, mesentery, dermis of skin (papillary layer)
• Functions: Provides metabolic support to overlying epithelium; site of immune surveillance (first defense against invading microorganisms that breach epithelial barriers); allows tissue swelling (edema) during inflammation; supports and connects other tissue components

3. Dense Connective Tissue

• Collagen fiber bundles are arranged in a random, interwoven, multi-directional pattern, allowing resistance to stress from multiple directions
• Few fibroblasts are scattered between the fibers
• Where found: Deep dermis of skin (reticular layer - provides the leather quality of skin), submucosa of the GI tract, capsules of organs (kidney, liver, testis), periosteum, perichondrium
• On a slide: Dense pink fibers running in all directions, very few cell nuclei visible

• Collagen fibers are arranged in parallel arrays, densely packed to provide maximum tensile strength in one axis (the direction of force)
• Fibroblasts (called tenocytes in tendons) are aligned in rows between fiber bundles; appear as flattened cells with elongated, dark nuclei
• Where found:
  • Tendons: Connect muscle to bone. Parallel bundles of Type I collagen. Primary bundles (fascicles) surrounded by loose CT (endotendineum); groups of fascicles enclosed by peritendineum; entire tendon enclosed by epitendineum.
  • Ligaments: Connect bone to bone. Similar structure to tendons but may contain more elastic fibers.
  • Aponeuroses: Flat, sheet-like tendons
• On a slide: Wavy, parallel pink bundles with elongated dark nuclei arranged in rows between them.

4. Reticular Connective Tissue

• A special type of loose CT in which reticular fibers (Type III collagen) form the predominant ECM component
• The cells are reticular cells (a type of fibroblast)
• Where found: Framework (stroma) of lymphoid organs - lymph nodes, spleen, thymus, bone marrow; liver lobules; kidney cortex
• Staining: Only visible with silver stains (black)

Special Connective Tissues

• Adipose tissue (covered tomorrow under Chapter 6/Day 2)
• Cartilage (covered tomorrow)
• Bone (covered tomorrow)
• Blood (covered tomorrow)

What's Covered Today

HISTOLOGY METHODS & MICROSCOPY

Exam Memorization Format

PART 1: WHAT IS HISTOLOGY?

Histology is the scientific study of the microscopic structure of tissues and organs using various staining and microscopy techniques.

PART 2: TISSUE PREPARATION - THE 4 STEPS

STEP 1 - FIXATION

1. Stops cell metabolism (arrests all enzymatic activity)
2. Prevents autolysis (self-digestion by lysosomal enzymes)
3. Kills pathogens (bacteria, fungi, viruses)
4. Hardens tissue (by cross-linking or denaturing proteins)

• Formalin = 37% aqueous formaldehyde solution, buffered to pH 7
• Mechanism: Reacts with amino groups of proteins (especially lysine residues) → forms cross-links → preserves 3D protein structure
• Advantages: Good tissue penetration, preserves antigenicity (proteins still react with antibodies - important for immunostaining)
• Disadvantage: Poor fixation of lipids/cell membranes

• Glutaraldehyde - used for electron microscopy (better ultrastructural preservation)
• Osmium tetroxide - fixes and stains lipids/membranes for EM
• Bouin's solution - used for reproductive tissue (testis, ovary)
• Carnoy's - rapid fixative, good for nucleic acids

STEP 2 - DEHYDRATION AND CLEARING

• Tissue washed after fixation
• Passed through ascending series of ethanol (70% → 80% → 90% → 95% → 100%)
• Then passed through a clearing agent (xylol/toluol) which is miscible with both alcohol and paraffin

STEP 3 - EMBEDDING

• Tissue infiltrated with molten paraffin wax (60°C), then allowed to harden into a block
• Sections cut at 5-15 micrometers thick using a MICROTOME
• Sections floated on warm water (unfurls wrinkles) → mounted on glass slides
• Slides deparaffinized (xylol again) and rehydrated before staining

Frozen sections = tissue frozen with liquid nitrogen instead of paraffin. Much faster (10 minutes total). Used during surgery when a rapid diagnosis is needed. Lower quality image compared to paraffin sections.

STEP 4 - STAINING

PART 3: STAINING - THE MOST IMPORTANT SECTION

H&E Staining (Hematoxylin and Eosin) - CORE EXAM TOPIC

Hematoxylin = Basic dye → stains ACIDIC structures BLUE/PURPLE Eosin = Acidic dye → stains BASIC structures PINK/RED

Special Stains - Table to Memorize

METACHROMASIA = When a dye stains a structure a different color than the dye itself. Classic example: Toluidine blue (blue dye) stains mast cell granules purple-red because the heparin in granules is so densely negatively charged that it alters the dye's absorption spectrum.

PART 4: MICROSCOPY TYPES

A. LIGHT MICROSCOPY (LM)

B. ELECTRON MICROSCOPY (EM)

TEM shows the inside of a cell (2D cross-section). SEM shows the outside surface (3D image).

C. COMPARISON TABLE: LM vs TEM vs SEM

PART 5: HISTOCHEMISTRY AND CYTOCHEMISTRY

Enzyme Histochemistry

• Uses substrates that produce a colored insoluble precipitate at the exact site where the enzyme is active
• Example: Alkaline phosphatase activity in bone (osteoblasts), small intestinal brush border
• Example: ATPase staining of muscle - distinguishes Type I (slow) from Type II (fast) fibers based on pH-dependent ATPase activity

Immunohistochemistry (IHC)

1. Direct method: Labeled antibody applied directly to tissue (less sensitive)
2. Indirect method: Unlabeled primary antibody applied first, then labeled secondary antibody against the primary antibody (more sensitive - amplification)

• Peroxidase-antiperoxidase (PAP) - enzyme produces brown color with DAB substrate
• Fluorescent dyes (FITC = green, rhodamine = red) - seen with fluorescence microscopy
• Colloidal gold - seen with electron microscopy

Autoradiography

• Radioactive precursor molecules (e.g., ³H-thymidine for DNA, ³H-leucine for protein) are injected into the animal or added to cell culture
• Radioactive molecules are incorporated into cells synthesizing DNA/protein
• Tissue is fixed, sectioned, and coated with photographic emulsion
• Radioactive decay exposes the emulsion → produces silver grains over sites of radioactive label
• Uses: Tracking cell proliferation (which cells are dividing?), studying secretory pathways (how long does it take a protein to go from RER → Golgi → secretory granule → exocytosis?)

In Situ Hybridization (ISH)

• A labeled complementary nucleic acid probe hybridizes to specific mRNA or DNA sequences within intact tissue sections
• Used to detect gene expression at the cellular level
• FISH (Fluorescence ISH) uses fluorescent probes to detect chromosomal abnormalities

PART 6: INTERPRETATION OF TISSUE SECTIONS - WHAT TO KNOW FOR PRACTICALS

3D → 2D Problem

Artifacts

• Shrinkage artifact: Formalin fixation causes slight tissue shrinkage; cells may pull away from each other
• Knife marks: Parallel lines across the section from a nicked microtome blade
• Bubbles: Appear as round empty spaces (different from real spaces like vessel lumens)
• Folding: Section folds on itself

QUICK RECALL SUMMARY - Write This in 5 Minutes

1. Fixation (formalin - stop autolysis, preserve structure)
2. Dehydration (ascending alcohol) → Clearing (xylol) → Embedding (paraffin)
3. Sectioning (microtome, 5-15 μm)
4. Staining (H&E most common)

• Hematoxylin (basic dye) → nuclei, RER, cartilage = BLUE
• Eosin (acidic dye) → cytoplasm, collagen, muscle, RBCs = PINK

• LM: 0.2 μm resolution; studies tissue/cell level
• TEM: 0.2 nm resolution; electrons through section; shows organelle ultrastructure (2D)
• SEM: electrons bounce off surface; shows 3D surface topography

• IHC = antibody + labeled secondary → localize specific proteins
• Autoradiography = radioactive labels → track cell division/secretion
• Enzyme histochemistry = substrate → colored precipitate at enzyme site

EPITHELIAL TISSUE

Exam Memorization Format

PART 1: DEFINITION AND KEY CHARACTERISTICS

Epithelial tissue (epithelium) is a continuous sheet of closely packed cells that covers all body surfaces, lines hollow organs and body cavities, and forms glands.

PART 2: CLASSIFICATION OF EPITHELIUM

Criterion 1: Number of cell layers Criterion 2: Shape of the surface cells

CRITERION 1 - Number of Layers:

CRITERION 2 - Shape of Surface Cells:

Rule: In stratified epithelia, the shape of the TOP (surface) layer determines the name - not the basal cells.

PART 3: ALL EPITHELIUM TYPES - ONE BY ONE

TYPE 1: SIMPLE SQUAMOUS EPITHELIUM

• Endothelium = lining of all blood vessels and lymphatics
• Mesothelium = lining of body cavities (pleura, pericardium, peritoneum)
• Bowman's capsule (parietal layer) in the kidney
• Alveoli of lungs (Type I pneumocytes)
• Thin segment of loop of Henle in the kidney

TYPE 2: SIMPLE CUBOIDAL EPITHELIUM

• Kidney tubules (proximal and distal convoluted tubules)
• Thyroid follicles (when not stimulated - becomes columnar when active)
• Small collecting ducts of exocrine glands
• Surface of the ovary (germinal/surface epithelium)
• Lens epithelium of the eye

TYPE 3: SIMPLE COLUMNAR EPITHELIUM

• Most of the GI tract (stomach, small intestine, large intestine)
• Cells have microvilli on apical surface = brush border / striated border
• Goblet cells are scattered among them (mucus-secreting unicellular glands - pale, cup-shaped)

• Uterine tubes (oviducts/fallopian tubes)
• Uterus (endometrium)
• Small bronchioles

The abomasum (4th/true stomach of ruminants) is lined by simple columnar epithelium with gastric glands - just like the stomach of monogastric animals. The forestomachs (rumen, reticulum, omasum) are lined by stratified squamous epithelium with NO gastric glands - they are fermentation chambers, not digestive glands.

TYPE 4: PSEUDOSTRATIFIED COLUMNAR EPITHELIUM

• Goblet cells secrete mucus which traps dust, debris, and pathogens
• Cilia beat rhythmically (12-15 beats/second) to sweep mucus toward the pharynx
• This is called the MUCOCILIARY ESCALATOR - a key innate defense mechanism
• In veterinary medicine: damage to the mucociliary escalator (by viruses like IBR, PI3 in cattle) predisposes to bacterial pneumonia

TYPE 5: STRATIFIED SQUAMOUS EPITHELIUM - THE MOST IMPORTANT TYPE

1. Stratum basale (germinativum) - single layer of cuboidal/columnar cells; stem cells here; mitosis occurs here; cells attached to BM by hemidesmosomes
2. Stratum spinosum - several layers of polygonal cells; cells are connected by DESMOSOMES (look like spines in EM - hence "spinosum"); keratin synthesis begins
3. Stratum granulosum - 3-5 layers of flattened cells; contain keratohyalin granules (dark-staining; contain filaggrin which cross-links keratin) and lamellar granules (contain lipids - discharged to create waterproof barrier)
4. Stratum lucidum - only in THICK SKIN (palms, soles, hoof sole); homogeneous, pale, glassy layer; cells are dead with no nucleus
5. Stratum corneum - outermost layer; dead, anucleate, keratin-filled cells called corneocytes; shed continuously (desquamation)

• Skin (epidermis) of all animals - the main example
• Hard palate, gums (gingiva)
• Hoof wall, claw, horn - heavily keratinized in large animals
• Esophagus - in ruminants and horses (keratinized); contrast with humans where it is non-keratinized

• Oral mucosa (cheeks, lips, floor of mouth) in most species
• Esophagus - in dogs, cats, humans
• Vagina
• Cornea of the eye
• Anal canal (distal part)

Rumen, reticulum, and omasum of ruminants = stratified squamous non-keratinized (partially keratinized in rumen) This is one of the most commonly asked veterinary histology questions. The forestomachs have NO GLANDS. They are lined by stratified squamous epithelium because they are fermentation/mixing chambers, not secretory organs.

TYPE 6: TRANSITIONAL EPITHELIUM (UROTHELIUM)

• Contains rigid protein plaques called uroplakins
• These form an impermeable barrier that prevents urine (which is toxic, hypertonic, and acidic) from leaking back into the tissue
• The apical membrane has asymmetric unit membrane (AUM) - thicker than a normal cell membrane

QUICK COMPARISON TABLE - ALL EPITHELIUM TYPES

PART 4: BASEMENT MEMBRANE

• PAS = magenta/purple (detects glycoproteins)
• Silver impregnation = black (detects reticular fibers)
• NOT visible on routine H&E unless thickened by disease

1. Mechanical support and anchorage of epithelium
2. Molecular filter between epithelium and CT
3. Regulates cell differentiation, proliferation, and polarity
4. Guides cell migration during wound healing
5. Tumor invasion barrier - carcinoma cells must break through the BM to become invasive (this is the definition of invasive carcinoma vs carcinoma in situ)

PART 5: CELL JUNCTIONS - THE JUNCTIONAL COMPLEX

"The Angry Dog Growls Hoarsely" Tight → Adherens → Desmosome → Gap → Hemidesmosome

JUNCTION 1: TIGHT JUNCTION (Zonula Occludens)

1. Seals the intercellular space - prevents molecules from passing BETWEEN cells (paracellular route)
2. Maintains cell polarity - prevents apical membrane proteins from migrating to the basal membrane (fence function)

JUNCTION 2: ADHERENS JUNCTION (Zonula Adherens)

• Extracellular: E-cadherin (calcium-dependent)
• Intracellular: linked to actin filaments via catenins (alpha, beta, gamma catenin)

1. Strong cell-to-cell adhesion
2. Structural integrity of epithelial sheet
3. Cell signaling (beta-catenin links to Wnt signaling pathway - important in development and cancer)

JUNCTION 3: DESMOSOME (Macula Adherens)

• Extracellular: Desmogleins and Desmocollins (cadherins)
• Intracellular: linked to intermediate filaments (cytokeratins) via desmoplakin and plakophilin

1. Strongest mechanical attachment between cells
2. Distributes mechanical forces through the intermediate filament network across the entire epithelium

Pemphigus vulgaris = autoimmune disease where antibodies attack desmogleins → desmosomes break down → cells separate (acantholysis) → fluid-filled blisters (bullae) form in skin and mucous membranes. Important in veterinary dermatology (dogs, horses).

JUNCTION 4: GAP JUNCTION (Nexus / Communicating Junction)

• Six connexin proteins assemble into a ring = connexon (hemichannel)
• Two connexons (one from each adjacent cell) align and dock = a complete gap junction channel
• Channel pore diameter = ~1.5 nm; allows passage of molecules < 1,000 Daltons

1. Direct cell-to-cell communication
2. Electrical coupling between cells (cardiac muscle - allows action potentials to spread)
3. Metabolic coupling (sharing of nutrients between cells)

JUNCTION 5: HEMIDESMOSOME

• Transmembrane: Integrins (α6β4)
• Intracellular: linked to intermediate filaments (cytokeratins)
• Extracellular: binds to laminin in the basal lamina

Bullous pemphigoid = autoimmune disease where antibodies attack hemidesmosome proteins (BP180, BP230) → epithelium detaches from BM → large tense blisters (bullae). Seen in dogs.

JUNCTION SUMMARY TABLE

PART 6: APICAL SURFACE SPECIALIZATIONS

1. MICROVILLI

• Finger-like projections of the apical plasma membrane
• Core of actin filaments cross-linked by villin and fimbrin
• Length: 1-3 μm | Diameter: 0.1 μm
• When densely and regularly packed → BRUSH BORDER (intestine) or STRIATED BORDER

• Small intestine enterocytes - brush border (increases surface area 20-40x)
• Proximal convoluted tubule (PCT) of kidney - very prominent brush border (distinguishes PCT from DCT on slides)
• Hepatocytes (along bile canaliculi)
• Gallbladder epithelium

2. STEREOCILIA (Stereovilli)

• Unusually long, irregular microvilli
• Core of actin (NOT microtubules - this is key difference from true cilia)
• Non-motile

• Epididymis - longest stereocilia in the body; function in resorption of fluid and sperm maturation
• Inner ear - sensory hair cells of the cochlea and vestibular apparatus; function as mechanosensors (convert sound vibrations or head movement into electrical signals)

3. CILIA

• Motile hair-like extensions; much longer and thicker than microvilli
• Core = AXONEME: 9 doublets of microtubules + 2 central single microtubules = 9+2 arrangement
• Axoneme arises from a basal body (modified centriole) at the cell surface
• Movement powered by dynein ATPase - dynein arms on outer doublets generate sliding force → bending motion
• Each ciliated cell may have hundreds of cilia beating in a coordinated wave

• Pseudostratified respiratory epithelium (trachea, bronchi, nasal cavity)
• Oviducts/uterine tubes - sweep ovum toward uterus
• Efferent ductules of testis
• Ependymal cells lining brain ventricles (circulate CSF)

• Arrangement: 9+0 (no central pair, no dynein arms)
• One per cell (monocilium)
• Found on almost EVERY cell in the body
• Function as sensory antennae - chemosensors, mechanosensors, osmosensors
• Example: Primary cilia on kidney tubule cells detect urine flow; dysfunction = polycystic kidney disease (PKD)

COMPARISON: MICROVILLI vs STEREOCILIA vs CILIA

PART 7: GLANDS

EXOCRINE GLANDS

• Secrete watery, protein-rich (enzymatic) fluid
• Cells: pyramidal, round basal nucleus, eosinophilic cytoplasm, zymogen granules at apex
• Lumen is small and clearly visible
• Examples: Parotid salivary gland, pancreatic acini (exocrine portion)

• Secrete thick, viscous mucus (glycoproteins)
• Cells: columnar, flat basal nucleus pushed to base, pale/empty-looking cytoplasm (mucin washes out in routine processing), large distended lumen
• Examples: Sublingual salivary gland, goblet cells in intestine and trachea

• Contain BOTH serous and mucous cells
• Mucous cells form the acinus; serous cells cap the end = SEROUS DEMILUNES (look like half-moons of dark-staining cells at the edge of a mucous acinus)
• Examples: Submandibular (submaxillary) salivary gland, submucosal glands of the trachea

Vet note: In dogs, cats, and horses, the main sweat glands are APOCRINE (located deep in hair follicles) - unlike humans who rely primarily on eccrine glands. This is why animals cool down differently than humans.

ENDOCRINE GLANDS

QUICK RECALL SUMMARY - Write This in 5 Minutes

• Simple squamous → blood vessels, body cavities, alveoli
• Simple cuboidal → kidney tubules, thyroid
• Simple columnar → GI tract (with goblet cells + brush border)
• Pseudostratified ciliated → trachea, bronchi (respiratory epithelium)
• Stratified squamous keratinized → skin, hoof, horn
• Stratified squamous non-keratinized → esophagus (dogs/cats), mouth, vagina, rumen/reticulum/omasum
• Transitional → bladder, ureter

• Microvilli (actin, short) → intestine, PCT kidney (brush border)
• Stereocilia (actin, long, non-motile) → epididymis, inner ear
• Cilia (9+2 microtubules, motile) → trachea, oviducts

• Merocrine = exocytosis (most glands)
• Apocrine = apical cytoplasm pinched off (mammary, sweat in animals)
• Holocrine = cell disintegrates (sebaceous glands)