[Placentation in the human: a transplantation or tumor model?]

In human placentation, anchoring villi develop to attach the placenta to the wall of the uterus. This attachment is brought about by extensive infiltration of the maternal tissue by cytotrophoblast cells of fetal origin. As trophoblast cells do not express classical histocompatibility antigens (HLA antigens), increasing doubt has been cast upon the "transplantation" model of pregnancy. However, more recently discovered, previously unknown HLA antigens on the invasive cytotrophoblast cells could lead to maternal immune responses similar to those observed following organ transplantation. Nevertheless, the biological behaviour of the invasive cytotrophoblast cells suggests more parallels with the processes of invasion and metastasis seen in malignant tumours, although there is regulation of the timing and extent of cytotrophoblast invasion of the uterus in normal pregnancy. In this survey, findings relating to the control of infiltration by the cytotrophoblast are reviewed, with particular reference to immunology and tumour biology. Possible effects on pregnancy and parturition due to failure of the regulatory processes involved in placentation are discussed.