Which of the following molecules primarily mediate blastocyst adhesion?

Amniotic fluid plays an essential role in fetal development, particularly in the respiratory system. It provides a fluid medium that fills the fetal lungs, necessary for lung tissue expansion and maturation. The fetus practices breathing movements by inhaling and exhaling amniotic fluid, which stimulates the growth and differentiation of the pulmonary alveoli. Normal amniotic fluid volume ranges from 500 to 1000 mL in the third trimester, and abnormalities in volume can impair lung development, leading to conditions like pulmonary hypoplasia.

Rationale for correct answers

B. Amniotic fluid allows the fetus to perform breathing movements, critical for stimulating lung expansion and promoting structural and functional maturation of the lungs.

E. It provides the necessary fluid medium inside the lungs, which maintains lung distension and facilitates alveolar growth, essential for postnatal respiratory function.

Rationale for incorrect answers

A. Acting as a mechanical buffer protects the fetus from external trauma but does not directly affect lung development.

C. While amniotic fluid supports symmetrical growth of the musculoskeletal system, this is unrelated to pulmonary maturation.

D. Prevention of umbilical cord compression ensures adequate blood flow and oxygen delivery but is not directly linked to lung development.

Take home points

• Amniotic fluid is essential for fetal lung growth by providing a fluid medium within the lungs.
• Fetal breathing movements in amniotic fluid stimulate pulmonary development.
• Mechanical protection and musculoskeletal support are important but not lung-specific functions.
• Adequate amniotic fluid volume is crucial to prevent pulmonary hypoplasia.

The chorion frondosum is the part of the chorion characterized by proliferation of vascularized villi adjacent to the decidua basalis, which contributes to the formation of the fetal placenta. These villi undergo extensive branching and vascularization to establish maternal-fetal exchange. In contrast, the chorion laeve is the smooth, avascular part away from the decidua basalis. Proper development of the chorion frondosum is essential for adequate placental function, supporting fetal nutrition and gas exchange throughout pregnancy.

Rationale for correct answers

B. The chorion frondosum forms from the proliferation of chorionic villi adjacent to the decidua basalis, developing into the functional fetal placenta with a dense vascular network enabling exchange of nutrients, gases, and waste. This structure is essential for sustaining fetal growth.

Rationale for incorrect answers

A. The chorion laeve, or smooth chorion, forms from villi on the side of the chorion away from the decidua basalis; these villi regress and become avascular, so it does not contribute to placental formation.

C. The yolk sac is an early embryonic structure involved in initial hematopoiesis and nutrient transfer but is not formed from chorionic villi and is distinct from the chorion.

D. The umbilical cord develops from the connecting stalk and contains blood vessels connecting fetus to placenta but is not formed by chorionic villi proliferation adjacent to the decidua basalis.

Take home points

• The chorion frondosum is the vascularized region of chorionic villi adjacent to the decidua basalis forming the fetal placenta.
• The chorion laeve is avascular and forms the smooth chorion on the opposite side of the placenta.
• Proper placental development depends on chorionic villi proliferation and vascularization in the chorion frondosum.
• The yolk sac and umbilical cord are distinct structures not formed from chorionic villi adjacent to decidua basalis.