*1.1.2. Ultra-structure*

346 Thyroid Hormone

**Figure 1. A.** Histological cross-section of mouse trachea, showing cartilage (black arrow) and mouse thyroid lobes (L) and isthmus (clear arrow) adhering to connective tissue between cartilage and thyroid. The thyroid parenchyma consists of follicles which look like spherical or oval structures. **B.** The appearance of pig follicles in the lobe's central region. Each follicle consists of simple cubic epithelium (black arrow) limits the follicle centre full of colloid (Co) which follicular cells or thyrocytes secrete. The capillaries surrounding the follicles can be seen (clear arrow). **C.** Rat thyrocyte ultra-structure or cytology. The rugose endoplasmic reticulum (RER) can be seen around the nucleus (N) and Golgi complex in supranuclear position; these organelles and lysosomes (L) occupy the cells' base region. Different vesicles can be seen at apical level, from exocytosis (ex) being denser than electrons and endocytosis (en) and having the same density as electrons and colloid (Co), and the apical membrane forming microvellosities (M) in contact with colloid in the centre of the follicles. The binding complex can be observed in the lateral membrane at apical level: tight junctions (TJ) followed by belt desomosome (BD) and spot desmosome (SD). The thyrocytes' basement membrane or basal lamina (white arrow) is in close contact with the endothelium's (E) fenestrated capillaries' basement membrane (black arrow); the endothelium

cell nucleus (EN) can be seen (A and B H-E, MO. Scale bar A 1mm, B 40 μm. C TEM. 7300X).

Endocrine gland cells are usually not polarised with their central nucleus and organelles; however, thyrocytes are exceptionally polarised endocrine cells and the nucleus and organelles are located in the cell's basal region like exocrine epithelium cells. This is due to the larynx's embryonic development by polarised cells involuting inside it. Such morphological and exocrine functional characteristics are conserved within follicles for thyroid hormone synthesis, storing and secretion (Figure 1C). The thyrocytes' basal membrane is directly related to the follicles' basement membrane or basal lamina (Figure 1C white arrow), in turn, being in direct contact with the fenestrated capillaries' endothelial (Figure 1C E) basement membrane (Figure 1C black arrow). The thyrocytes' apical membrane is in direct contact with colloid forming microvellosities (Figure 1C M) whose length and amount vary according to a gland's functional state [8]. Exocytotic vesicles (Figure 1C ex) can be seen in the apical region, some of them being more electron dense than less electron dense endocytic vesicles (Figure 1C en) and some coated at the base of microvellosities [5].

The thyrocytes' lateral membranes have binding complexes in the apical region formed by tight junction, belt desmosome and spot desmosome (Figure 1C TJ, BD, SD) isolating and separating colloid from the intercellular spaces and basolateral apical membranes [9].

The thyrocytes' nucleus is surrounded by abundant RER in the cells' basal region (Figure 1C RER) and the GC is in the supra-nuclear region in normal physiological conditions (Figure 1C N). The lysosomes are located in the thyrocytes' basal media region (Figure 1C L). The mitochondria are distributed throughout the whole cell.

The thyrocytes form depends on their functional state; they are cubic in normal conditions (euthyroid morphology) and have the aforementioned ultra-structure (Figure 1C). Without thyrotrophic or thyrotropin-stimulating hormone (TSH), endocytic vesicles disappear at the beginning and exocytosis increases and microvellosities become reduced. The lysosomes increase in size and have very heterogeneous content following several days without TSH; the follicular cavity increases after a few days and cells become thin and atrophied because the RER and GC become reduced. Such cells disappear following 20 or 30 days' suppression of TSH (hypothyroid morphology). A rapid increase in exocytic vesicles (micropendocytosis) occurs when the gland is stimulated by TSH and the apical membrane forms cytoplasmic expansions or pseudopods forming large macroendocytic vesicles called colloid droplets; such vesicles merge with lysosomes which migrate to the cells' apical region [10]. When TSH stimulation is sustained for more than 5 days, microvellosities' length and amount increase, follicular cavities become reduced and the thyrocytes become cylindrical and hypertrophied because the RER and GC increase, occupying almost the whole of a cell (hyperthyroid morphology) [2,8,10,11].
