|Abstrakt: ||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.|