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Endocrine system (systema endocrinum) 169

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9.9 Thyroid follicle with adjacent capillaries and nerve fibres embedded in loose connective tissue (schematic).

9.10 Functional stages of the epithelium of the thyroid gland: hormone synthesis, storage and resorption/
secretion (schematic).

· Storage phase: large, plump, colloid-filled follicles, The functional status and thus the morphology of the thy-
low epithelium, inactive (non-secretory) (Figure 9.10). roid gland undergoes large fluctuations (seasonal factors,
· Resorption/hormone secreting phase: as liquid reproductive cycle). Furthermore, at a given point in time,
colloid is taken up by follicular cells, the cells become not all follicles exhibit the same functional characteristics.
taller, their nuclei occupy a basal position, colloidal
resorption droplets appear in the apical cytoplasm Follicular cells (endocrinocytus follicularis)
and the adjacent colloid becomes interspersed with In follicular cells exhibiting a cuboidal morphology, the
resorption vacuoles. nucleus is round, centrally located and euchromatic. The
· Synthetic phase: the follicles are small due to a supranuclear Golgi fields enclose small secretory granules
reduction in colloid volume, and the cuboidal to (50–200 μm) associated with the early stages of colloid
columnar epithelial cells release colloid into the fol- production. Lysosomes and phagolysosomes are present.
licular lumen. The apical cell surface features microvilli and pseudopo-

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