Oogenesis in ovipositing females of the microtrombidiid mite Platytrombidium fasciatum (C.L. Koch) (Acariformes: Microtrombidiidae)

Abstract

Summary The process of oogenesis in ovipositing females of the microtrombidiid mite Platytrombidium fasciatum (C.L. Koch, 1836) was investigated by means of transmission electron microscopy. Two ovaries do not fuse at their posterior ends and may be conventionally divided into centrally located compact germarium where oocytes undergo previtellogenesis and prominent peripheral vitellarium consisting of growing oocytes protruding on either sides of the ovaries into surrounding tissues are encompassed only by a basal lamina (tunica propria) (solitary type of oogenesis). Previtellogenic oocytes subsequently grow and contain multiplying mitochondria and prominent Golgi complexes and are characterized by a high rate of synthetic activity. To the end of previtellogenesis, large oval oocytes, tightly packed with elements of rough endoplasmic reticulum and free ribosomes and containing large nucleus, gradually migrate to the ovarian boundaries, coming into contact with a basal lamina. At the onset of vitellogenesis, oocytes form short microvilli on their surface. The processes of yolk accumulation and vitelline envelope formation begin simultaneously and carry out due to endogenous sources. Yolk granules are deposited within the membrane profiles of endoplasmic reticulum and originally occupy a zone just beneath a rim of the cortical ooplasm. In addition to yolk bodies, lipid inclusions are accumulated in the cytoplasm. In the late vitellogenesis, the rarely observed process of extraoocytic obtaining of yolk precursors via pinocytotic activity adds to the main mode of yolk accumulation. The vitelline envelope formation proceeds also intraoocytically due to activity of Golgi complexes, which produce electron-dense globules during early vitellogenesis and small vesicles in the late period. They fuse with plasmolemma to form a vitelline envelope that possesses two distinct layers and in mature egg is devoid of pore canals.