Binding and internalization of gold-labeled insulin complex by thymic cells.

Colloidal gold-labeled insulin complex was used for the ultrastructural visualization of insulin binding sites on the cell surface and to study the intracellular pathway of insulin. Thymic epithelial cells and thymic lymphoid cells from mammalian fetuses (mouse, rat, man) were investigated. The results show that all epithelial cells and some lymphoid ones bind insulin during fetal life. Certain fine structural features (single gold particles, patches and clusters of gold granules on the cell membrane) which are common for both kinds of thymic cell lines are established. Uncoated pinocytotic invaginations (more rarely via coated pits), tubulo-vesicles, uncoated vesicles, intermediate filaments and microtubules are related to insulin gold complex intracellular pathway in the epithelial cells. However, these peculiarities as a whole are noted typically in the lymphoid cells. These data suggest that the ability to cap and to internalize IGC is directly associated with thymic cell type and similar in principle to both cellular types (epithelial and lymphoid cells) in all mammalian fetuses studied.