Ovary organization and oogenesis in the tardigrade Macrobiotus polonicus Pilato, Kaczmarek, Michalczyk & Lisi, 2003 (Eutardigrada, Macrobiotidae): ultrastructural and histochemical analysis

Abstract

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.

The role of autophagy in the midgut epithelium of Parachela (Tardigrada)

“…Apoptosis was not observed in the midgut epithelium of any of the species examined here. However, we have observed apoptosis in the gonads of Isohypsibius granulifer granulifer (Poprawa et al 2015), H. dujardini, Thulinius ruffoi and Richtersius coronifer (our studies, data not published). Therefore, we can state that apoptosis is a process that is common for tardigrade tissues.…”

“…Dehydration and embedding were performed as described earlier (Rost-Roszkowska et al 2013a, b; Poprawa et al 2015). Semi- and ultrathin sections were cut on a Leica ultracut UCT25 ultramicrotome.…”

Body cavity cells of Parachela during their active life

“…During oogenesis, the accumulation of the reserve material (yolk) in the cytoplasm of the oocyte (future egg) occurs. In tardigrades, the precursors of yolk are primarily synthesized by the oocyte and the trophocytes, which are sister cells of the oocytes (they originate from the germ line; Węglarska, , ; Poprawa, ; Poprawa et al ., ; Poprawa, Hyra & Rost‐Roszkowska, ; Poprawa et al ., ). Additionally, the cells of the midgut epithelium can participate in the synthesis of vitellogenins.…”

“…Isohypsibius g. granulifer is found in freshwater habitats (Thulin, ; Węglarska, ), whereas H. dujardini is commonly present in mosses in damp and shady areas (Kinchin, ). Macrobiotus polonicus is collected from mosses (Pilato et al ., ; Poprawa et al ., ), whereas X. pseudohufelandi is present in drier habitats such as sandy soil (I. Poprawa, pers. observ).…”

“…Twenty‐five adult specimens of each species ( H. dujardini , I. g. granulifer , M. polonicus , and X. pseudohufelandi ) were fixed in 2.5% glutaraldehyde in a 0.1 M phosphate buffer, pH 7.4, and postfixed in 2% osmium tetroxide in the same buffer. The fixation and subsequent dehydration were carried out in the same manner as was described previously (Rost‐Roszkowska, Poprawa & Kaczmarek, ; Rost‐Roszkowska et al ., ; Poprawa, Schlechte‐Wełnicz & Hyra, ) and lastly the material was embedded in an Epoxy Embedding Medium Kit (Sigma, St Louis, MO). Semi‐ (800 nm thick) and ultra‐thin (50 nm thick) sections were cut on a Leica Ultracut UCT25 ultramicrotome.…”