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Migration across a stationary pH gradient results in the electrophoretic titration of a protein's dissociable groups. From the resulting curves, some properties of the protein may be derived, including overall amino acid composition and type of mutation between polymorphic variants, as well as range of stability or, for enzymes, of catalytic activity. Analysis with this technique is a stringent purity criterion; other applications allow the study of interacting systems and the planning of chromatographic fractionations based on differences in surface charge.

The European chub, Leuciscus cephalus (Cyprinidae), is a fish species employed in bioindication for the effects of endocrine disruption chemicals (EDC) upon the reproductive system. In applied histopathology, a useful tool in aquatic ecotoxicology, detailed knowledge on the intact structural integration of an organism is an indispensable prerequisite for the diagnosis of pathological alterations. This is particularly evident for gonadal structures in bioindication for the effects of EDCs interacting with the hypothalamus‐pituitary‐gonadal‐liver axis. However, little is known about the histology of L. cephalus until now, especially the extratesticular duct system is widely disregarded. Therefore, in the present study the testicular efferent duct system was described for three phases of testicular development (pre‐spawning, spawning, post‐spawning). Testicular main ducts were analyzed by means of histology and transmission electron microscopy. Additional techniques applied were lectin histochemistry to determine the secretory activity of epithelial cells, as well as immunohistochemistry to demonstrate cell proliferation activity and muscle actin to show distribution and amount of contractile cells. Remarkable variations of the extratesticular ducts mainly concerning the epithelium, were observed during the reproductive phases. Contribution of the main ducts’ epithelia (secretion and cellular detachment) and of degenerating spermatocytes, as sources of carbohydrates and phospholipids, to the seminal fluid were confirmed. Epithelial cell proliferation and secretory activity were dependent on the spawning phase of the specimen. Lectin binding experiments in the present study demonstrated sugar residues (mannose, glucose, galactose, N‐acetyl‐glucosamine) in the epithelial cell cytoplasm and apical glycocalyx, which is important in cell recognition. In the stroma of the testicular efferent ducts, a high number of cells expressed muscle actin and tropomyosin indicating the ducts’ contractile potential for the transportation of seminal fluid towards release. Adjacent to these contractile cells, numerous nerve fibres were found indicating neuronal control of sperm fluid flow.