Ultrastructure and lectin-binding pattern of the infundibulum of mare oviduct

The oviduct plays an essential role in the mammalian reproduction. The epithelium of the entire oviduct is of the simple columnar type and consists of ciliated (CCs) and non-ciliated cells (NCs). The CCs play a role in the transport of germinal cells, whereas NCs are considered secretory cells. Oviductal secretory glycoproteins are involved in the constitution of the luminal fluid together with a selective trasudate of serum as well as in the egg-epithelium interaction. The fimbriated infundibulum, is considered to be primarly responsible for introducing the ovulated eggs to oviductal fluid and for transport the ovulated eggs to the ampulla, where fertilization and early cleavage-stage embryonic occur. The objective of the present study was to investigate the ultrastructural features and the glycoconjugate pattern of the epithelium lining the infundibulum of mare oviduct. Fimbriae fragments from oestrus mares were processed for scanning (SEM) and transmission (TEM) electron microscopy, and for lectin histochemistry. SEM and TEM evidenced NCs covered either with short microvilli or apical protrusions. Both CCs and NCs showed electron-dense and electron-lucent cytoplasm. The prominent ultrastructural feature of CCs cytoplasm was the presence of numerous mitochondria in the apical region. The apical region and the protrusions of NCs contained secretory granules with different electron-dense matrices, although in some tract of the infundibulum the NCs lacked granules. The cytoplasm of secretory cells showed RER and a well developed Golgi complex. Lectin histochemistry revealed the presence of glycans with terminal Neu5acα2,3Galβ1,4GlcNAc, Neu5Acα2,6Gal/GalNAc, Galβ1,3GalNAc, Forssman pentasaccharide (FP), α/βGalNAc, αL-Fuc as well as oligosaccharides with terminal/internal αMan, and GlcNAc (MAL II, SNA, PNA, DBA, SBA, HPA, UEA I, Con A, WGA, and GSA II reactivity) on the luminal surface of both CCs and NCs. The apical protrusions of NCs did not express glycans terminating with FP (DBA affinity). The presence of lectin-binding sites in the apical protrusions suggests that NCs secrete glycoproteins into the oviduct. These molecules may contribute to the functional specificity of the infundibular segment. Sialoglycoconjugates could regulate egg-epithelium interactions by virtue of their negative charge during flow of the oocyte to the ampulla, whereas asialoglycoproteins could modify the zona pellucida properties during the oocyte transit.