--- module: 083-01 language: en chapter: 83 title: "General Histology: Epithelium, Connective Tissue, Muscle, and Nerve" module_title: "Microscopy, tissue preparation, epithelial organisation, glands, and surface specialisations" source_sha256: 96c2752fe2eb07d6cd40032402a600b4c5523fd052cb781c92994993d14b8ccf --- # Microscopy, epithelium, glands, and surface specialisations ## Reading a section ### Thin processed sample, not the tissue itself ### Identify artefacts and orientation first ### Relate cells, matrix, lumina, vessels, stroma ### Combine pattern with mechanism ## Tissue preparation ### Formaldehyde fixation cross-links proteins ### Dehydrate, embed in paraffin, section, stain ### Frozen sections made rapidly during surgery #### Preserve lipids and enzyme activity #### Morphology usually less crisp ### Decalcification allows mineralised tissue #### May alter cellular and matrix detail ## Stains and imaging ### Haematoxylin colours acidic structures blue-purple #### Basophilia reflects abundant nucleic acid ### Eosin colours proteins pink #### Eosinophilia reflects protein-rich cytoplasm or matrix ### Intensity is not an absolute measurement ### Special stains: periodic acid–Schiff, trichrome, silver ### Immunohistochemistry localises proteins ### In situ hybridisation localises nucleic acids ### Transmission microscopy: membranes, organelles, junctions ### Scanning microscopy shows surface topography ### Digital pathology: sampling and interpretation stay human ## Resolution and sectioning ### Magnification enlarges, resolution separates points #### Numerical aperture and wavelength set resolution #### Excess magnification only enlarges blur ### Low, intermediate, and high power for different levels ### Three-dimensional structures become flat profiles #### Tube: circle, oval, or elongated by plane #### Gland looks solid if lumen missed ### Artefacts mimic pathology #### Shrinkage spaces, folds, hyperchromatic crushed cells #### Consistent relationships separate biology from artefact ## Epithelial organisation ### Closely apposed cells with little matrix ### Avascular, nourished by diffusion ### Apical-basal polarity and basement membrane ### Classified by layers and surface cell shape #### Pseudostratified: every cell touches basement membrane ### Simple squamous minimises diffusion distance #### Endothelium is an active regulatory organ #### Mesothelium lines serous cavities ### Simple cuboidal: small ducts, tubules, thyroid follicles ### Simple columnar: gut and gallbladder ### Airway: cilia move trapped mucus to pharynx ### Stratified squamous protects against abrasion #### Keratinised: limits water loss, resists injury #### Non-keratinised lines moist surfaces ### Urothelium: distensible yet impermeable #### Umbrella cells, membrane plaques, tight junctions ## Surface specialisations ### Microvilli: actin projections enlarge surface area #### Brush border in intestine and proximal tubule ### Stereocilia: long actin projections ### Motile cilia: microtubules move fluid ### Goblet cells: unicellular mucous glands #### Excess obstructs airways, deficiency weakens defence ## Glands and secretion ### Arise by epithelial downgrowth ### Exocrine keep ducts, endocrine release to blood ### Simple or compound, tubular or acinar ### Serous watery, mucous viscous, mixed both ### Merocrine, apocrine, holocrine: release mechanism ### Protein cells: basal basophilia, Golgi, granules ### Steroid cells: smooth reticulum, lipid, tubular cristae ### Ion transport: basolateral folds, mitochondria ### Myoepithelial cells expel secretion ### Ducts modify ions and water ## Basement membrane, renewal, and reading ### Basement membrane anchors, filters, polarises #### Breach marks invasive malignancy ### Stem cells in protected niches renew epithelium ### Metaplasia swaps programme under stress #### May not perform the original function ### Read surface, layers, basal contact, apex, ducts #### Predict function and vulnerabilities