Figure 1: Proposed mechanisms of tubal ectopic pregnancy in humans. Bidirectional inputs—stimulation (yellow arrow) and inhibition (red bar)—may affect output responses of the Fallopian tube during early embryo transport. Note that not all proposed cellular and paracrine processes are necessary for embryo implantation in the Fallopian tube under the same tubal environment. CEC, ciliated epithelial cells; SEC, secretory epithelial cells; SM, smooth muscle cells; BV, blood vessel; Lu, lumen; Epi, epithelium.