Transplacental crossing is not seen in –
**Core Concept**
Transplacental crossing refers to the passage of substances from the mother's bloodstream into the fetus through the placenta. This process is crucial for the exchange of oxygen, nutrients, and waste products between the mother and the fetus. The placenta acts as a selective barrier, allowing certain substances to cross while preventing others.
**Why the Correct Answer is Right**
The placenta has a unique structure that prevents certain substances from crossing the placental barrier. The syncytiotrophoblast, a layer of multinucleated cells, forms the outer layer of the placenta and is responsible for the transfer of substances between the mother and the fetus. However, certain substances like insulin and heparin are bound to proteins in the maternal bloodstream and are not able to cross the placental barrier due to their large molecular size and charge.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is actually a substance that can cross the placental barrier. Insulin is essential for fetal growth and development, and it is transferred from the mother to the fetus through the placenta.
* **Option B:** This option is incorrect because heparin, an anticoagulant, can also cross the placental barrier, albeit in small amounts. However, it is not a substance that is typically associated with transplacental crossing.
* **Option C:** This option is incorrect because it is a substance that is actually transferred from the mother to the fetus through the placenta. Glucose is essential for fetal growth and development, and it is transferred from the mother to the fetus through the placenta.
**Clinical Pearl / High-Yield Fact**
The placental barrier is a selective barrier that allows certain substances to cross while preventing others. This is crucial for the exchange of oxygen, nutrients, and waste products between the mother and the fetus. However, certain substances like insulin and heparin are bound to proteins in the maternal bloodstream and are not able to cross the placental barrier due to their large molecular size and charge.
**Correct Answer: C. Glucose**