THE ULTRASTRUCTURE OF THE TRUNK GANGLIA IN MACROBIOTUS RICHTERSI (TARDIGRADA)

The current importance of investigating the anatomical and ultrastructural organization of Tardigrada nervous system is determined in relationship to the early evolution of the arthropods. According to the data by Dewel and Dewel (1996, 1999), the organization of the Tardigrada brain could be best understood by analyzing the pattern of the nervous system in the trunk. On this basis, we analyzed the ultrastructure of the trunk ganglia in the eutardigrade Macrobiotus richtersi. The trunk ganglion is located ventrally under the midgut beneath the delicate visceral muscles and surrounded by the body-cavity. The ganglion has an oval compact shape and distinct basal lamina. Eight perikarya have been found in the investigated portion of the ganglion (25 serial sections were examined). Most of them are located at the periphery and they contact basal lamina directly. At least one of the neurons is situated to some extent deeper and it contacts fringe cells. This neuron projects the process to the center of the neuropile and then passes into the lateral nerve. Usually, perikarya have an oval nucleus and neuroplasm with dense packed organelles such as mitochondria, reticulum and vesicles. Only one cell has irregular-shaped nucleus and a lucent cytoplasm. It is unclear if the Tardigrada nervous system has glial cells or not. Perhaps this lucent cell has a glia-like function in the trunk ganglion of M. richtersi.A pair of nerves goes out from the ganglion and innervates the lateral area of the segment. These nerves have no neuron cell bodies and any other cells like glial cells. The nervous processes are slim, dense-packed and contain mitochondria and synaptic vesicles. A basal lamina surrounds not only the ganglion but nerves too, and consists of fine dense filaments. Rough fibres have not been found. There are numerous neurites containing small clear and rare dense vesicles in the neuropile. Besides in the ganglion there is one large axon which contains clear cytoplasm and dense-core vesicles. Its perikaryon is not found in the ganglion. This axon goes through the ganglion and passes out into a nerve. Many processes in the neuropile have a specific dense neuroplasm with clear small vesicles. We have found a similar pattern in neurites innervating the salivary duct and muscles of the salivary glands and perhaps in the circumesophageal connective of another eutardigrade, Ramazzottius tribulosus. Investigations partially supported by INTAS Grant № 04-83-3807 to A.A.