1997 - Volume: 38 Issue: 2 pages: 141-151
Date published: 1997-05-31
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Keywords: Trombiculidae, cytology, ultrastructure, oogenesis
The developing oocytes in ovipositing females of the trombiculid mite Hirsutiella zachvatkini (Schluger, 1948) have been investigated during the course of vitellogenesis, using transmission electron microscopy. The external surface of the egg-shell was also examined under the scanning electron microscope. Oocyte growth may be divided into small growth, or previtellogenesis, and great cytoplasmic growth, or vitellogenesis. Growing oocytes subsequently migrate to the periphery of the ovaries, rupture the basal membrane and often protrude into the surrounding tissues. Late previtellogenic oocytes are rich in mitochondria and are characterized by the formation of curved microvilli on their surfaces. A fine-granular substance of low electron density, referred to as primary vitelline envelope, begins to deposit above the tips of microvilli. The short initial step of yolk accumulation is carried out intraoocytically by small Golgi complexes, scattered throughout the cytoplasm, during late previtellogenesis. Later in vitellogenesis, yolk precursors are accumulated by the oocyte from an extraoocytic source by high endocytotic activity, which is reflected in the presence of large numbers of coated pits and vesicles in the cortical ooplasm. Proteinaceous yolk bodies are formed by the fusion of endosomes and finally, as mature yolk bodies, they have crystalline cores. In addition to yolk bodies, lipid inclusions are deposited in the ooplasm. At the onset of vitellogenesis, Golgi complexes migrate peripherally to a position near a rim of cortical cytoplasm and give rise to many electron-dense vesicles which seem to deposit the secondary vitelline envelope, initially in the form of electron-dense spots between the microvilli. The external layer of the secondary vitelline envelope is formed by the secretory activity of the somatic cells which encircle the oocyte towards the end of vitellogenesis.