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.

This quiz focuses on the anatomy of fetal membranes, including the amnion, yolk sac, and allantois. Students will explore their development, functions, and any potential anomalies. Prepare to deepen your understanding of these vital structures in human development.