Ultrastructural changes in the midgut epithelium of Hypsibius dujardini (Doyère, 1840) (Tardigrada, Eutardigrada, Hypsibiidae) in relation to oogenesis

In Hypsibius dujardini (Doyère, 1840), the endodermal region of the digestive system, which is called the midgut, spreads along the entire length of the body. Its wall is formed by a simple epithelium that is composed of digestive cells. In this paper, we present the first report on the presence of two groups of midgut regenerative cells that form two ‘epithelial rings’ – anterior and posterior. Additionally, we observed the proliferative abilities of the midgut regenerative cells, thus confirming the statement that they play the role of midgut stem cells. The precise ultrastructure of the digestive and regenerative cells was determined using transmission electron microscopy. Changes in the digestive and regenerative cells were correlated with the different stages of oogenesis. The process of oogenesis in H. dujardini took 4 days (at a temperature of 16 °C). Reserve material gradually accumulated in the cytoplasm of the digestive cells and histochemical staining showed that it primarily contained proteins, polysaccharides and a small quantity of lipids. The reserve material accumulated during vitellogenesis, and it began to decrease during choriogenesis. During the simplex stage, when the entire buccal–pharyngeal apparatus was expelled from the body, the stages of oogenesis were advanced, the midgut was much reduced, and the reserve material was exploited by the animal.