Stricker, S. A., Smythe, T. L., Miller, L. and Norenburg, J. L. 2001. Comparative biology of oogenesis in nemertean worms. —Acta Zoologica (Stockholm) 82: 213–230



In order to supplement previous analyses of oogenesis in nemertean worms, this study uses light and electron microscopy to compare the ovaries and oocytes in 16 species of nemerteans that represent various taxa within the phylum. Nemertean ovaries comprise serially repeated sacs with an ovarian wall that characteristically includes myofilament-containing cells interspersed among the germinal epithelium. Each oocyte can attach to the germinal epithelium by a vegetally situated stalk and resides in the ovarian lumen without being surrounded by follicle cells. In the ovary, oocytes arrest at prophase I of meiosis and contain a hypertrophied nucleus (‘germinal vesicle’) that often possesses multiple nucleoli. Intraovarian growth apparently involves an autosynthetic mode of yolk formation in most nemerteans and generates oocytes that measure ~60 µm to 1 mm. When fully developed, oocytes can be discharged through a short gonoduct and are either spawned freely or deposited within egg cases. In most species, oocytes released from the ovary possess extracellular coats and resume maturation by undergoing germinal vesicle breakdown (GVBD). Such post-GVBD specimens also form a punctate endoplasmic reticulum that may facilitate fertilization and development.

Comparative biology of oogenesis in nemertean worms

A modification of the silver staining procedure of Howell and Black (1980) is reported which makes use of teleostean gelatin as protective colloid and renders a high signal-to-noise ratio. We demonstrate that this ratio can be further increased by subsequent attenuation with a chemical reducer consisting of a mixture of potassium ferricyanide (III) and sodium thiosulphate. It is shown that slight changes of the concentration of the reactive compounds of the chemical reducer make the protocol applicable to human, plant (Aliium cepa, Rhinanthus minor) as well as meiotic insect (Acheta domesticus) chromosome preparations. Due to its broad applicability, the method could find utilization in studies on chromatin and chromosome functions in many species.

Increased specificity of colloidal silver staining by means of chemical attenuation.

Structural and functional relationships between nuclear bodies and the nucleolus-DNA body complex in the oocyte of Amphiporus lactifloreus

The nucleus of the premeiotic oocyte from the hoplonemertine Amphiporus lactifloreus contains a characteristic DNA-rich body that rapidly generates a peculiar nucleolar apparatus previously described as the nucleolus-DNA body complex. During early vitellogenesis, the nucleolar complex is centrally located in the nucleus and consists of many spherical and ribbon-shaped components-the nucleolar spherulae and lamellae-which produce hundreds of small peripheral nucleoli and several granular bodies as vitellogenesis progresses. Distributions of the nucleolar organizer regions (NORs) in various nucleolar components were studied by silver staining, and results were compared with previous findings obtained by cytochemical and autoradiographic techniques.The core of each spherula in the nucleolus-DNA body complex is strongly Ag-positive, and so, later on, is a minor part of each peripheral nucleolus. In contrast, the cortices of the nucleolar spherulae, in which slightly decondensed chromatin is distributed uniformly, and the nucleolar lamellae produced through self-peeling of the spherulae, do not react with silver. Thus, the NOR proteins identified by silver deposits correspond to numerous loci at which the most decondensed, transcriptionally active chromatin is made visible by the osmium-ammine reaction. These results are discussed in relation to the functional evolution of the initial DNA body of the oocyte from A. lactifloreus, the transcriptional abilities of its by-products, and the general process of the amplification of ribosomal genes.

Localization of NORS in the nucleolus-DNA body complex of the oocyte from the nemertine Amphiporus Lactifloreus

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Localization of NORS in the nucleolus-DNA body complex of the oocyte from the nemertine Amphiporus Lactifloreus