Exogenous thyroid hormone affects myoepithelium and proliferation in the developing rat parotid gland.

In the mature rat parotid gland, myoepithelial cells (MEC) invest intercalated ducts, but not acini. During postnatal development, however, these cells differentiate around both intercalated ducts and acini, then translocate to only intercalated ducts during weaning. Previously, we found that thyroxine (T(4)) accelerates translocation of cells with small secretory granules from acini into intercalated ducts and the number of apoptotic cells increased tremendously with high doses. We present here additional analysis of the effects of T(4) on developing rat parotid gland, namely, the distribution of MEC and the proliferation of parenchymal cells. Beginning at age four days, pups were given daily subcutaneous injections of low, medium, and high doses of T(4) or vehicle or no injection. At ages 4, 7, 10, and 15 days, glands were excised and processed for light microscopy. Sections were double-immunostained with antibodies against proliferating cell nuclear antigen (PCNA) and actin, and counterstained with hematoxylin. Proliferative activity was assessed via PCNA histochemistry and MEC were identified using actin histochemistry. MEC in the T(4) groups invested mostly acini at 15 days in vehicle/normal glands and mostly intercalated ducts after 10 days in the T(4) groups. The proliferative activity of acinar cells and MEC in vehicle/normal glands declined progressively with age and T(4) increased the rate of this decline in the MEC in a dose-dependent manner. We conclude that T(4) accelerates the translocation of MEC from acini to intercalated ducts and that an important mechanism is the more rapid decline in the proliferative activity of MEC than in acinar cells in the T(4) groups. Some of the decline in the proliferative activity of all cells in the high and medium dose T(4) groups after seven days may have been due to dose-related thyroxine toxicity.