Which of the following is present at the beginning of 3rd week?
**Core Concept**
During embryonic development, the formation of the neural tube is a critical event that occurs early in gestation. The neural tube gives rise to the brain and spinal cord. The process of neural tube formation is influenced by various genetic and environmental factors.
**Why the Correct Answer is Right**
The closure of the neural tube begins at the beginning of the 3rd week of gestation, around day 22-23. This process is triggered by the convergence of the neural folds and the formation of the neural plate. The closure is facilitated by the presence of specific molecules, such as Sonic hedgehog (Shh), which play a crucial role in the patterning and morphogenesis of the neural tube. The closure of the neural tube is a complex process that involves the coordinated action of multiple cellular and molecular mechanisms.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the formation of the heart begins slightly later, around day 22-23, but it is not the primary event occurring at the beginning of the 3rd week.
**Option B:** This option is incorrect because the development of the limb buds occurs later, around day 26-28, and is not the primary event occurring at the beginning of the 3rd week.
**Option C:** This option is incorrect because the formation of the gut begins slightly later, around day 22-23, but it is not the primary event occurring at the beginning of the 3rd week.
**Clinical Pearl / High-Yield Fact**
The neural tube defects, such as spina bifida and anencephaly, can occur if the closure of the neural tube is disrupted. These defects are often associated with genetic mutations or environmental factors that interfere with the normal development of the neural tube.
**Correct Answer:** C.