What component of the menstrual cycle, dominated by estrogen, causes the uterine wall to shed and become engorged with blood, specifically referring to the Follicular (Follicular) phase or the Luteal (Luteal) phase?

From the Guidelines

The follicular phase of the menstrual cycle, dominated by estrogen, is responsible for causing the uterine wall to shed and become engorged with blood. This phase is characterized by the growth and thickening of the endometrium (uterine lining) in preparation for a potential pregnancy, typically occurring from days 5-14 of a 28-day cycle 1. The hormone estrogen plays a crucial role in this process, stimulating the growth and thickening of the endometrium, causing it to become engorged with blood vessels and glands 1. Key points to consider include:

• Estrogen levels rise steadily during the follicular phase, reaching their peak just before ovulation 1
• The hormone works by binding to receptors in the endometrial cells, triggering cellular proliferation and vascular development 1
• This creates a nutrient-rich environment that could support embryo implantation if fertilization occurs
• If pregnancy doesn't occur, the subsequent drop in estrogen and progesterone at the end of the cycle will trigger the shedding of this built-up lining during menstruation, beginning the cycle again 1. It is essential to note that the effects of estrogen on the uterine lining are critical in understanding the menstrual cycle and its various phases, particularly in the context of female athletes and their nutritional needs 1.

From the Research

Menstrual Cycle Phases

The menstrual cycle is divided into two main phases: the Follicular phase and the Luteal phase.

• The Follicular phase is dominated by estrogen and is characterized by the growth and thickening of the uterine wall, preparing it for a potential pregnancy 2, 3.
• The Luteal phase is dominated by progesterone and is characterized by the preparation of the uterine wall for implantation of a fertilized egg 4, 5.

Uterine Wall Changes

During the menstrual cycle, the uterine wall undergoes significant changes, including:

• Shedding of the outer layer of the endometrium during menstruation, triggered by falling progesterone and estrogen levels 2.
• Regeneration and thickening of the endometrium during the Follicular phase, driven by rising estrogen levels 2, 3.
• Preparation of the uterine wall for implantation during the Luteal phase, under the influence of progesterone 4, 5.

Hormonal Regulation

The changes in the uterine wall are regulated by hormonal fluctuations, including:

• Estrogen, which promotes the growth and thickening of the endometrium during the Follicular phase 3, 5.
• Progesterone, which prepares the uterine wall for implantation during the Luteal phase 4, 5.
• Other non-steroidal substances, such as inhibin A and B, and gonadotrophin surge-attenuating factor (GnSAF), which also play a role in regulating the menstrual cycle 5.

Estrogen's Role

Estrogen plays a crucial role in the Follicular phase, causing the uterine wall to become engorged with blood and preparing it for a potential pregnancy 2, 3.

• Estrogen levels rise during the Follicular phase, promoting the growth and thickening of the endometrium 2, 3.
• Estrogen also regulates the proliferation of endothelial progenitor cells, which are involved in neovascularization and the maintenance of uterine blood flow 3.