In the human placenta, besides the fetal blood vessel system a second extravascular contractile system exists. It is localized in the chorionic plate and runs in a longitudinal direction and adjacent to fetal blood vessels into the stem villi, where it forms perivascular contractile sheaths. Characteristically, cells of the extravascular contractile system are extremely long and spindle-shaped and give rise to fine cell processes, by which they obviously contact each other or insert into the basement membrane of the trophoblast. They show immunoreactivity with desmin, vimentin, alpha-actin, myosin, nitric oxide synthase type I (brain form) and dipeptidyl peptidase IV. The ultrastructure suggests that cells of the extravascular contractile system are related to smooth muscle cells, including subpopulations with myofibroblastic features. In stem villi a few cells are nitric oxide synthase type I immunoreactive. These cells are thought to be specialized smooth-muscle-like cells of the extravascular contractile system or cells of the extravascular contractile system related to paraneurons that generate nitric oxide, which, in turn, may modulate the tone of perivascular contractile sheaths. The high dipeptidyl peptidase IV activity suggests that modulation of the extravascular contractile system may also occur by substance P.
In 200 healthy nulliparous women the mean arterial blood pressure in the second trimester (MAP-2 value) was calculated. 85 women (42 %) had a MAP-2 value of greater than or equal to 90 mmHg (positive test result), but only 27 women (32 %) developed a hypertensive complication. Conversely, 113 of the 115 (98 %) women with a negative test result (MAP-2 value less than 90 mmHg) remained normotensive. Only two women of this group (2 %) later showed a mild pregnancy-induced hypertension. Thus, the MAP-2 value has a high sensitivity (93 %) and a high predictive value of negative test results (98 %). On the other hand, there is a high rate of false-positive results (68 %) and thus a low predictive value of positive test results (32 %). It is concluded that the MAP-2 value is a simple method for selecting pregnant women who should be examined with other more specific predictive tests. Alternatively, weekly measurements of blood pressure are recommended for early diagnosis of hypertensive disorders of late pregnancy in all women with a MAP-2 value of 90 mmHg or more.
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