Mature eggs from T. solium and T. saginata are indistinguishable. As shown in Figure 23.3, they can be seen under the light microscope as 50 ^ diameter spheres, with a striated border corresponding to the keratin envelope and frequently a triple set of hooks within. Their presence in stool is diagnostic of Taenia sp. Electron micrographs of sections through these eggs illustrate the oncospheres with a number of cells; the hooks and several cell types have been identified (Figure 23.4). In addition, the embryo is surrounded by several membranes. The protective envelope (embryophore) is made up of keratin blocks, which are cemented together and become unglued when the egg comes into contact with hydrochloric acid, digestive enzymes and bile in the small intestine, thus liberating the onco-sphere, which can penetrate the intestinal wall and reach the blood or lymphatic vessels of the mesentery, from where it is passively transported to the host tissues.
Once the oncosphere has reached an extracellular site (the mechanisms by which the
oncosphere traverses the intestinal and vessel walls are not understood), it develops to the larval stage, a process that takes about 8 weeks.
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