No abstract

The body cavity cells (storage cells, storage bodies) of four species of Parachela (hermaphroditic Isohypsibius granulifer granulifer, parthenogenetic Hypsibius dujardini, gonochoristic Xerobiotus pseudohufelandi, gonochoristic Macrobiotus polonicus) were analysed during their active life using light, confocal (laser scanning), and scanning and transmission electron microscopy. The ultrastructure of the storage cells confirmed previous studies suggesting a high level of metabolic activity. Additionally, we revealed the participation of the storage cells of H. dujardini, I. g. granulifer, and M. polonicus in the synthesis of vitellogenins. This did not seem to apply for X. pseudohufelandi. All of the species that were examined in this study accumulated polysaccharides, proteins, and lipids in their body cavity cells, but the amount of these components differed in each species. Isohypsibius g. granulifer accumulated a huge amount of polysaccharides and smaller amounts of lipids and proteins, H. dujardini and M. polonicus primarily accumulated lipids and small amounts of polysaccharides and proteins, whereas X. pseudohufelandi primarily accumulated polysaccharides and lipids, and a small amount of proteins.

Tardigrades represent an invertebrate phylum with no circulatory or respiratory system. Their body cavity is filled with free storage cells of the coelomocyte-type, which are responsible for important physiological functions. We report a study comparing the ultrastructure of storage cells in anhydrobiotic and hydrated specimens of the eutardigrade Richtersius coronifer. We also analysed the effect of temperature stress on storage cell structure. Firstly, we verified two types of ultrastructurally different storage cells, which differ in cellular organelle complexity, amount and content of reserve material and connection to oogenetic stage. Type I cells were found to differ ultrastructurally depending on the oogenetic stage of the animal. The main function of these cells is energy storage. Storage cells of Type I were also observed in the single male that was found among the analysed specimens. The second cell type, Type II, found only in females, represents young undifferentiated cells, possibly stem cells. The two types of cells also differ with respect to the presence of nucleolar vacuoles, which are related to oogenetic stages and to changes in nucleolic activity during oogenesis. Secondly, this study revealed that storage cells are not ultrastructurally affected by six months of desiccation or by heating following this desiccation period. However, heating of the desiccated animals (tuns) tended to reduce animal survival, indicating that long-term desiccation makes these animals more vulnerable to heat stress. We confirmed the degradative pathways during the rehydration process after desiccation and heat stress. Our study is the first to document two ultrastructurally different types of storage cells in tardigrades and reveals new perspectives for further studies of tardigrade storage cells.

The process of cell death has been detected in the midgut epithelium of four tardigrade species which belong to Parachela: Macrobiotus diversus, Macrobiotus polonicus, Hypsibius dujardini and Xerobiotus pseudohufelandi. They originated from different environments so they have been affected by different stressors: M. polonicus was extracted from a moss sample collected from a railway embankment; M. diversus was extracted from a moss sample collected from a petrol station; X. pseudohufelandi originated from sandy and dry soil samples collected from a pine forest; H. dujardini was obtained commercially but it lives in a freshwater or even in wet terrestrial environment. Autophagy is caused in the digestive cells of the midgut epithelium by different factors. However, a distinct crosstalk between autophagy and necrosis in tardigrades’ digestive system has been described at the ultrastructural level. Apoptosis has not been detected in the midgut epithelium of analyzed species. We also determined that necrosis is the major process that is responsible for the degeneration of the midgut epithelium of tardigrades, and “apoptosis–necrosis continuum” which is the relationship between these two processes, is disrupted.

Encystment in tardigrades is relatively poorly understood. It is seen as an adaptive strategy evolved to withstand unfavorable environmental conditions. This process is an example of the epigenetic, phenotypic plasticity which is closely linked to the molting process. Thulinius ruffoi is a freshwater eutardigrade and a representative of one of the biggest eutardigrade orders. This species is able to form cysts. The ovoid-shaped cysts of this species are known from nature, but cysts may also be obtained under laboratory conditions. During encystment, the animals undergo profound morphological changes that result in cyst formation. The animals surround their bodies with cuticles that isolate them from the environment. These cuticles form a cuticular capsule (cyst wall) which is composed of three cuticles. Each cuticle is morphologically distinct. The cuticles that form the cuticular capsule are increasingly simplified. During encystment, only one, unmodified and possibly functional buccal-pharyngeal apparatus was found to be formed. Apart from the feeding apparatus, the encysted specimens also possess a set of claws, and their body is covered with its own cuticle. As a consequence, the encysted animals are fully adapted to the active life after leaving the cyst capsule.