- Human placenta is discoid, hemochoroidal, deciduate
- The development of the placenta begins at 6th week of gestation and is well established by the 12th week
- The placenta at term:
- At term, four-fifths of the placenta is of fetal origin.
- Only the decidua basalis and the blood in the intervillous space are of maternal origin.
- Line of separation of placenta is through the decidua spongiosum.
- Nitabuch's membrane is the fibrinoid deposition in the outer syncytiotrophoblast. It limits the further invasion of the deciduas by the trophoblast. Absence of the membrane causes placenta accreta.
- During the early weeks of pregnancy, there is a space between the decidua capsularis and decidua parietalis because the gestational sac does not fill the entire uterine cavity. By 14-16 weeks, the expanding sac has enlarged enough to fill the uterine cavity.
- The uteroplacental circulation is established 9-10 days after fertilization.
- Fetoplacental circulation is established 21 days post fertilization.
- Chorionic villi can first be distinguished in the human placenta on about the 12th day after fertilization.
- FFN (fetal fibronectin) has been called trophoblast glue to suggest a critical role for this protein in the migration and attachment of trophoblasts to maternal decidua.
- The presence of FFN in cervical or vaginal fluid can be used as a prognostic indicator for preterm labor.
NOTE: The tumor which can metastasize to placenta is melanoma.
Decidual Spiral Artery Invasion by Trophoblast
- The timing of the development of the uteroplacental vessels has been described in waves, or stages, over the course of gestation. The first wave occurs before 12 weeks postfertilization and consists of invasion and modification of the spiral arteries of the decidua. Between 12 and 16 weeks postfertilization, the second wave occurs. This involves invasion of the intramyometrial parts of the spiral arteries, converting narrow lumen, muscular spiral arteries into dilated, low-resistance uteroplacental vessels. If this fails to happen, the mother is more prone to develop preeclampsia (theory of improper trophoblastic invasion) and fetus may develop IUGR.
- Hofbauer cells, representing fetal macrophages, increase in numbers and maturation state as pregnancy progresses. Although phagocyte, they have an immunosuppressive phenotype.