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Data accumulated over several years of investigating implantation in macaque monkeys have been used to estimate the rate of expansion of the initial implantation site, the increase in volume of the site, and the rate of arterial invasion by cytotrophoblast columns. In addition the expansion of the secondary implantation site has also been estimated. The primary implantation site expands from an average diameter of 0.268 mm on day 10 to 4.93 mm on day 16–17. It expands in thickness from 0.064 mm on day 10 to 0.96 mm on day 16–17, and in volume from 0.0036 mm 3 on day 10 to 18.34 mm 3 on day 16–17. During this period of rapid expansion in extent and volume of the implantation site, trophoblast invades the endometrium, forms a lacunar stage, and initiates villus formation; consequently these very considerable changes in structure occur when the implantation is still very small yet growing rapidly. The secondary site expands from 0.23 mm in diameter on day 12 to 2.8 mm on day 17. The rate of penetration of cytotrophoblast into endometrial arteries diminishes from 0.602 mm per day on day 12 to 0.171 mm per day on day 16, using the straight-line method of estimation. This diminution in rate is consistent with the hypothesis that cytotrophoblast cells generated at the anchoring villi migrate over cytotrophoblast cells that have attached to the endothelium of the endometrial arteries in advancing the intraluminal columns of trophoblast. It is hoped that the summaries provided will be useful to investigators using macaque monkeys to analyze aspects of implantation in primates.

This study examines endovascular trophoblast invasion in pregnancies complicated by complete hydatidiform mole (CM), partial hydatidiform mole (PM) and non-molar abortions (HA). Two hundred consecutive cases from a supra-regional referral centre for suspected trophoblastic disease were examined histologically with particular regard to the presence or absence of endovascular trophoblast invasion of decidual vessels. There were 57CM, 75PM and 68HA. The prevalence of normal endovascular invasion of decidual vessels was significantly lower in CM compared to all other clinical groups, amongst which there were no significant differences. Endovascular trophoblast was identified in about 80 per cent of HA and PM moles, compared to only around 25 per cent of CM (Z=−4.0, P 0.0001). The majority of cases of complete mole demonstrated a similar appearance, of implantation site showing florid interstitial extravascular trophoblast invasion with surrounding of decidual vessels but without normal endovascular trophoblast invasion or ‘plugging' seen. In many cases, there also appeared to be destruction of decidual vessels with interstitial haemorrhage and extensive fresh blood clot present in the histological sections. These findings may provide some explanation regarding the mechanism of clinical findings in molar pregnancy.

The Workshop discussed five topics which were considered to be topical. 1) A comparison was made between rodents and primates regarding how uNK cells are distributed at the implantation site in these species. 2) The possibility that the functions of uNK cells may be different in mice and humans are discussed. 3) Trophoblast MHC expression in the Rhesus monkey (haemochorial) and the sheep (epitheliochorial) was compared in order the gauge the importance of these molecules in these two fundamentally different types of placentation. 4) New data were presented on the intriguing molecule, HLA-G, whose function is still unclear. 5) A discussion was initiated to debate the role of the innate and the adaptive immune systems in human implantation.

Summary Implantation in the dusky leaf monkey, Presbytis obscura , is superficial; the placenta is of the double discoid villous haemochorial type. Immediately prior to implantation the thick endometrium contains highly active uterine glands. There is an early uterine epithelial proliferation at the implantation site but no stromal decidual response. The fetal villi are evenly spread over that part of the chorion in contact with the endometrium and consist of a mesenchymal core covered by an inner layer of cytotrophoblast and an outer layer of syncytiotrophoblast. The endothelial cells of the maternal vessels undergo swelling before disruption of the vessel walls and the establishment of a circulation, through the intervillous spaces.

Introduction Extracellular matrix proteins play a crucial role in influencing the invasion of trophoblast cells. However the role of collagens and collagen type IV (col-IV) in particular at the implantation site is not clear.