Special Histology and Embryology: Practical Course - V. K. Napkhanyuk 2001

Endocrine system
Thyroid gland

The Thyroid Gland (glandula thyroidea) is a peripheral endocrine organ that regulates basal METABOLISM and maintains Blood Calcium Homeostasis.

Externally, the thyroid gland is covered by a Connective Tissue capsule, from which septa extend into the interior of the organ, dividing it into lobes.

Its parenchyma contains two main Cell types, distinguished by their morphological features, origin, and Functions:

— follicular endocrinocytes, or thyrocytes (endocrinocyti folliculares);

— parafollicular endocrinocytes, or calcitoninocytes (endocrinocyti parafolliculares).

Thyrocytes originate from the epithelium of the ventral wall of the Pharynx and form the walls of the thyroid follicles. They produce the Hormones thyroxine and triiodothyronine.

Calcitoninocytes (C-Cells) can be located within the follicular wall or between follicles. They are of neural origin. In hematoxylin-eosin-stained sections, calcitoninocytes appear lighter than thyrocytes; however, upon silver impregnation, they become darker due to brown granules in their Cytoplasm that are absent in thyrocytes. The specific granularity of calcitoninocytes is also visible under Electron Microscopy. The hormone produced by calcitoninocytes, Calcitonin, lowers blood calcium levels.

Slide 3. Thyroid gland (Figs. 16, 17).

Low power. At this magnification, it can be seen that the thyroid gland is externally enclosed by a capsule composed of dense Fibrous connective tissue. Interlobular connective tissue septa extend from the capsule into the parenchyma, dividing the gland into lobes. Blood Vessels run within these septa. Rounded structures—the follicles—are clearly visible within the lobes.

The follicular wall is formed by a single layer of cubic cells, and the follicular lumen is filled with colloid. During histological preparation, the volume of the colloid decreases, which may cause a cleft to form between the colloid and the follicular wall. Interfollicular epithelial cell islands are located between the follicles.

High power. At this magnification, the detailed Structure of a follicle must be examined. Most of the cubic cells in the follicular wall are follicular endocrinocytes. The follicular colloid, along with the endocrinocytes, contains small vacuoles. In the thin connective tissue layers surrounding the follicles, the Vessels of the perifollicular capillary network can be distinguished.

Label the following on the diagram: 1) capsule; 2) lobe; 3) interlobular connective tissue septa; 4) blood vessels; 5) thyroid follicles; 6) thyroid colloid; 7) vacuoles in the colloid; 8) interfollicular islands; 9) thyrocytes; 10) parafollicular cells; 11) interfollicular cells.

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Fig. 16. Thyroid gland. Hematoxylin-eosin staining. × 56:

1 — capsule; 2 — lobe; 3 — interlobular septa; 4 — blood vessel; 5 — follicles; 6 — thyroid colloid; 7 — vacuoles in the colloid; 8 — interfollicular islands

Fig. 17. Thyroid gland. Hematoxylin-eosin staining. Fragment of the previous slide. × 400:

1 — thyrocytes; 2 — colloid; 3 — fibrous Connective tissue with blood vessels; 4 — follicle; 5 — parafollicular cells