Hormonal Control of the Menstrual Cycle

Introduction

The menstrual cycle is a complex and tightly regulated process that occurs in females of reproductive age. It involves a series of hormonal changes that prepare the female reproductive system for possible pregnancy. The cycle typically lasts approximately 28 days, with the first day of menstruation considered day one. This section will elucidate the hormonal control of the menstrual cycle, encompassing the roles of follicle-stimulating hormone (FSH), luteinising hormone (LH), oestrogen, and progesterone.

Menstrual Cycle Phases

1. Menstruation

The menstrual cycle begins with menstruation, where the inner lining of the uterus, known as the endometrium, is shed. This phase usually lasts for a few days.

2. Follicular Phase

Following menstruation, the follicular phase commences. FSH plays a pivotal role in this phase, stimulating the development of a follicle within one of the ovaries. This follicle contains an immature ovum (egg).

3. Ovulation

Around the mid-point of the menstrual cycle, ovulation occurs. Peak levels of oestrogen stimulate a surge in the secretion of LH, triggering the release of the mature ovum from the follicle. Ovulation is the release of the egg from the follicle, making it available for potential fertilisation.

4. Luteal Phase

After ovulation, the luteal phase begins. The ruptured follicle transforms into a structure called the corpus luteum, which secretes progesterone. Progesterone promotes further development and vascularisation of the endometrium, preparing it for potential implantation of a fertilised egg.

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Hormonal Control of the Menstrual Cycle

Role of FSH and Oestrogen

Follicular Phase

During the follicular phase, FSH stimulates the development of the ovarian follicle. As the follicle matures, it produces increasing amounts of oestrogen.

Oestrogen's Effects

Oestrogen has several effects:

• It stimulates the proliferation of the endometrial lining, thickening it in preparation for possible implantation.
• Oestrogen affects the consistency of cervical mucus, making it more hospitable to sperm penetration, aiding in fertilisation.

Luteal Phase and Progesterone

Luteal Phase

In the luteal phase, the corpus luteum, which is the remnants of the ruptured follicle, secretes progesterone.

Progesterone's Functions

Progesterone has several functions:

• It promotes further development and vascularisation of the endometrium, making it suitable for embryo implantation if fertilisation occurs.
• Progesterone helps maintain the thickened endometrial lining, ensuring it remains receptive to a potential embryo.

Negative Feedback Mechanisms

Ovarian Hormones and Pituitary Gland

Ovarian hormones (oestrogen and progesterone) exert a negative feedback effect on the pituitary gland.

FSH and LH Regulation

The negative feedback prevents excessive secretion of FSH and LH by the pituitary gland. This regulation is essential to prevent further follicles from developing once one has reached maturity.

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Hormonal Control of the Menstrual Cycle

Graph Interpretation and Ovulation

1. Graphs Showing Hormone Concentrations

Interpretation of graphs depicting changes in FSH, LH, oestrogen, and progesterone concentrations throughout the menstrual cycle is an important aspect of understanding hormonal control. These graphs reflect the dynamic hormonal fluctuations during different phases of the cycle.

2. Ovulation

Ovulation is a crucial event in the menstrual cycle, marked by the release of a mature egg from a follicle in the ovary. It usually occurs around the midpoint of the menstrual cycle.

3. Role in Pregnancy

If fertilisation occurs and an embryo is successfully implanted in the thickened endometrium, the corpus luteum does not degenerate. Consequently, progesterone levels remain elevated, supporting early pregnancy.

Summary