| Abstrakt: | Tardigrades are poorly known group of microscopic Ecdysozoa. Together with
Onychophora and Arthropoda, tardigrades belong to monophyletic group called Panarthropoda.
Up till now tardigrades do not play any important role in practical or economical meaning.
However, this small animals attract more and more scientist due to their special abilities. Thanks to
stabilization of lipid membranes and nucleic acids they are able to withstand freezing, extreme
dehydration, exposure on vacuum or ionizing radiation. Better understanding of their biology
and how they are built is essential not only for tardigradologists, zoologists or evolutionary
biologist, but also for their potential applications for instance medicine, pharmacology, food
industry, pharmaceutical industry or even astrobiology. Our knowledge about the mechanisms of
tardigrade anabiosis has led scientists to the development of dry vaccines that are used widely in Third
World countries.
Species Hypsibius exemplaris, examined in this PhD thesis, has been recognized in
2007 as a new model organism. The material was analyzed by using different methods such as
a light microscope, scanning electron microscope, transmission electron microscope, confocal
microscope as well as a new method called SBEM - a serial block face scanning electron
microscope. H. exemplaris belongs to class Eutardigrada, order Parachela. It possesses a single,
sac-like ovary, located above a midgut. The top part of the ovary is attached to a body wall by
two ligaments of terminal filament. The wall of the ovary consists of flat epithelial cells
supported on non-basal lamina. The apical part of these cells is localized to the ovary interior.
There can be distinguished two zones in the ovary: the smaller one, situated in the top part of
the ovary called germarium, and the second - bigger one called vitellarium. In the ovary of H.
exemplaris there are only germ line cells such as: oogonias, oocytes and trophocytes. In
vitellarium cells create a germ cell clusters branched type, thanks to incomplete cytokinesis.
During oogenesis process only one cell of germ cell cluster develop into oocyte while the rest
cells support oocyte development as a trophocytes. In H. exemplaris occur a mixed
vitellogenesis. Yolk precursors occur thanks to authosynthesis in oocyte as well as thanks to
heterosynthesis - synthesized in trophocytes, storage cells and in epithelial cells of midgut. Just
before oviposition cell bridges which connect oocytes with trophocytes became closed, then a
cell death of trophocytes occur thanks apoptosis. Choriogenesis begins during the late
vitellogenesis. As first envelope is deposited chorion due to oocyte and ovary wall activity, then
vitelline envelope is synthesized thanks to oocyte activity. According to egg envelope
classification established by Ludwig (1874) chorion is a mixed type - primary (secreted by the
oocyte) and secondary (secreted by the cells of ovary wall) while vitelline envelope is a primary
type. A complete egg capsule of H. exemplaris is built of thin vitelline envelope adhering to
oolemma and three-layer chorion. Oviposition of investigated species is connected with
moulting. A female shed an old cuticule and laid eggs inside it. During moulting tardigrade
get rid of cuticule covered body outside as well as cuticule covered foregut and hindgut. H.
exemplaris eggs are covered with smooth chorion, it doesn’t have any conical processes
distinctive for tardigrade ornamented eggs. |