Different Calcium Requirements for Low Iodine Diet (LID) Induced and Developmental Growth of the Rat Thyroid: Evidence from in Culture Experiments
In vivo, in post-embryonic life, most mammalian cells are growth arrested. Considered a few years ago as the main target of extracellular Ca2+ on the cell cycle, initiation to division is not believed to be extracellular Ca2+-dependent (1). Initiation covers one quarter to one third of G1, the phase of the cycle that precedes DNA synthesis (2). Presence of the cation is required in the incubating media over the second half of G1 (3). Once initiation is accomplished, each phase of the cycle proceeds within a determined time and, if adequate Ca2+ concentration is not provided over the Ca2+sensitive period, initiation vanishes. No DNA synthesis at all occurred with BALBc/3T3 mouse cells, initiated in very low Ca2+, when mMolar Ca2+ was given past three lengths of G1 (3). That in vivo hypocalcemia suppressed cell proliferation in some instances and not in others (4) could be interpreted in two different ways: either that Ca2+ requirements for cell division are different for divisions induced by different mitogens, since it was shown that epidermal growth factor (EGF) reduced Ca2+ requirements for growth of the normal epithelial cell line NP-2s by two orders of magnitude (5); or that alteration of other parameters, such as permeability of the plasma membrane (PM) to the ion and/or cell geometry, affected the rate of Ca2+ collection.
KeywordsThyroid Cell Cell Coat Developmental Growth Thyroid Cell Line Thyroid Research
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