Biological Systems MCAT Practice Exam

What primary effect do estrogen and progesterone have on the endometrium?

1. Inducing apoptosis

2. Thickening of the endometrial lining

3. Reduction of blood flow

4. Activation of mitochondria

The correct answer is: Thickening of the endometrial lining

Estrogen and progesterone play crucial roles in the menstrual cycle, particularly concerning the endometrium, which is the inner lining of the uterus. The primary effect of these hormones, especially during the luteal phase of the menstrual cycle, is to prepare the endometrium for potential implantation of a fertilized egg. Estrogen stimulates the growth of the endometrial lining, promoting cellular proliferation and thickening the endometrium in preparation for possible pregnancy. This thickening is essential because it creates a nutrient-rich environment conducive to the implantation of an embryo. Following ovulation, progesterone is secreted by the corpus luteum and further enhances the development of the endometrial lining that was initiated by estrogen. It also promotes the secretion of various substances that support potential implantation and early embryonic development. The combination of these actions results in a well-prepared endometrium, making option B the correct choice. Understanding the roles of both estrogen and progesterone is critical for grasping how they regulate the menstrual cycle and support reproductive processes.