Structure and Appearance of the Maternal and Fetal Sides of the Placenta
The placenta is a specialized organ with distinct maternal and fetal surfaces that serve critical functions in supporting fetal development through separate but interconnected circulatory systems.
Placental Structure Overview
The placenta contains two distinct circulatory systems that separate maternal and fetal blood:
- Feto-placental circulation: Blood volume increases with gestational age, comprising approximately 110-120 mL/kg of fetal weight in the second trimester, decreasing to 80-85 mL/kg at term 1
- Utero-placental circulation: The proportion of blood in the feto-placental circulation residing in the placenta varies from approximately 50% at 30 weeks gestation to around one-third at term 1
Maternal Surface (Basal Plate)
The maternal side of the placenta has several distinctive features:
- Appearance: Rough, red surface divided into 15-20 cotyledons (lobes) separated by grooves or septa
- Decidual layer: Normally present at the uteroplacental interface; its absence is the defining histologic feature of placenta accreta spectrum disorders 1
- Uteroplacental interface: In normal placentas, shows a thick undulating layer of fibrinoid and dilated vessels along the interface 1
- Vascular features: Contains maternal blood vessels and spiral arteries that have been remodeled by extravillous trophoblast cells
Pathological Considerations
In placenta accreta spectrum disorders, the maternal surface shows:
- Loss of the normal undulating/corrugated contour of the uteroplacental interface 1
- Chronic inflammation in the myometrium with edema and myocyte injury 1, 2
- Massively dilated anastomosing vasculature in the mid-myometrium 1
Fetal Surface (Chorionic Plate)
The fetal side of the placenta has these key characteristics:
- Appearance: Smooth, shiny, gray-white surface covered by the amnion
- Umbilical cord insertion: Usually central or slightly off-center
- Vascular pattern: Chorionic vessels (branches of umbilical vessels) radiate from the umbilical cord insertion point across the fetal surface
- Functional units: Contains chorionic villi, which are the main functional units where exchange between maternal and fetal circulation occurs 3
Placental Membrane Structure
The placental membrane separates maternal and fetal blood and consists of several layers:
- Syncytiotrophoblast: Multinucleated outer epithelial layer of chorionic villi formed by fusion of villous cytotrophoblast cells 3
- Cytotrophoblast: Mononuclear cells that serve as stem cells for the syncytiotrophoblast
- Basement membrane: Supports the trophoblast layers
- Villous stroma: Contains fetal blood vessels
- Fetal endothelium: Lines the fetal blood vessels within villi
Functional Significance
The structural organization of the placenta facilitates:
- Exchange function: Terminal branches of chorionic villi are the primary sites of maternal-fetal exchange 3
- Barrier function: The placenta protects the fetus against certain xenobiotic molecules, infections, and maternal diseases 3
- Endocrine function: Releases hormones into both maternal and fetal circulations 3
- Metabolic function: Metabolizes substances and releases products into maternal and/or fetal circulations 3
Clinical Implications
Understanding normal placental structure is essential for:
- Identifying abnormal placentation, such as placenta accreta spectrum disorders
- Recognizing the significance of placental pathology in pregnancy complications
- Appreciating the role of the placenta in maternal-fetal exchange and fetal development
The placenta's complex architecture and dual circulatory systems allow it to perform multiple vital functions simultaneously, supporting fetal growth and development while maintaining the physiological separation between maternal and fetal blood.