Ultrastructural studies of early stages of oogenesis in a trichuroid nematode,Trichuris muris

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Abstract
Oogenesis within the hologonic ovary of the trichuroid nematode, Trichuris muris, was observed by light and electron microscopy. Early germinal stages in the form of oogonia and young primary oocytes were characterised by a high nuclear-cytoplasmic ratio, numerous ribosomes and several mitochondrial clusters. Previtellogenic primary oocytes contained a prominent nucleus with a nuclear envelope punctuated by pores. They also contained increased amounts of granular endoplasmic reticulum (GER), often arranged as annulate lamellae, several Golgi complexes and limited amounts of lipid. The appearance of three types of cytoplasmic inclusion, in the form of lipid, dense yolk granules and reticulate granules, indicated the onset of vitellogenesis. At this stage of oogenesis, all three types were distributed throughout the ooplasm. The possible role of the granules is discussed. During passage along the oviduct the oocyte was coated by an additional unit membrane and associated fibrillar layer external to the oolemma. It is suggested that this may be synthesised by the oocyte.