A major step in the evolution of eutherian mammals was the formation of the trophoblast, a specialised layer of cells derived early in embryogenesis. From this separate compartment, trophoblast is able to organise its own programme of development within a well-defined time span that is independent of the embryo, thereby enabling it to fulfil unique functions during ontogeny. While trophoblast contributes to the formation of the placenta in all eutherians, the manner by which it does so varies significantly among species (Carter 2001). It is important to recognise the pattern and extent of these variations if dialogue between investigators using human and those using animal material is to be meaningful. The trophoblast cell itself occurs in different forms ranging from uninuclear to multinuclear varieties, the latter appearing either as large giant cells or as a syncytium. Some of the giant cells are polytenic whilst others are polyploid but it is not known why these occur in different locations and time points of gestation in different species.

Trophoblast cells have remarkable growth and invasive properties in vivo, so much so that they resemble neoplastic cells, yet the in vitro culture and propagation of human trophoblast cells have still not met with much success. The recent realisation that some ‘trophoblast’ cell lines presently available are not what they seem to be has raised questions about how these cells should be characterised (King et al. 2000).

Introduction

In the light of the discovery of the major histocompatibility complex (MHC) and its role in transplantation, the seminal essay written by Medawar drew a logical comparison between an allograft and a fetus (Medawar 1953). Despite being non-self, the fetus survives while the transplant is rejected. Medawar himself pointed out that the placenta must play a central role in fetal acceptance as it is the placental trophoblast cells that interface with the mother. Now, over 50 years later, the question how the allogeneic trophoblast survives in the potentially immunological hostile uterine environment remains unanswered. Why is the solution to this problem so elusive? I would argue that comparing the placental/maternal relationship with the graft/host relationship is misleading because the analogy between the two is not as close as it appears to be.

The extent of cellular contact between trophoblast and maternal tissue will vary significantly between species depending on the type of placentation. The immunological problem is likely to be most acute in the deeply invasive haemochorial placenta used in humans so that the adaptation required and the strategy employed in human reproduction would be expected to be different from those of other species. For this reason, animal models are not very useful and extrapolation of data between species has led to much confusion. The present paper is focused on human placentation.