Fetal Membranes and Placentation

Fetal Membranes & Placentations

• Fetal membranes are tissue layers that cover the fetus, separating it from the mother's endometrial lining.
• Fetal membranes originate from the zygote.
• Key functions of fetal membranes include protection, nutrition, respiration, excretion, and hormone production.
• The placenta and fetal membranes are expelled from the uterus after birth.
• The outcomes covered are formation, key roles during pregnancy, eventual fate, and related clinical aspects.

Fetal Membrane Components

• Amniotic membrane (Amnion)
• Yolk sac (umbilical vesicle)
• Allantois
• Chorionic plate

The Amniotic Membrane

• It forms within the inner cell mass during the 2nd week of development.
• Amnioblast cells migrate from the epiblast layer to form it.
• It secretes amniotic fluid into the amniotic cavity.
• The amniotic sac grows bigger as the pregnancy progresses.

Sources of Amniotic Fluid

• Embryonic period: Amnioblast cells and embryonic surfaces
• Early fetal period: Amnioblast cells, embryonic surfaces, fetal urine, and fetal lung secretions
• Late fetal period: Fetal urine, fetal lung secretions, fetal oral secretions, and fetal nasal secretions

Functions of the Amniotic Sac

• Protects the fetus from trauma to the maternal abdomen
• Cushions the umbilical cord from compression
• Has antibacterial properties
• Serves as a reservoir of fluid and nutrients for the fetus
• Provides the environment for normal development of the fetal lungs, musculoskeletal system (MSK), and gastrointestinal tract (GIT)
• Thermoregulation
• Lubricates fetal skin and the birth canal

Fate of the Amniotic Sac

• Tears around the time of delivery, causing "rupture of membranes"
• Expelled after birth along with the placenta

Oligohydramnios

• Occurs when amniotic fluid volume is less than expected for the gestational age
• Generally, the Amniotic Fluid Index (AFI) is less than 5cm
• Possible causes: fetal demise, drugs, renal abnormalities, intrauterine growth restriction (IUGR), premature rupture of membranes (PROM), placental insufficiency, chromosomal anomalies (Trisomy 13, Trisomy 18)
• Can lead to complications like pulmonary hypoplasia, fetal limb anomalies, or fetal demise

Polyhydramnios

• Common fetal causes: CNS anomalies, gastrointestinal obstruction, multiple pregnancy, cardiac anomalies, Trisomy 21 (or 18 & 13)
• Other causes: Idiopathic in more than 50% of cases, maternal causes such as diabetes mellitus (DM) or congestive heart failure (CCF)

Amniotic Band Disruption Syndromes

• Results from entrapment of fetal body parts in a disrupted fibrous amnion
• Involves a wide spectrum of abnormalities
• Multiple defects can occur in this condition

The Yolk Sac (Umbilical Vesicle)

• Forms in the 2nd week of development
• Forms by migrating cells from the hypoblast layers.
• Forms later by endodermal cells.

Functions of the Yolk Sac

• Nutrient supply
• Site of early hemopoiesis
• Gives rise to primordial germ cells (PGCs)
• Fate of the Yolk Sac
• Its dorsal part is incorporated into the embryo during embryonic folding, becoming the primordial gut
• Its ventral part degenerates, initially as the vitelline duct

The Allantois

• Extension of the yolk sac into the connecting stalk
• Has similar functions as the yolk sac, like nutrient transfer, hematopoiesis, and giving rise to primordial germ cells (PGCs)

Fate of Allantois

• Lower part gets incorporated to form the urinary bladder
• Upper part degenerates as the urachus
• The remaining structure becomes the median umbilical ligament

Clinical correlates of the Allantois

• Persistence of the allantois causes urachal anomalies

The Chorionic Plate

• COMPONENTS
  • Somatic layer of the extraembryonic mesoderm
  • Cytotrophoblast layer
  • Syncytiotrophoblast

Functions of the Chorion

• Chorion frondosum forms the fetal component of the placenta
• Protects the embryo
• Serves as a hemopoietic center during the yolk sac phase
• Forms the fetal components of the placenta

Fate of Chorion

• The villous chorion forms the fetal components of the placenta

The Placenta

• Primary site of nutrient and gas exchange between mother and fetus
• Feto-maternal organ with 2 components:
  • Fetal part: Develops from the chorionic sac
  • Maternal part: Derived from the endometrium
• The placenta and umbilical cord form a transport system.
• Some functions include protection, nutrition, respiration, excretion, and hormone production.

The Decidua

• Also known as the gravid endometrium; is the functional layer of the endometrium
• Separates from the remainder of the uterus after parturition
• Consists of three regions:
  • Decidua basalis: deep to the conceptus
  • Decidua capsularis: superficial part overlying the conceptus
  • Decidua parietalis: The remaining parts
• Due to elevated progesterone, decidua enlarge, forming pale-staining decidual cells that accumulate glycogen and lipids.
• Cellular and vascular changes in the endometrium due to blastocyst implantation constitute the decidual reaction.
• The decidual reaction protects maternal tissue against uncontrolled invasion by the syncytiotrophoblast
• Ultrasonography can be used for early detection of pregnancy

Development of the Placenta

• Involves rapid trophoblast proliferation and chorionic sac/villi development
• By the end of the third week, the anatomy enables physiological exchanges
• A complex vascular network forms by the end of the fourth week, facilitating maternal-embryonic exchanges
• Chorionic villi cover the entire chorionic sac until the eighth week
• As the sac grows, villi associated with the decidua capsularis are compressed, resulting in the smooth chorion
• Villi associated with the decidua basalis increase, branch and enlarge forming the villous chorion
• The uterus, chorionic sac, and placenta enlarge with embryo growth, with placenta growth rapid by 18 weeks.
• A fully developed placenta covers 15%-30% of the decidua and weighs 1/6 that of the fetus.
• The placenta consists of a fetal part and a maternal part, the latter replaced by the fetal part by the fourth month.
• The fetal part of the placenta attaches to the maternal part (decidua basalis) via the cytotrophoblastic shell, with the chorionic villi anchoring to the decidua basalis.
• Endometrial arteries and veins pass through cytotrophoblastic shell gaps to interface with the intervillous space.
• Chorionic villi are showered with maternal blood that circulates through the intervillous space
• Maternal blood carries both oxygen and nutrients for support
• Maternal blood also holds fetal salts, carbon dioxide, waste and products of protein metabolism

Functions of the Placenta

• Three main functions: metabolism, transport, and endocrine secretion, which ensure normal fetal development