Placenta

Extended Modular Program: Placenta

Introduction to Placenta

  • Presented by: Dr. Shereen Adel
    Professor of Anatomy & Embryology

Questions Regarding the Placenta

  1. Types of chorionic villi formed.

  2. Steps of placenta formation.

  3. Structure of the placenta.

  4. Structures forming the placental barrier.

  5. Placental functions.

  6. Placental anomalies.

  7. Types of twins.

Development of the Placenta

  • Definition: A feto-maternal organ that represents the primary site of nutrient and gas exchange between the mother and fetus.

Steps of Formation

  1. By the beginning of the 4th month, the placenta has developed from two components:

    • Fetal Part: Called chorion frondosum.

    • Maternal Part: Called decidua basalis.

  2. During the 4th to 5th months, the decidual cells form several septa that project into the intervillous space but do not reach the chorionic plate.

    • These septa divide the placenta into compartments called Cotyledons.

    • A cotyledon is the area between two septa.

    • The placenta expands due to fetal growth and uterine enlargement.

Gross Appearance of Full-term Placenta

  • Shape: Flattened disc.

  • Measurements:

    • Diameter: 15-25 cm.

    • Thickness: 3 cm.

    • Weight: 500-600 g.

  • Coverage: The placenta covers 25-30% of the internal surface of the uterus.

Surfaces of the Placenta

  1. Fetal Surface:

    • The inner smooth surface covered by the amnion.

    • The umbilical cord is attached near its center.

  2. Maternal Surface:

    • The outer rough surface showing elevated areas called cotyledons (15-20), separated by grooves indicating the sites of the placental septa.

Microscopic Structure of the Placenta

  1. Tertiary Chorionic Villi of Chorion Frondosum:

    • Changes to form the fetal part of the placenta:

      • Stem or Anchoring Villi: Extend from the chorion to attach to the decidua basalis.

      • Free or Absorbing Villi: Extend laterally to increase surface area for fetal and maternal blood exchange.

  2. Intervillous Space:

    • Lined completely with syncytial trophoblast filled with maternal blood from maternal vessels opening at the roof of the spaces.

  3. Cytotrophoblastic Shell:

    • The cytotrophoblast at the anchoring villus penetrates the syncytiotrophoblast to form a continuous shell surrounding the embryonic vesicle, preventing further penetration by syncytiotrophoblast.

Placental Barrier

  • Definition: A membrane separating maternal blood in the intervillous space from fetal blood in fetal vessels within the free villi.

  • Composition during the First Half of Pregnancy:

    • Syncytiotrophoblast

    • Cytotrophoblast

    • Primary mesoderm

    • Endothelium of fetal blood vessels

  • Changes in the Second Half of Pregnancy:

    • Layers of cytotrophoblast and mesoderm disappear, making the barrier thinner and more permeable to allow rapid exchange.

    • Note: Maternal and fetal blood never mix except during birth.

Functions of the Placenta

  1. Nutritive Function:

    • Transports water, glucose, amino acids, antibodies, antigens, free fatty acids, neutral fat, electrolytes, and vitamins to the fetus.

  2. Respiratory Function:

    • Exchange of oxygen and carbon dioxide occurs here.

  3. Excretory Function:

    • Removes a few constituents of urine secreted by the fetal kidney.

  4. Secretory Function:

    • Secretes chorionic gonadotrophins, oestrogen, progesterone, and placental lactogen.

    • Chorionic gonadotrophins appear in urine starting from the 9th day of pregnancy, detectable by pregnancy tests.

  5. Protective Barrier:

    • Protects against infection transmission from mother to fetus.

    • Antibodies can pass from mother to fetuses, providing immunity against infections such as diphtheria, measles, and smallpox.

    • Some viruses such as German measles, syphilis, poliomyelitis, and AIDS can pass through.

    • Rh factor antibodies and certain drugs can also cross this barrier.

Placental Circulation

  • Deoxygenated fetal blood arrives through 2 umbilical arteries, branching to form capillaries in the villi for gas exchange with maternal blood in the intervillous space.

  • Oxygenated blood returns to the fetus via the left umbilical vein.

Anomalies of the Placenta

  1. Abnormal Attachment:

    • Placenta is attached to the lower uterine segment (placenta previa):

    1. Placenta Praevia Lateralis: Above internal os.

    2. Placenta Praevia Marginalis: Partially covers internal os.

    3. Placenta Praevia Centralis: Completely covers internal os.

  2. Abnormal Distribution of Chorionic Villi:

    • Diffuse Placenta (Placenta Membranacea): Extensive chorion frondosum area.

    • Separate Accessory Lobule (Placenta Succenturiata).

    • Divided Placenta: Either bipartite or tripartite.

  3. Abnormal Sites of Umbilical Cord Attachment:

    • Battledore Placenta: Umbilical cord attached to the margin rather than the center of the placenta.

    • Velamentous Insertion of the Cord: Attached to the membranes away from the placenta's margin.

  4. Chorion Epithelioma: Malignant tumor of the placenta.

Types of Twins

  1. Monozygotic Twins (Uni-ovular, Identical):

    • One zygote forms two embryos, identical except for fingerprints.

    • Same sex with a single placenta and one or two amnion but two cords.

    • Incidence: 30% of all twins.

  2. Dizygotic Twins (Binovular):

    • Results from two ova and two sperms, forming two zygotes, non-identical.

    • Possible same or opposite sex.

    • Two placentas, two amnions, and two cords.

    • Incidence: 70% of all twins.

  3. Conjoined Twins:

    • Result from incomplete separation of identically fused twins.

    • Types include:

      • Craniopagus: Fused at the head.

      • Thoracopagus: Fused at the chest.

      • Pygopagus: Fused at the sacral region.

Conclusion

  • Key points include the formation, functions, abnormalities of the placenta, and types of twins.

  • Thank you!