Capillary invasion of corpus luteum begins
**Core Concept**
Capillary invasion of the corpus luteum is a critical process that occurs during the luteal phase of the menstrual cycle. It involves the growth of new capillaries into the corpus luteum, a temporary endocrine structure that forms in the ovary after ovulation. This process is essential for the maintenance of corpus luteum function and the subsequent production of progesterone.
**Why the Correct Answer is Right**
The correct answer is **Day 21**. This is because capillary invasion of the corpus luteum begins around this time, allowing for the increased blood supply necessary for the continued production of progesterone. The corpus luteum relies on a rich blood supply to synthesize progesterone, which is essential for preparing the uterine lining for potential implantation of a fertilized egg.
**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because capillary invasion begins after ovulation, not immediately after follicular rupture.
**Option B:** This is incorrect because capillary invasion is not a feature of the follicular phase, which occurs before ovulation.
**Option C:** This is incorrect because capillary invasion begins after ovulation, not before ovulation.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the corpus luteum's function is tightly regulated by the hypothalamic-pituitary-gonadal axis. The production of progesterone by the corpus luteum is essential for preparing the uterine lining for potential implantation of a fertilized egg, and any disruption to this process can lead to issues with conception or implantation.
**Correct Answer:** D. Day 21