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RECENT advances in fixation (Palade, 1951) embedding (Newman, Borysko and Swerdlow, 1949) and sectioning (Pease and Baker, 1948; Latta and Hartman, 1950; Dempsey and Lansing, 1953) have made it possible to prepare thin sections of almost any mammalian tissue for use in the electron microscope. With the current techniques and instruments, resolutions of 100 A are easily possible, so that approximated twenty times the definition available with the ordinary microscope can now be attained. We have initiated, in this laboratory, a program of electron microscopy of various tissues and organs the physiological activity of which can be controlled by experimental means, with the intent of correlating changes in function with whatever alteration occurs in the morphological elements of cells. The present investigation concerns the submicroscopic structure of the rat’s thyroid gland. In the following passages we shall describe the appearance of the thyroid follicular cells in states of inactivity and in stages during...

In the course of the last 20 years the world medical literature has reported several instances of the difficulties of interpreting microscopic alterations in thyroid tissue when the patient had been previously treated with radioactive iodine. Nonetheless, these microscopic findings have never been adequately described or produced experimentally. It is generally felt that in the human patient they are not preneoplastic; however, in the rat, many authors have reported the production of thyroid neoplasms by the use of iodine 131.In the present paper the authors have given a clear description of the microscopic changes involved, and report on their experimental production in the rat thyroid.They discuss the significance of these alterations in human patients and in the rat.