Which of the following molecules primarily mediate blastocyst adhesion?

The amnion is a thin, transparent membrane that forms the innermost layer of the fetal membranes, creating a protective sac filled with amniotic fluid. By 12 weeks gestation, the amnion has expanded to enclose the fetus completely, providing a cushioning environment, temperature regulation, and protection against infection and mechanical injury. Amniotic fluid volume normally ranges from 50 ml at 12 weeks to approximately 800 ml at term. The amnion does not produce hormones or directly facilitate nutrient exchange, which is the role of the placenta.

Rationale for correct answers

B. The amnion forms a protective sac that contains amniotic fluid, which cushions the fetus and permits movement. At 12 weeks, this sac is fully formed, surrounding the embryo/fetus with fluid essential for normal development and mechanical protection.

Rationale for incorrect answers

A. The amnion does not produce hormones; hormone production to maintain pregnancy is primarily by the trophoblast and placenta, including human chorionic gonadotropin and progesterone.

C. Nutrient exchange occurs at the placenta via chorionic villi, not the amnion, which acts as a barrier and container for amniotic fluid but does not participate in exchange functions.

D. The umbilical cord develops from the connecting stalk and allantois, not from the amnion. The amnion lines the amniotic cavity but does not differentiate into the umbilical cord.

Take home points

• The amnion forms a protective sac filled with amniotic fluid by 12 weeks gestation.
• Amniotic fluid cushions and protects the fetus, volume increases from 50 ml at 12 weeks to 800 ml at term.
• Hormone production for pregnancy maintenance occurs in the placenta, not the amnion.
• Nutrient and gas exchange occurs through the placenta, not the amnion.

The acrosomal reaction is a crucial event in human fertilization involving the release of hydrolytic enzymes from the sperm’s acrosome. These enzymes allow the sperm to penetrate the zona pellucida, a glycoprotein-rich extracellular matrix surrounding the oocyte. Hyaluronidase and acrosin are the two main enzymes involved. The reaction begins upon binding of the sperm to the ZP3 glycoprotein receptor on the zona pellucida. The acrosomal reaction does not involve genetic fusion but is essential to reach the oocyte membrane. This reaction precedes cortical reaction and oocyte activation.

Rationale for correct answers

B. The acrosomal reaction is defined by the release of hydrolytic enzymes like acrosin and hyaluronidase from the sperm’s acrosomal cap. These enzymes digest the cumulus oophorus and the zona pellucida, allowing sperm to reach the oocyte membrane. This event is molecularly triggered by the binding of sperm to ZP3 receptors on the zona pellucida.

Rationale for incorrect answers

A. Fusion of the male and female pronuclei marks syngamy, which is the final step of fertilization after the sperm has already entered the oocyte. This event occurs after both the acrosomal and cortical reactions and does not define the acrosomal reaction itself.

C. A rapid depolarization of the oocyte membrane is part of the “fast block to polyspermy,” which is associated with the cortical reaction, not the acrosomal reaction. This block prevents multiple sperm from fertilizing the oocyte after one sperm has succeeded, but it happens after the acrosomal reaction is complete.

D. Migration of sperm through the cervical mucus and uterus is a part of sperm transport and capacitation, which occur prior to the acrosomal reaction. This is necessary for the sperm to even reach the site of fertilization, but it is not the defining event of the acrosomal reaction.

Take home points

• The acrosomal reaction is triggered by sperm binding to ZP3 in the zona pellucida.
• Hydrolytic enzymes like acrosin are released to penetrate the zona pellucida.
• The acrosomal reaction is essential for fertilization but occurs before sperm-oocyte membrane fusion.
• It is distinct from the cortical reaction and sperm transport.