The endothelium of initial lymphatics during postnatal development of the rat tongue.

The luminal surface of the subepithelial lymphatic plexus in the tongue of rats was investigated with SEM at different stages of postnatal development. In newborn and infant animals prominent and branched endothelial cells exhibit a conspicuous phenomenon producing a very irregular inner profile of the vascular wall. Among these cells the spindle-shaped type proves to be an essential component of the valve structures already found in few-day-old animals. There are also prominent cells with a polygonal appearance resembling histioblasts which form with their manifold processes, that partly extend into the lumen, an interlacing cellular pattern. The special morphological characteristics of the endothelium of the initial lymphatics already occurring in early postnatal development point to several particular functional activities such as controlled permeability, contractility, phagocytotic property, and demonstrate the exceptional position of this kind of vessel among the structures of the lymph- and blood-draining system.