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

Abstract

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 (T4) 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 T4 on developing rat parotid gland, namely, the distribution of MEC and the proliferation of parenchymal cells. Beginning at age 4 days, pups were given daily subcutaneous injections of low, medium and high doses of T4 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 T4 groups invested mostly acini at 15 days in vehicle/normal glands and mostly intercalated ducts after 10 days in the T4 groups. The proliferative activity of acinar cells and MEC in vehicle/normal glands declined progressively with age and T4 increased the rate of this decline in the MEC in a dose-dependent manner. We conclude that T4 accelerates the translocation of MEC from acini to intercalated ducts, with an important mechanism being the more rapid decline in the proliferative activity of MEC than in acinar cells in the T4 groups. Some of the decline in the proliferative activity of all cells in the high and medium dose T4 groups after 7 days may have been due to dose-related thyroxine toxicity.

Effect of Thyroid Hormone on the Histology of Rat Submandibular Salivary Gland During Postnatal Development

“…It has been reported that to function as a mature gland, the rat salivary glands complete their morphological development postnatally by 7 to 10 weeks 6,21 and during this period alteration in the hormones such as thyroid, androgens and growth hormones also have an influence on the normal completion of salivary gland development. 18 The gross examination of submandibular salivary glands showed no morphological changes in postnatal development. Thyroid hormone administration to the animals did not affect the weight and volume of both the salivary glands suggesting that exogenous T3 administration did not have any significant morphological and histological changes in the submandibular salivary glands of both control and experimental animals.…”

“…5 As the rodent salivary glands complete their development postnatally, any alterations in hormones such as T3 can also have an influence on the normal completion of salivary gland development. 18 Keeping in view the possible hormonal changes induced by T3 on differentiation of salivary GCT's, 5 the present study was designed to investigate the effects of exogenous administration of thyroid hormone on the histology of submandibular gland during postnatal development.…”