In vitro models used to study implantation, trophoblast invasion and placentation : A review

Abstract

hnplantation In The Human Is Unique Despite the fact that some examples of viviparity exist in invertebrates, fish, amphibians and reptiles, implantation is a relatively new acquisition in evolution. The establishment of an intimate trophic co1~nection between mother and embryo is a characteristic of mammals. Implantation is a new strategy in reproduction which allows the development of a small number of embryos in the protective maternal organism. This became possible with the establishment of a functional uterus. Although viviparity is an evolutionary advantage, it has one important limitation: the absolute necessity of a synchronization between embryonic and uterine development. The uterus of almost all mammals is ready for implantation only during a limited period of time (implantation window). Before or after that period the uterus is either indifferent or hostile to the embryo. The period during which an embryo is capable of implanting is usually much longer but varies considerably from species to species. Implantation can be superficial or interstitial according to the extent of blastocyst penetration into the endometrium. Implantation is said to be lateral, mesometriaI, or antimesometrial according to the orientation of the embryo in the uterine cavity. In humans, implantation is interstitial and antimesometrial: the embryo implants deeply in the body of the uterus, most frequently in the upper part of the posterior wall near the mid sagittal plane (Boyd and Hamilton, 1970). In all mammals, the implantation process can be divided in two phases: 1) an attachment phase and 2) a penetration phase. The attachment phase consists of 2 steps: apposition and adhesion. During apposition, no visible connections are established between the blastocyst and the endometrium, and the blastocyst can be dislodged by simple washing of the uterine cavity. Adhesion is the step during which functional connections are established although the nature of these connections is still very speculative. The attachment phase is common to almost all mammals with noticeable differences: in rhesus monkeys attachment occurs simultaneously at two opposite poles of the blastocyst (Enders et al., 1983). The penetration phase proceeds along three general lines with minor modifications. In intrusive penetration as in the ferret, the guinea pig, or the rhesus monkey penetration occurs by thin folds of trophoblast (called invadopodia) progressing between adjacent epithelial cells. In displacement penetration as in the rat and mouse, the trophoblast phagocytes dead epithelial cells and progresses to the basal lamina. Epithelial cells die by