[Specificity of thyroid follicular cells]

Thyroid follicular cells, as polarised endocrine cells in fish, amphibia, reptile, birds and mammalia, release Tg molecules, mainly via apical cytoplasm. These products known as colloid, the amount, contents, staining affinity, density and absorptive capacity, differs clearly even in follicules of the same gland. The pathways of secretion by "a regulated type of exocytoses" are clear signs of cell memory acquired during evolution and genetical program. However, the character of thyroid follicles appearance, as well as structural and ultrastructural organization of follicular cells, proliferative rate, nature of their internal membranes and the other organelles, might be altered by numerous endogenous and/or exogenous factors. Our earlier data, obtained examining thyroid follicular cells in the experiments as are: experimental goitre and iodine prophylaxis, enzootic goitre in the animals, after internal or X-rays irradiation, tissue culture and/or transplantation, might be summarized as follow. Thyroid follicular cells are polarized and their synthetic capacity and pathways of Tg molecules secretion are closely related with proliferative rates of granular and agranular ER, nature of their cavities and vesicles as well as localization of both ER and Golgi complexes. The main pathway of Tg and the other substances are released via apical cells by exocytosis. Their activity, storage capacity, the amount and properties of colloid are dependent on the balanced neuroendocrine cells activities. The character of colloid absorption, expressed in the number and size of microvillis and protrusion, as well as cytoskeleton, colloid droplets and the appearance of other apical cytoplasmic organelles, could be considered as a signs of stimulative hormonal or/and neuronal influences. The fate of reabsorbed Tg molecules, as dependent on the lysosomal activities, and release of thyroid hormones, is closely related on ultrastructural follicular cells properties. However, many endogenous and/or exogenous factors might affect both synthetic and secretory activities by stimulation, or retardation and even inhibition modifying genetic program and cell memory. As a results Tg molecule synthesis and maturation, as well as both Tg and thyroid hormones release, are released directly into blood circulation, as are usually a cases under conditions of thyroid hormones deficiency.