Ultrahistology of oogenesis and vitellogenesis in the spider mite Tetranychus urticae.

Histology of the ovary of the spider mite Tetranychus urticae is described light and electron microscopically with special reference to oogenesis and vitellogenesis of this mite. Morphology of the ovary is comparable to the typical sac-like chelicerate ovary with oocytes protruding from the ovarian surface, thus resulting in a grape-like appearance. According to different oogenetic stages, a germ, pre-vitellogenic and vitellogenic region can be observed. Oogonia and primary oocytes characterized by extranuclear material or 'yolk nuclei' are situated in the germ region. Primary oocytes develop into three-nucleated nurse cells situated in the periphery of the pre-vitellogenic region, and into pre-vitellogenic oocytes protruding from the ovarian surface. Growth of oocytes is performed while they are in ovarian pouches by uptake of nurse cell cytoplasm and following extraovarian yolk precursors. Intraoocyte yolk synthesis interpreted from altered cytoplasmic organelles also occurs. Processes taking place during oogenesis and vitellogenesis in T. urticae are compared to published information on yolk synthesis of other animal species.