Systems Biology
A typical menstrual cycle is 28 days, but can range from 21 to 35 days. The cycle starts on the first day of a period and ends just before the bleeding begins during the next period. The cycle is divided into the follicular phase (characterized by egg maturation within ovarian follicles) and the luteal phase (charaterized by presence of the corpus luteum, the eggless remnant of the follicle that forms after the egg is discharged). During the follicular phase, follicle stimulating hormone (FSH) is released, stimulating follicles in the ovaries to grow and releasing estradiol, the body's most active form of estrogen, which thickens the uterine lining (endometrium). Rising estradiol levels trigger a surge of luteinizing hormone (LH) around day 11 to 13, leading to ovulation.
After ovulation, the remnants of the follicle transform into the corpus luteum, entering the luteal phase. The corpus luteum secretes progesterone, the dominant hormone during this part of the cycle, which causes the endometrial lining to become more favorable for implantation. If fertilization and implantation do not occur, the corpus luteum degenerates and estrogen and progesterone levels go down, leading to the breakdown of endometrial tissue and bleeding during menses. The decline in estrogen and progesterone removes their negative feedback effect on gonadotropin-releasing hormone (GnRH), allowing FSH and LH levels to rise, thus recruiting a new set of follicles and returning to day one of the cycle.
Lesson Outline
<ul> <li>Introduction to typical menstrual cycle</li> <ul> <li>Normal cycle lasts 21 to 35 days</li> <li>Bleeding lasts four to six days</li> <li>Abnormal cycles could indicate underlying problems</li> </ul> <li>Two phases of the menstrual cycle</li> <ul> <li>Follicular phase</li> <li>Luteal phase</li> <li>Variations in cycle lengths mostly due to follicular phase length</li> </ul> <li>Follicular phase</li> <ul> <li>FSH is released due to GnRH secretion</li> <li>Stimulates follicles in ovaries to grow</li> <li>Dominant follicle develops, releasing estradiol (estrogen)</li> <li>Estradiol causes uterine lining to proliferate and thicken</li> <li>Luteinizing hormone (LH) levels rise</li> <li>LH surge around day 11 to 13, triggering ovulation</li> </ul> <li>Ovulation</li> <ul> <li>Dominant follicle releases egg</li> <li>Follicle remnants transform into corpus luteum</li> <li>Luteal phase begins</li> </ul> <li>Luteal phase</li> <ul> <li>Corpus luteum secretes progesterone</li> <li>Progesterone causes the endometrial lining to change, preparing for potential implantation</li> <li>If fertilization does not occur, corpus luteum degenerates</li> <li>Decline of estrogen and progesterone levels</li> <li>Endometrial tissue breaks down and is shed during menses</li> </ul></ul>
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FAQs
The two main phases of the menstrual cycle are the follicular phase and the luteal phase. The follicular phase starts on the first day of menstruation and ends with ovulation. It is characterized by the stimulation of follicle growth under the influence of follicle-stimulating hormone (FSH). The follicles produce estradiol, which helps in the development of the endometrial lining. The luteal phase, on the other hand, occurs after ovulation and involves the formation of the corpus luteum, the remnant of the follicle that secretes progesterone. This phase is characterized by the thickening of the endometrial lining in preparation for implantation of a fertilized egg.
Throughout the menstrual cycle, hormone levels fluctuate to regulate different processes. In the follicular phase, estradiol levels rise as the follicles develop, with a significant increase right before ovulation. FSH stimulates growth and maturation of the follicles, with levels rising in the first half of the follicular phase and then decreasing as a dominant follicle is selected. Luteinizing hormone (LH) stays relatively low during the early follicular phase but experiences a surge about 24 to 48 hours prior to ovulation, triggering the release of the mature egg. In the luteal phase, both estradiol and progesterone levels increase in response to the formation of the corpus luteum.
The corpus luteum is a temporary endocrine structure that forms from the remnants of the ruptured follicle after ovulation. Its primary function during the luteal phase is to produce and secrete progesterone, a hormone that is essential for maintaining the thickened endometrial lining and preparing the uterus for potential implantation of a fertilized egg. The corpus luteum also produces some estradiol. If implantation does not occur, the corpus luteum degenerates, causing a decrease in progesterone and estradiol levels that lead to the onset of menstruation.
The endometrial lining undergoes several changes during the menstrual cycle in order to prepare for the possible implantation of a fertilized egg. During the follicular phase, the rising levels of estradiol stimulate the thickening and proliferation of the endometrial lining. In the luteal phase, the corpus luteum produces progesterone, which further matures the endometrium, making it more glandular and vascular. These changes ensure that the endometrial lining can provide optimal support and nourishment for a developing embryo if implantation occurs.
Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are two key hormones involved in the regulation of the menstrual cycle. Both are gonadotropins (hormones that stimulate the gondas) produced and released by the anterior pituitary gland. FSH stimulates the growth and maturation of ovarian follicles during the follicular phase. As the follicles develop, they release estradiol, which provides negative feedback to the pituitary gland, decreasing FSH levels. When the dominant follicle is selected, estradiol levels rise significantly, causing a rapid surge in LH levels. This LH surge is responsible for the final maturation of the egg and triggers its release from the ovary during the process of ovulation.
