Oviduct: roles in fertilization and early embryo development

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   Figure 1

   Oviduct-guided fertilization. The oviduct regulates fertilization through sperm guidance and sperm hyperactivation. The sperm guidance is achieved through rheotaxis, thermotaxis and chemotaxis. Rheotaxis is created by tubal fluid, which generates a current flow from the ampulla toward the isthmus of the oviduct. Sperm swim against this current based on the physical rotation of the flagella upon CatSper (Cation channel of Sperm) activation. Thermotaxis is mediated through a Ca²⁺-sensing transient receptor potential channel (TRPM8) and G protein-coupled receptor (opsins). This thermal sensibility of human sperm can detect a difference of 0.006°C (Bahat et al. 2012). Temperatures depicted (37°C vs 39°C) are from the finding in rabbits (Bahat et al. 2003). Chemotaxis is driven through progesterone (P₄) released from the cumulus cells and through small cytokines found in the follicular fluid. Together, these processes provide guidance for sperm to swim toward the eggs and be competent for fertilization. COC, cumulus-oocyte complex.
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   Figure 2

   The influence of the oviduct on embryo development. (A) The main energy supply for the embryos is pyruvate and lactate. During the very early stage of embryo development, pyruvate and lactate are provided by the oviductal fluid as energy sources for oxidative metabolism. The oviduct can also supply the glycogen as an energy source for the embryos during the cleavage stage. Amylase (AMY2B) is produced within the oviductal epithelial cells and converts glycogen to sugar. At this stage, the mitochondria of the embryos are immature and do not function. During morula and blastocyst stages, the mitochondria are fully mature and can use oxygen and glucose to produce their own energy via glycolysis as they leave the oviduct. (B) At the same time, oviductal epithelial cells provide embryotrophic factors, such as growth factors, to promote cleavage and embryo development. EGF, epidermal growth factor; FGF, fibroblast growth factor; IGF, insulin-like growth factor; TGF, transforming growth factor.
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   Figure 3

   Oviduct-mediated embryo protection against stress and immune responses. Oviductal epithelial cells produce heat shock protein family (HSP25 and HSP70) to handle heat stress. Oviductal fluid also contains catalase, superoxide dismutases (SOD1/2) and glutathione peroxidase (GPX4) to reduce stress of the embryos from reactive oxygen species. In addition to stress, the oviduct also protects the embryos against their own immune system, partly through the E₂/ESR1 signaling, by inhibiting the production of antimicrobial peptides and excess protease activity. ESR1, estrogen receptor α.
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   Figure 4

   Roles of the oviduct during embryo transport. Embryo transport is composed of tubal muscle contraction and the motility of ciliated epithelial cells. Estrogen (E₂) generally increases the tubal muscle contraction, tubal fluid secretion, and ciliary beat frequency (CBF), which accelerate the embryo transport rate. Opposite to E₂, progesterone (P₄) causes muscle relaxation and decreases CBF to reduce the embryo transport rate. In addition, P₄ also inhibits E₂-induced tubal fluid production. Prostaglandins (PGs) can be produced in the oviductal epithelial cells or induced by E₂ treatment. PGE₂ and PGF_2α stimulate muscle contraction in both human and bovine oviducts. Endothelin 1 and 2 are expressed and contribute to the oviduct contraction. IP3, inositol triphosphate; PGE₂, prostaglandin E₂; PGF_2α, prostaglandin F₂; PGR, progesterone receptor.
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   Figure 5

   Essential roles of the oviduct in assisting embryo development. Schematic cross-section of the oviduct and the embryo with their interaction regulating embryo development. The interactions are divided into five domains: protection, cleavage and development, transport, nutrients and epigenetic regulation. E₂, estrogen.

• © 2017 Society for Endocrinology