Abstract
Vitellogenin is the main yolk precursor protein in insect oocytes. It is synthesized in the fat body and released into the hemolymph. To reach the oocyte surface, vitellogenin must cross a single layer of follicular epithelium cells. The transport of vitellogenin across the follicular epithelium has been suggested to occur through the enlarged intercellular spaces (patency) by a paracellular route or by endocytosis by follicular cells and release onto oocyte surface in a transcelluar route. In this study, we investigated whether vitellogenin transport in the meroistic telotrophic ovary of Podisus nigrispinus (Hemiptera) occurs via a paracellular or transcellular route. Light and transmission electron microscopies showed that short cell–cell contacts with well-developed occluding septate junctions were present in follicular cells with patency. Immunofluorescence microscopy revealed the presence of vitellogenin receptors in the plasma membrane and of vitellogenin in the cytoplasm of follicular cells. Data suggest that cell–cell contacts serve as a barrier to large vitellogenin molecules and that this protein is transported via a transcellular route of receptor-mediated endocytosis.
Highlights
Vitellogenin is the main yolk precursor protein in insect oocytes
Our findings show that the follicular epithelium in vitellogenic follicles of P. nigrispinus is differentiated into columnar and juxtaposed cells at the anterior and posterior poles, whereas follicular cells in lateral regions are cubic with dilated intercellular spaces
A similar morphology was observed in R. prolixus (Hemiptera) at the onset of vitellogenesis, when cells of the follicular epithelium at the anterior and posterior poles are morphologically distinct from cells in the lateral portion[41]
Summary
Vitellogenin is the main yolk precursor protein in insect oocytes It is synthesized in the fat body and released into the hemolymph. Insects have two LBDs with different numbers of class A cysteine-rich ligand binding repeats (LBRs)[16,17,18,19,20] These VgRs have been reported to combine with some different ligands in vertebrates[21], and their mRNA are detected in the hypopharingeal glands[22] and fat body[16,23] in insects, suggesting that VgR may recognize other ligands, but in insect ovary vitellogenin is the unique ligand of VgR endocytosed by oocytes[11,13,14,15,24]. To reach the oocyte surface, vitellogenin must be transported through the follicular epithelium[25], which surrounds the oocyte[2,6] and regulates the flow of vitellogenin to the oocyte[5,7]
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