Lymphatic system of the thyroid gland in the rat

Intraglandular thyroid lymph vessels in the rat were studied by qualitative and quantitative analyses in order to obtain information regarding their structure, distribution, relationships, and possible mechanisms of lymph formation. Ultrastructurally, the lymphatic vessels were similar to those described in other organs. The volume density of the lymphatic vessels was 0.007, the profile density 5.68 mm2 and the maximum diameter 17.87 nm. Ultrastructurally, visible transport pathways across the vessels appear to be represented by intracytoplasmic vesicles and channels between endothelial cells. The mean maximum diameter of the vesicles was 96 nm and they occupied 6.9% of the cytoplasm. They were equally distributed between luminal, abluminal, and intracytoplasmic positions. Open junctions (greater than 30 nm) were not seen between endothelial cells, but dilations along part of the length of interdigitating and overlapping contacts were frequent. It was concluded that the mechanism of lymph formation in the thyroid is similar to that in the kidney and liver, but differs from that in the dermis or diaphragm. However, the volume density of the vesicles of the thyroid was twice that of the liver and more than twice that of the kidney. This finding is consistent with an increase in transendothelial vesicular transport of macromolecules.