Development of respiratory trees in the holothurian Apostichopus japonicus (Aspidochirotida: Holothuroidea).

The development of respiratory trees in the holothurian Apostichopus japonicus has been studied using light and electron microscopy. Primordial respiratory trees emerge in 2-3-mm-long animals (2 months after fertilization). They arise as two independent outgrowths from the dorsal wall of the anterior part of the cloaca. Upon first emerging and throughout the course of development, the left respiratory tree is longer than the right one. A common base develops in 4-mm-long animals (2-3 months after fertilization). In yearlings, the left respiratory tree grows into gaps between the loops of the intestinal tube interlaced with intestinal hemal vessels. The developing coelomic and luminal epithelia have ultrastructural peculiarities. The luminal epithelium of respiratory trees has been shown for the first time to comprise nerve cells and their processes. Characteristic structural features of the epithelia are shown to begin developing as early as in 4-mm-long animals (2-3 months after fertilization). In yearlings, the respiratory trees demonstrate definitive structural patterns and are entirely functional.