Introduction

The preimplantation phase of pregnancy occurs from the time of fertilization until the embryo implants into the endometrium of the uterus. Implantation occurs on days 5–6 of pregnancy in humans. Implantation triggers differentiation of the embryo's trophoblast layer, leading to a sequence of developmental changes that produce the placenta. Prior to implantation, the embryo exists in a freeliving state, initially within the oviduct (fallopian tube) and then in the lumen of the uterus. It has generally been considered that the embryo is passive during this time, relying upon the reproductive tract for the provision of its nutrients and a benevolent environment. However, it is now becoming clear that the local environment of the reproductive tract is conditioned and modified by the early embryo and these modifications are important for normal embryo development. These modifications include the activation of blood platelets within the reproductive tract and are elicited by the production of soluble and diffusible factors by the early embryo. The best characterized of these is platelet-activating factor (PAF).

Embryo-derived platelet-activating factor

The well-known ether phospholipid 1-o-alkyl-2-acetyl-snglyceryl- 3-phosphocholine (PAF) is a potent pleiotrophic mediator (Fig. 58.1). It is produced by the preimplantation embryo of many species soon after fertilization (mouse; human; rabbit; and sheep). It accumulates to significant concentrations within the embryo and is also released into the embryo's surrounding fluid. The production of embryo-derived PAF seems to be largely de novo occurring from simple substrates.