The oviduct: A key organ for the success of early reproductive events

Fertilization in a large number of animals occurs in a specific region of the female genital tract called the oviduct (uterine tube or Fallopian tube), which adjoins into the uterus and is located close to the ovary (Figures 1 and 2). The oviduct is a complex fibromuscular conduct with several layers comprising the mucosa, muscular layer, and a connective serosa. The size of these different layers depends on the anatomical region of the oviduct observed. In the ampulla, where the fertilization takes place, a highly folded mucosa was observed; however, the size and the number of the folds are reduced in the isthmus region and even more so at the uterine tubal junction (Figure 2c). The isthmus region is commonly associated with the storage of sperm before ovulation. Very important events for fertilization take place in the oviduct. For example, the oviductal environment is responsible of the final maturation of female and male gametes, fertilization, and early embryo development. It is important to take into consideration that early embryos spend several days in the oviduct before they reach the uterus, where implantation occurs. Consequently, the oviduct is a dynamic organ adapted to different situations that are mainly regulated by the varying hormone levels in the blood. The understanding of the oviductal secretions in which gametes and embryos are temporarily placed is continually