The cytotrophoblastic shell and complications of pregnancy.

Many complications of pregnancy have their pathophysiological roots in the early stages of placentation. Impaired trophoblast invasion and deficient remodelling of the maternal spiral arteries are a common feature. While malperfusion of the placenta may underpin cases of fetal growth restriction and early-onset pre-eclampsia, the mechanistic links to spontaneous miscarriage, pre-term labour and premature rupture of the membranes are less obvious. Here, we speculate that formation of a well-developed cytotrophoblastic shell at the maternal-fetal interface is crucial for pregnancy success. Initially, extravillous trophoblast cells differentiate from the outer layer of the shell in contact with the endometrium. Impaired development may thus contribute to reduced invasion and deficient remodelling. In addition, the extent of the shell influences the timing and spatial configuration of onset of the maternal arterial circulation. A thin and fragmentary shell results in premature and disorganised onset, leading to spontaneous miscarriage. In less severe cases it may predispose to haemorrhage at the interface and formation of intrauterine haematomas. If pregnancy continues, these haematomas may act as a source of oxidative stress, promoting senescence and weakening of the membranes, and stimulating inflammation in the uterine wall and premature contractions. Formation of the shell is dependent on proliferation of cytotrophoblast progenitor cells during the first weeks after implantation, when the developing placenta is supported by histotrophic nutrition from endometrial glands. Hence, we propose the fitness of the endometrium prior to conception, and the peri-conceptional dialogue between the endometrium and the trophoblast is critical for avoidance of later complications of pregnancy.