Effects of Epidermal Growth Factor,Phorbol Ester,and Retinole Acid on Hormone Synthesis and Morphology in Porcine Thyroid Follicles Cultured in Collagen Gel

Epidermal growth factor (EGF), phorbol esters (PEs), and retinoic acid (RA) inhibit differentiated functions of thyrocytes. In the present study the inhibitory effects of these growth-promoting factors on hormone synthesis were studied in thyroid follicles cultured in type-I collagen gel, and morphologic alteration by these factors was examined by light and electron microscopy (EM). Porcine open thyroid follicles obtained by treatment with 0.1% collagenase were embedded in collagen gel and cultured in Ham's F12 medium supplemented with 6H (insulin, hydrocortisone, somatostatin, transferrin, glycyl-his-lys, and thyrotropin) + 0.5% fetal bovine serum (FBS). After 1 week these open follicles developed to closed follicles, and the medium was changed to one containing 6H + 0.5% FBS + 0.1 μM sodium iodide (NaI). Some media were supplemented with either EGF, phorbol 12-myristate 13-acetate (PMA), or all-trans RA. The closed follicles retained ability for hormone synthesis for 2 weeks after the medium change in the presence of 6H + FBS + NaI. The amounts of T₄ and T₃ secreted into the culture medium from day 9 to day 12 after the medium change were 60% and 45% of those from day 0 to day 4, respectively. EGF reduced production of T₄ and T₃ by 61% and 69%, respectively; PMA, by 87% and 99%; and RA, by 55% and 44%.

In the medium supplemented with 6H + 0.5% FBS, the follicles exhibited intact polarity. Apical surfaces with microvilli were oriented to the follicular lumen and tight junctions were on the apical side of cell-to-cell contacts. Desmosomes were found on both the apical and basal halves of the cell contacts. When Nal (1 μM) was present in the medium supplemented with 6H + 0.5% FBS, desmosomes disappeared, and the follicular epithelial cells were connected only by the tight junctions. Colloidal substance was formed in 84% of the follicles. After treatment with EGF, some of the follicular cells migrated to form aggregates of cells or new follicles. With PMA treatment they formed small satellite follicles around the main ones. After RA treatment the tight junctions were located on both the apical and basal sides of the cell contacts. Since these growth factors or their intracellular mediators exert effects on cell differentiation and embryogenesis in many organ systems, they might be involved in folliculogenesis of the thyroid in vivo.