15.2 Normal Histology & Diagnostic Tissue Identification
Key Takeaways
- Skeletal muscle is striated with peripheral nuclei, cardiac muscle is striated with central nuclei and intercalated discs, and smooth muscle is non-striated with a central cigar-shaped nucleus.
- Basement membranes are periodic acid-Schiff positive and are demonstrated in black by the Jones methenamine silver method, which makes glomerular and tubular basement membranes the internal control on a renal biopsy.
- Normal hepatocyte plates are one cell thick on a reticulin stain, and plates two or more cells thick indicate regenerative nodules.
- Recognizing the expected internal control structure on a section is how a technologist decides whether a weak special stain reflects true negativity or a failed reaction.
- Orientation errors are diagnostic errors: a tangentially embedded skin or gastrointestinal biopsy can make normal epithelium appear invasive and forces a re-embed.
1. Why Tissue Identification Is Tested
Each of the five ASCP BOC content areas opens with a Tissues subsection. For Fixation and Staining the outline adds Pathology and Biochemistry principles as HTL-only items. The examination does not ask the histotechnologist to render diagnoses, but it does assume the technologist can recognize what tissue is on the block, decide whether the diagnostic plane has been reached, and judge whether a special stain reacted with the structure it was supposed to react with.
2. The Four Basic Tissue Types
2.1 Epithelium
Epithelium is classified by number of layers and cell shape, and it always sits on a basement membrane.
| Type | Location Example | Recognition Cue |
|---|---|---|
| Simple squamous | Alveolus, endothelium, mesothelium | Single flat layer, nuclei bulging into the lumen |
| Simple cuboidal | Renal tubule, thyroid follicle | Single layer, height about equal to width |
| Simple columnar | Stomach, intestine, gallbladder | Tall cells, basally aligned oval nuclei |
| Pseudostratified ciliated columnar | Trachea, bronchus | Nuclei at several levels but every cell touches the basement membrane; cilia and goblet cells |
| Stratified squamous, non-keratinized | Esophagus, ectocervix, vagina | Multiple layers, flattened surface cells retaining nuclei |
| Stratified squamous, keratinized | Skin | Anucleate surface keratin layer |
| Transitional (urothelium) | Bladder, ureter | Multiple layers with large dome-shaped umbrella cells |
2.2 Connective tissue
- Collagen type I — the coarse pink bundles of dermis, tendon, and scar; blue with Masson trichrome
- Collagen type III (reticulin) — fine argyrophilic fibers of liver sinusoids, lymphoid organs, and marrow; black with silver impregnation
- Collagen type IV — basement membrane collagen; periodic acid-Schiff positive and black with Jones methenamine silver
- Elastic fibers — refractile, wavy; black with Verhoeff-van Gieson
- Ground substance — glycosaminoglycans and proteoglycans; alcian blue and colloidal iron positive
- Resident cells — fibroblasts, mast cells (metachromatic with toluidine blue), plasma cells (clock-face chromatin, perinuclear hof), macrophages
- Specialized forms — hyaline, elastic, and fibrocartilage; compact and cancellous bone with unmineralized osteoid versus mineralized matrix; adipose tissue with signet-ring lipocytes
2.3 Muscle
| Muscle | Striations | Nuclei | Distinguishing Feature |
|---|---|---|---|
| Skeletal | Present, cross-striated | Multiple, peripheral | Large parallel fibers; fiber typing by ATPase |
| Cardiac | Present, cross-striated | Usually one, central | Intercalated discs, branching fibers |
| Smooth | Absent | One, central, cigar-shaped | Interlacing fascicles; red with Masson trichrome |
2.4 Nervous tissue
Neurons carry RNA-rich Nissl substance and long processes; myelin wraps axons and is demonstrated by Luxol fast blue; astrocytes express GFAP, oligodendrocytes myelinate the central nervous system, and microglia are the resident phagocytes.
3. Organ Recognition Anchors
| Organ | Hallmark Structure | Confirming Stain |
|---|---|---|
| Kidney | Glomeruli, tubules, basement membranes | Jones / PAMS, PAS |
| Liver | Hepatocyte plates one cell thick, portal triads, central veins | Reticulin, trichrome |
| Lung | Alveolar septa, respiratory epithelium, cartilage plates in bronchi | Elastic, GMS for organisms |
| Thyroid | Colloid-filled follicles lined by cuboidal cells | PAS-positive colloid |
| Pancreas | Serous acini with interspersed islets of Langerhans | Trichrome, immunohistochemistry for islet hormones |
| Spleen | White pulp lymphoid nodules within red pulp sinusoids | Reticulin, Prussian blue |
| Lymph node | Cortical follicles with germinal centers, subcapsular sinus | Reticulin |
| Gastrointestinal tract | Mucosa, submucosa, muscularis propria, serosa in sequence | Alcian blue-PAS for goblet cells |
| Skin | Stratified squamous epidermis with adnexa in dermis | Fontana-Masson for melanin |
| Bone marrow | Hematopoietic cells alternating with fat, bony trabeculae | Reticulin, Perls |
| Prostate | Glands with a two-cell layer, basal plus secretory | Basal cell immunohistochemistry |
| Testis | Seminiferous tubules with spermatogenic maturation | Bouin fixation preferred |
| Uterus | Endometrial glands in stroma over myometrial smooth muscle | Trichrome |
4. Internal Control Structures
A technologist reading a special stain has to answer one question before anything else: did the reaction work at all? The most reliable answer comes from a structure inside the patient's own section that must react.
| Stain | Internal Control Structure | If It Fails |
|---|---|---|
| PAS | Basement membranes, glycogen, goblet cell mucin | The negative finding cannot be reported |
| Jones / PAMS | Tubular basement membranes on a renal biopsy | Repeat; the glomerular reading is invalid |
| Alcian blue pH 2.5 | Normal goblet cells adjacent to the lesion | Repeat before calling intestinal metaplasia absent |
| Masson trichrome | Vessel wall smooth muscle red, adventitial collagen blue | Differentiation was mistimed |
| Verhoeff-van Gieson | Internal elastic lamina of any artery in the field | Under- or over-differentiated |
| Prussian blue | Any known hemosiderin-laden macrophage present | Reagent or acid problem |
| Reticulin | Portal tract and vessel wall reticulin | Silver or oxidation step failed |
| Immunohistochemistry | Normal ducts, lymphocytes, or endothelium expressing the marker | Pre-analytic failure invalidates a negative result |
[!CRITICAL] This is the practical reason a histotechnologist must know normal histology. A negative result on a stain whose internal control also failed is not a negative result; it is a void run.
5. Pathology Patterns the Technologist Should Recognize
The Pathology subsection is marked HTL only, and the level expected is pattern recognition sufficient to anticipate and verify stains.
- Fibrosis — expanded collagen; confirm with trichrome and grade marrow fibrosis with reticulin
- Granuloma — nodular epithelioid histiocytes, often with giant cells; triggers acid-fast and GMS stains
- Amyloid — amorphous eosinophilic extracellular material; triggers Congo red at 8 to 10 micrometers
- Steatosis — clear cytoplasmic vacuoles displacing the hepatocyte nucleus; confirmed on frozen section with Oil Red O
- Iron overload — golden-brown granules; confirm with Perls
- Cholestasis — green-brown canalicular plugs; confirm with Fouchet or Stein
- Neoplasia — architectural disorder, nuclear enlargement and hyperchromasia, mitoses; drives the immunohistochemical panel
- Dysplasia versus reactive change — the reason section quality and orientation matter so much
6. Artifact Versus Pathology
| Appearance | Artifact Explanation | Genuine Finding It Mimics |
|---|---|---|
| Brown-black refractile granules near blood | Formalin pigment, birefringent, removed with alcoholic picric acid | Hemosiderin or malarial pigment |
| Clear space around cells | Shrinkage from over-dehydration or delayed fixation | Clear cell change, lacunar cells |
| Crowded, hyperchromatic small nuclei at a tissue edge | Crush or cautery artifact from forceps or electrosurgery | Small cell carcinoma |
| Nuclear bubbling and pallor centrally with a crisp rim | Under-fixation with alcohol coagulation of the unfixed core | True necrosis |
| Epithelium appearing to lie in the stroma | Tangential sectioning of a mis-oriented biopsy | Invasion |
| Tissue floating above the section plane, different staining | Extraneous floater from bath or forceps | A true second lesion |
Recognizing these six patterns prevents both unnecessary recuts and, more importantly, the delivery of a slide whose artifact could be read as disease.
A section shows cross-striated fibers that branch, contain a single central nucleus per cell, and are joined end to end by dark transverse bands. What tissue is this?
A Jones methenamine silver stain on a renal biopsy shows glomerular basement membranes that are entirely unstained. The tubular basement membranes in the same section are also unstained. How should the technologist proceed?
A gastric biopsy shows crowded, dark, elongated nuclei in a narrow band along one edge of the tissue, with the remainder of the mucosa appearing normal. What is the most likely explanation?