Identifying feature of thyroid gland
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The thyroid gland's identifying histological feature is the presence of follicles - spherical structures lined by follicular epithelial cells and filled with colloid (thyroglobulin). This is the classic hallmark under microscopy.
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Here is a complete account of the identifying features of the thyroid gland, drawn from authoritative histology and physiology textbooks.
Identifying Feature of the Thyroid Gland
The Thyroid Follicle - The Key Identifying Feature
The thyroid follicle is the structural and functional unit of the thyroid gland - this is its single most important identifying feature under both gross and microscopic examination.
Each follicle is:
- A roughly spherical, cyst-like compartment (diameter 0.2-1.0 mm)
- Walled by a simple cuboidal to low columnar epithelium (follicular epithelium)
- Filled with a gel-like material called colloid (composed predominantly of the glycoprotein thyroglobulin)
Hundreds of thousands of these follicles constitute nearly the entire mass of the human thyroid gland.
Histology (H&E stain):
H&E stain of human thyroid. The large pink spaces are colloid-filled follicles, each surrounded by a single layer of follicular epithelial cells. Arrows indicate blood capillaries between follicles. x500. - Histology: A Text and Atlas
Two Cell Types in the Follicular Epithelium
| Cell Type | Also Called | Function | Appearance (H&E) |
|---|---|---|---|
| Follicular cells | Principal cells / Thyrocytes | Produce T3 and T4 | Cuboidal; slightly basophilic cytoplasm; spherical nucleus with prominent nucleoli |
| Parafollicular cells | C cells | Secrete calcitonin | Pale-staining; located at periphery of follicular epithelium, never exposed to follicle lumen |
Functional State Changes the Appearance
The appearance of follicles changes with thyroid activity - this is a classic exam point:
| State | Follicle Size | Epithelial Cell Shape | Colloid | Feature |
|---|---|---|---|---|
| Inactive (resting) | Large | Flat / squamous | Abundant | No reabsorption lacunae |
| Active (stimulated) | Small | Cuboidal to columnar | Reduced | Reabsorption lacunae visible at colloid-cell interface |
Reabsorption lacunae (also called "scalloping") - small punched-out spaces at the periphery of the colloid next to follicular cells - are the hallmark of an actively secreting thyroid gland.
Summary of Key Identifying Features
| Feature | Detail |
|---|---|
| Follicles with colloid | Spherical structures filled with pink, homogeneous colloid on H&E |
| Simple cuboidal epithelium | Lines each follicle; varies with activity |
| Colloid = thyroglobulin | PAS-positive material; precursor of T3/T4 |
| Parafollicular (C) cells | Pale, intercalated between follicular cells; secrete calcitonin |
| Scalloping/Reabsorption lacunae | Seen at colloid periphery in active gland |
| Rich capillary network | Fenestrated capillaries between follicles |
The colloid-filled follicle lined by follicular epithelium is what immediately identifies the thyroid gland in any histological section - no other endocrine gland has this arrangement.
Sources: Histology: A Text and Atlas with Correlated Cell and Molecular Biology; Ganong's Review of Medical Physiology, 26th Edition
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