In vitro conversion of porcine thyroid cells growing in monolayer into functional follicular cells.

Porcine thyroid cells in primary cultures form either monolayers or, when they are stimulated by thyrotropin (TSH), follicles. This system, monolayer-follicle associated cell culture, determines two morphofunctional states of the thyroid cell in vitro. These cells divide, when grown in monolayer. In this article we describe the precise conditions which allow the conversion of monolayer cells into follicles. The rate of cell growth was lowered using a serum-free medium. Cells were concentrated, stimulated by TSH and cultured on poly-L-lysine pretreated plastic flasks. Light and electron microscope studies show that cells reorganize into follicles containing thyroglobulin. Active iodide transport by the cells, as well as detection of thyroid hormones in the cell culture media, demonstrate that these follicles are functional. Formation of monolayers from follicles is in vitro a spontaneous phenomenon. It is linked to the loss of cell polarity, iodide transport, synthesis of hormones and to the decrease of the number of TSH receptor-sites. These main characteristics of differentiation may be regained in vitro after conversion of monolayer thyroid cells into active follicles up to at least generation five.