Fetal Membranes Anatomy Unit

Fetal Membranes Overview

• Fetal membranes are structures derived from the fertilized ovum and do not contribute to the embryo's formation.
• They include: Amnion, Yolk sac, Chorion, Allantois, Connecting stalk, Umbilical cord, and Placenta.

Amnion

• Amnion is a sac filled with fluid that surrounds the embryo, appearing around the 7th or 8th day post-fertilization.
• Initially bounded by amnioblast (roof) and epiblast (floor), it eventually completely encases the embryo.

Amniotic Fluid

• Amniotic fluid fills the amniotic cavity, with a typical volume of one liter at birth.
• Composed of 99% water, desquamated epithelial cells, organic constituents (carbohydrates, enzymes, fats, hormones), pigments, and inorganic salts.
• Source includes amnioblasts, placenta, and fetal urine.

Functions of Amnion

• Facilitates fetal movement and symmetrical growth.
• Prevents adhesion between fetal parts and acts as a barrier against infection.
• Helps maintain the fetus's body temperature and fluid/electrolyte balance.
• Provides cushioning against trauma.

Birth Process Role of Amnion

• Cervical dilatation during labor is aided by the bulging amniotic sac.
• When the amniotic sac ruptures, fluid acts as an antiseptic in the birth canal.

Amniotic Fluid Anomalies

• Oligohydramnios: Defined as less than ½ liter of amniotic fluid, causes include placental insufficiency and renal agenesis (absent kidney).
• Polyhydramnios: Defined as more than 2 liters of amniotic fluid, possibly caused by idiopathic factors, fetal esophageal atresia, or maternal diabetes.

Yolk Sac

• The yolk sac appears on the 9th day post-fertilization and is located ventrally to the embryo.
• Initially bounded by hypoblast (roof) and Heuser's membrane (floor), it contributes to the formation of the primitive gut and definitive yolk sac post-folding.

Functions of Yolk Sac

• Nutrient transfer to the embryo occurs primarily in the 2nd and 3rd weeks.
• Endodermal cells provide the epithelium for the trachea, bronchi, lungs, and digestive tract by the 4th week.
• Primordial germ cells form and migrate to the developing gonads around the 3rd week.
• Plays a role in the development of blood cells and vessels.

Anomalies of Yolk Sac

• Vitelline Fistula: Occurs due to failure in obliteration of the vitellointestinal duct, creating an abnormal connection between midgut and umbilicus.

Allantois

• The allantois is a diverticulum from the caudal end of the yolk sac that extends into the connecting stalk.
• Its proximal part contributes to the urinary bladder's formation, while the distal part becomes the urachus, which forms the median umbilical ligament post-birth.

Anomalies of Allantois

• Urachal Fistula: Resulting from failure of the urachus to obliterate, leads to communication between the urinary bladder and umbilicus.
• Urachal Cyst: Occurs when the middle part of the urachus persists.
• Urachal Sinus: Resulting from persistence of the distal part of the urachus.

Case Scenarios

• A pregnant woman experiencing a sudden large fluid passage may indicate rupture of membranes.
• Newborn fluid draining from the umbilicus identified as urine suggests a urachal anomaly, most likely a urachal fistula.