Teiary chorionic villi develop in which intrauterine week?
## **Core Concept**
The development of tertiary chorionic villi is a critical step in the formation of the placenta during early pregnancy. Chorionic villi are projections of the chorion (the outer membrane of the embryo) into the uterus, playing a key role in maternal-fetal exchange. Tertiary chorionic villi are characterized by a core of mesenchymal tissue surrounded by a layer of cytotrophoblast and then syncytiotrophoblast.
## **Why the Correct Answer is Right**
Tertiary chorionic villi begin to develop around the 3rd week of gestation. This developmental stage is crucial for the establishment of a functional placenta. The mesenchymal core of the tertiary villi is vascularized, which is essential for the exchange of gases, nutrients, and waste products between the mother and the fetus. The presence of tertiary villi significantly increases the surface area available for exchange, supporting the growing needs of the embryo.
## **Why Each Wrong Option is Incorrect**
- **Option A:** Development of tertiary chorionic villi does not occur as early as the first week of gestation. Implantation of the blastocyst occurs around 6-7 days post-fertilization, and it takes a couple of weeks for the chorionic villi to develop and mature.
- **Option B:** Secondary chorionic villi, which develop around the 2nd week of gestation, are not the final form of chorionic villi; they need to further develop into tertiary villi.
- **Option D:** By the 5th week of gestation, tertiary chorionic villi are already established, and the development of the placenta is well underway.
## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the development and maturation of chorionic villi are critical for a healthy pregnancy. Abnormalities in villous development, such as those seen in molar pregnancies or certain placental insufficiency conditions, can have significant clinical implications. Understanding the timeline of chorionic villi development is essential for assessing fetal development and placental function.
## **Correct Answer:** C. 3rd week.