Yolk organelles and their membranes during vitellogenesis ofXenopus oocytes |
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Authors: | H -P Richter |
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Institution: | (1) Department of Physiology I, University of Saarland, D-6650 Homburg/Saar, Federal Republic of Germany |
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Abstract: | Summary The yolk platelets ofXenopus laevis have been studied by thin-section and freeze-fracture electron microscopy to characterize the boundary membrane during yolk
formation. Throughout vitellogenesis, large yolk platelets are in close contact with smaller nascent yolk organelles. Two
types of primordial yolk platelets (I and II) have been discriminated. After membrane fusion these precursors can be completely
incorporated into the main body of existing platelets, numerous yolk crystals then merge and form one uniformly stratified
core. Lipid droplets are tightly attached to the membrane at all developmental stages of yolk platelets. A direct connection
of endoplasmic reticulum to the membranes of yolk platelets was not observed. On freezeetching replicas, yolk-platelet membranes
present fracture faces with intramembranous particles (IMP) of various sizes and a heterogeneous distribution of approximately
200–600 IMP/μm2 at the E face, and 1200–2100 IMP/μm2 at the P face. Again, this presentation of the membrane exhibits neither anastomoses to the endoplasmic reticulum, nor caveolae
that exclude the uptake of yolk-containing vesicles into these yolk organelles. Proteinaceous yolk platelets tend to fracture
along their periphery through the superficial layers. |
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Keywords: | Vitellogenesis Xenopus oocyte Yolk-platelet membrane Ultrastructure |
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