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 (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.

Fig. 1

Serum total T₄ (μg/dl) by age and treatment. The scale for the vehicle/normal (vehicle-injected, non-injected) pups is on the right multiplied × 40, because the increase with age would appear barely above baseline if set to the scale of the T₄-treated pups. Reprinted from Ikeda et al. (2008).

Fig. 2

Sections of parotid glands from vehicle/normal (column on left; A,C,E) and low T₄-treated (column on right; B,D,F) pups at ages 7 (top row), 10 (middle row) and 15 (bottom row) days. The cytoplasm of MEC and smooth muscle cells in large blood vessels (arrows) are stained red (aminomethylcarbozol chromogen) from immunohistochemical reactions with antibodies to muscle actin. Nuclei in which reactions with antibodies to PCNA have occurred are brown (DAB chromogen). Insert to A) Higher magnification of the two labeled MEC on the right side (the tips of the arrows are retained). The nuclei of both of these MEC are surrounded by anti-muscle actin-positive (red) cytoplasm and thus clearly illustrate an essential criterion for identifying MEC in our study. The MEC on the right has a very brown nucleus, thus is clearly PCNA-labeled, while the nucleus of the MEC on the left is not stained brown and so just as clearly is PCNA-negative. ID, intercalated ducts; LD, large (striated and excretory) ducts. Hematoxylin counterstain. For A-F (except the insert to A), scale bar = 50 μm.

Fig. 3

Sections of parotid glands from medium (left column; A,C,E) and high (right column; B,D,F) T₄-treated pups at ages 7 (top row), 10 (middle row) and 15 (E) and 12 (F) days. The cell identification labels, colors resulting from immunohistochemical reactions and counterstain are the same as in Fig. 2. For A-F. Scale bar = 50 μm.