The mammalian oviductal epithelium: regional variations in cytological and functional aspects of the oviductal secretory cells.

The secretory cells in the epithelium of mammalian oviducts produce and release various secretory materials into the lumen. Secretions from such cells provide a suitable environment for the events that occur in the oviductal lumen. This review focuses on the regional differentiation of the secretory cells in mammalian oviducts. Many histological studies have demonstrated regional variations in both the morphological and ultrastructural features of the secretory cells in the oviductal epithelium. Regional differences have been found, for example, in the number of putative secretory granules in the oviductal secretory cells. Histochemical and immunocytochemical studies have also revealed regional differences in the localization of various materials in the oviductal epithelium, suggesting the possibility of regional specificity in the production of various secretory materials by the oviductal epithelial cells. Recent biochemical and immunoelectron microscopical studies have shown that biosynthesis of specific proteins or glycoproteins is associated with region-specific variations in epithelial cells in different oviductal segments. In particular, certain oviduct-specific glycoproteins are produced by secretory cells in specific regions of the oviduct and these glycoproteins may affect fertilization, embryonic development, and sperm functions. The oviductal epithelial cell also provide physiological support to gametes and embryos. The interactions of oviductal epithelial cells with gametes support the development of embryos and the maintenance of sperm functions in vitro. Some studies using coculture systems have suggested regional differences associated with such physiological support by oviductal epithelial cells. Moreover, the expression of functional proteins, such as growth factors, show segmental variations within the oviduct. The regional variations demonstrated in these studies may reflect distinct functional differences among the various segments of the mammalian oviduct. The proposal is presented that despite the fact that the mammalian oviductal tissue is not very complex in terms of structure, the oviductal secretory cells may be highly differentiated along the length of the oviduct.