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.

In Hypsibius dujardini (Doy ere, 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 buccalpharyngeal 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.

The female reproductive system, the process of oogenesis, and the morphology of the egg capsule of Macrobiotus polonicus were analyzed using transmission and scanning electron microscopy and histochemical methods. The female reproductive system of Macrobiotus polonicus consists of a single ovary and a single oviduct that opens into the cloaca. The seminal receptacle filled with sperm cells is present. The ovary is divided into two parts: a germarium that is filled with oogonia and a vitellarium that is filled with branched clusters of the germ cells. Meroistic oogenesis occurs in the species that was examined. The yolk material is synthesized by the oocyte (autosynthesis) and by the trophocytes and is transported to the oocyte through cytoplasmic bridges. The process of the formation of the egg envelopes starts in the late vitellogenesis. The egg capsule is composed of two envelopes-the vitelline envelope and the three-layered chorion. The vitelline envelope is of the primary type while the chorion is of a secondary type. The surface of the chorion is covered with conical processes that terminate with a strongly indented terminal disc.

Germ cell cluster organization and the process of oogenesis in Dactylobiotus parthenogeneticus have been described using transmission electron microscopy and light microscopy. The reproductive system of D. parthenogeneticus is composed of a single, sac-like, meroistic ovary and a single oviduct that opens into the cloaca. Two zones can be distinguished in the ovary: a small germarium that is filled with oogonia and a vitellarium that is filled with germ cell clusters. The germ cell cluster, which has the form of a modified rosette, consists of eight cells that are interconnected by stable cytoplasmic bridges. The cell that has the highest number of stable cytoplasmic bridges (four bridges) finally develops into the oocyte, while the remaining cells become trophocytes. Vitellogenesis of a mixed type occurs in D. parthenogeneticus. One part of the yolk material is produced inside the oocyte (autosynthesis), while the second part is synthesized in the trophocytes and transported to the oocyte through the cytoplasmic bridges. The eggs are covered with two envelopes: a thin vitelline envelope and a three-layered chorion. The surface of the chorion forms small conical processes, the shape of which is characteristic for the species that was examined. In our paper, we present the first report on the rosette type of germ cell clusters in Parachela.