Thyrotrophin‐Induced Aggregation and Reorganization into Follicles of Isolated Porcine‐Thyroid Cells in Culture

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Porcine thyroid cells isolated by trypsinization grow in culture as a monolayer. At high cell concentration and in the presence of thyrotrophin, cells aggregate and rearrange in structures resembling cross‐sections of intact thyroid tissue. Longitudinal and transverse sections of thyrotrophin‐stimulated and non‐stimulated cell cultures have been examined by electron microscopy after fixation of the cell layers in the culture flasks. Cells cultured in the presence of thyrotrophin undergo an organizational rearrangement into typical three‐dimensional follicles as shown by the presence of junctional complexes, differentiated apical poles with microvillae, follicular lumen sometimes containing material dense to electrons, and apical vesicles. In contrast, cells cultured in the absence of thyrotrophin develop as a two‐dimensional layer. No differentiation of the plasma membrane is visible. Cells are flat and partially overlap each other. All cells are linked by junctional complexes. Except for this zone of association, the undifferentiated plasma membranes of the contiguous cells are not adherent and limit more or less distended intercellular spaces. Cells associated in follicles show the ultrastructural aspects of polarized and stimulated follicular cells. The apical pole is rich in organelles such as lysosomes, vesicles, Golgi complexes and rough endoplasmic reticulum. To the contrary in monolayer cells, the organelles are perinuclear; Golgi complexes are scarce and a very dense network of microfilaments and microtobules is visible in the peripheral part of the cytoplasm. Therefore, thyroid cells cultured with or without thyrotrophin, each form a homogeneous population of cells in a given organizational arrangement and at different level of differentiation.