Ultrastruktura nabłonka jelita środkowego wybranych gatunków dwuparców (Myriapoda, Diplopoda) ze szczególnym uwzględnieniem procesów jego degeneracji oraz regeneracji

The monophyletic taxon Diplopoda, which is classified as a class, is one o f the four taxa within the monophyletic subphylum Myriapoda that plays an important role in terrestrial ecosystems. Millipedes represent important members o f soil macrofauna due to their role in the soil forming processes that are connected with the biodégradation and fragmentation of dead plant material and other organic matter. As the material for study, two millipede species: Archispirostreptus gigas and Julus scandinavius have been selected. A. gigas commonly called as the giant millipede, is one o f the largest known millipedes and is widespread in the south-eastern regions of Africa. J. scandinavius is distributed mainly in Central and Northern Europe with a marginal extension into the Balkan and East European regions, so it is a representative o f our native fauna. Millipedes are an object of many toxicological and ecophysiological studies, however, knowledge about the ultrastructure and physiology o f the digestive system of these animals is still incomplete. The research was carried out to obtain information on the details o f the ultrastructure o f midgut epithelium and to describe the physiological processes which take place in the midgut of above-mentioned species. The material was analyzed using light, fluorescence and transmission electron microscopies. The results o f this study have shown that in both millipedes examined here, the midgut has the tube-shaped internal duct that spreads along the entire length of the middle region of the body. It is surrounded by visceral muscles and hepatic cells (liver cells) distributed along its length. Hepatic cells form the fusiform junctions that protrude into the basal regions of the midgut epithelium. These cells play an important role in the storage and transport o f backup materials and are responsible for the accumulation and neutralization o f harmful substances that originate from the metabolism or food. The midgut is lined with a pseudostratified columnar epithelium in which all cells come into contact with the basal lamina. The midgut epithelium is lined with coated multi-layer perytrophic membrane with a regular grid pattern. Three types o f cells can be distinguished among the epithelial cells: digestive cells, secretory cells and regenerative cells. The most numerous cells are digestive cells which have a columnar shape and a distinct regionalization in organelles distribution as has been described for other arthropods. The basic role o f these cells is the digestion and absorption as well as the transport and accumulation. They are responsible for the synthesis and secretion. Substances that are produced and accumulated in the cytoplasm o f the digestive cells are released into the midgut lumen due to microapocrine secretion in both species, while the apocrine secretion was observed, only in J. scandinavius. Histochemical stainings showed that lipids, polysaccharides and glycolipids accumulated in both o f the species that were analyzed. However, proteins were detected only in J. scandinavius. The second type of cells in midgut epithelium are regenerative cells, which are individually distributed along the entire length o f the midgut in the basal regions of digestive cells. The mitotic divisions o f the regenerative cells in J. scandinavius and A. gigas result in the formation of two cells. One o f them is the regenerative cell as it is, and another differentiates into the digestive cell. Thus, the divisions of regenerative cells are asymmetric. Regenerative cells in both species could play the role o f unipotent midgut stem cells. Secretory cells are also individually dispersed among the basal regions o f the digestive cells. They do not contact the midgut lumen, and therefore they are o f a closed type. Their ultrastructure is similar with ultrastructure of endocrine cells in insects. They are characterized by the presence of numerous granules o f different electron density in the entire cytoplasm. In J. scandinavius, all secretory cells have the same structure, whereas in A. gigas two types of these cells have been distinguished. The processes o f degeneration o f the central epithelium occur through autophagy, apoptosis and necrosis. These types o f cell death are typical, also described in other invertebrates. In both the species examined here, the autophagy could be induced by changes in the ultrastructure and function o f cell organelles and the presence o f potentially toxic substances. It is the kind o f strategy o f cell survival. Apoptosis is the most common type o f cell death in the midgut epithelium o f J. scandinavius. It is a major process o f removing disrupted and damaged cells from the epithelium and it participates in homeostatic maintenance. In A. gigas mainly necrosis is responsible for described function, while the apoptosis is less frequent