What is the sequence of the menstrual cycle, including the follicular phase, involved tissues, cells, and hormones, such as Gonadotropin-Releasing Hormone (GnRH), Follicle-Stimulating Hormone (FSH), and Luteinizing Hormone (LH)?

From the Guidelines

The menstrual cycle is a complex hormonal sequence that begins with the follicular phase, where the hypothalamus releases Gonadotropin-Releasing Hormone (GnRH), stimulating the anterior pituitary to secrete Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH), as described in the most recent study 1. The cycle starts with menstruation, which is the shedding of the endometrial lining, and is followed by the follicular phase (days 1-14), where FSH promotes the development of ovarian follicles containing eggs, while estrogen levels rise as these follicles mature.

• The hypothalamus plays a crucial role in regulating the menstrual cycle by releasing GnRH, which stimulates the anterior pituitary to secrete FSH and LH 1.
• The anterior pituitary gland secretes FSH and LH, which stimulate the development of ovarian follicles and the release of estrogen and progesterone.
• The ovaries contain follicles that produce eggs and secrete estrogen and progesterone, which regulate the menstrual cycle.
• The uterus lining, or endometrium, thickens in response to increasing estrogen levels during the follicular phase and prepares for potential pregnancy. Around day 14, a surge in LH triggers ovulation, releasing a mature egg from the dominant follicle.
• The surge in LH is triggered by the peak levels of estrogen, which stimulates the release of LH from the anterior pituitary gland 1. The post-ovulatory luteal phase (days 15-28) begins as the ruptured follicle transforms into the corpus luteum, which secretes progesterone and estrogen to maintain the thickened endometrial lining.
• The corpus luteum plays a crucial role in maintaining the pregnancy by secreting progesterone, which helps to thicken the endometrial lining and prepare it for implantation of the fertilized egg 1. If fertilization doesn't occur, the corpus luteum degenerates, hormone levels drop, and the endometrium sheds during menstruation, marking the beginning of a new cycle.
• The degeneration of the corpus luteum is triggered by the decrease in LH levels, which causes a decrease in progesterone production, leading to the shedding of the endometrial lining 1. This intricate hormonal interplay involves the hypothalamus, pituitary gland, ovaries, and uterus working in coordination to regulate reproduction, as described in the study 1.

From the Research

Menstrual Cycle Sequence

The menstrual cycle is a complex process involving the coordinated action of various tissues, cells, and hormones. The sequence of the menstrual cycle can be described as follows:

• The cycle begins with the follicular phase, during which the hypothalamus secretes Gonadotropin-Releasing Hormone (GnRH) 2, stimulating the pituitary gland to release Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH) 3, 2.
• FSH stimulates the growth of follicles in the ovary, while LH stimulates the production of androgens by theca cells in the follicle 2.
• As the follicle grows, it produces estradiol, which inhibits GnRH secretion but increases the gonadotropin pool and prepares the follicle for LH influence 4.
• The increasing levels of estradiol trigger a positive feedback loop, causing a surge in LH secretion, which leads to ovulation 2, 4.
• After ovulation, the follicle transforms into the corpus luteum, which secretes progesterone, preparing the endometrium for implantation of a fertilized egg 2, 5.
• If pregnancy does not occur, the corpus luteum regresses, and progesterone levels drop, leading to menstruation 5.

Involved Tissues, Cells, and Hormones

The menstrual cycle involves the coordinated action of various tissues, cells, and hormones, including:

• Hypothalamus: secretes GnRH, which stimulates the pituitary gland to release FSH and LH 2.
• Pituitary gland: releases FSH and LH in response to GnRH 3, 2.
• Ovary: contains follicles that produce estradiol and respond to FSH and LH 3, 2.
• Follicle: composed of theca cells, granulosa cells, and the oocyte, produces estradiol and responds to FSH and LH 2.
• Endometrium: prepares for implantation of a fertilized egg in response to progesterone 2, 5.
• GnRH: stimulates the pituitary gland to release FSH and LH 2.
• FSH: stimulates the growth of follicles in the ovary 3, 2.
• LH: stimulates the production of androgens by theca cells and triggers ovulation 2, 4.
• Estradiol: produced by the follicle, inhibits GnRH secretion and prepares the follicle for LH influence 4.
• Progesterone: produced by the corpus luteum, prepares the endometrium for implantation of a fertilized egg 2, 5.