[Fine structure of venules and lymphatics of the dog thyroid gland]

The endothelial cells of the venules (collecting venules) of the thyroid gland of the dog are very thin and show fenestrations. In their wall, however, more 'primitive' smooth muscle cell layers are recognizable. The endothelial cells in the lymphatic vessels are also thin, but continuous. They have all known morphological characteristics of these vessels. The fenestrae of the venules' endothelium could facilitate hormone passage from the interstitial tissue into the blood stream.