13.14: Hormonal Regulation of the Female Reproductive Cycles
- Page ID
- 121777
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The ovarian and uterine cycles are hormonally coordinated, each with two main phases, to regulate ovulation, prepare the uterus for pregnancy, and reset the system when pregnancy does not occur.
- Describe the ovarian and uterine cycles and explain how they work together to form the menstrual cycle.
- Identify the two phases of the ovarian cycle and the two phases of the uterine cycle, including the major events that occur in each phase.
- Distinguish between the outcomes of the luteal phase when pregnancy occurs versus when pregnancy does not occur.
Female Reproductive Cycles: Dual Cycles with Paired Phases
The menstrual cycle is the roughly monthly sequence of coordinated hormonal and tissue changes that prepares the body for pregnancy. It includes
- the ovarian cycle, in which an ovarian follicle matures, ovulation occurs, and the corpus luteum forms and then regresses if pregnancy does not occur, and
- the uterine cycle, in which the endometrium thickens to support possible implantation and then sheds as menstruation if implantation does not happen.
The Two Cycles: Ovarian and Uterine Cycles
The ovarian and uterine cycles are two coordinated, repeating cycles that occur in the female reproductive system and together make up the menstrual cycle. The ovarian cycle takes place in the ovaries and focuses on the maturation and release of an oocyte, while the uterine cycle (also called menstrual cycle) occurs in the uterus and prepares the endometrium for a possible pregnancy. Although these cycles happen in different organs, they are tightly linked by hormones and occur on the same timeline. Changes in ovarian hormones drive predictable structural and functional changes in the uterus, ensuring that ovulation and uterine preparation are synchronized. Understanding how these two cycles work together is essential for understanding fertility, menstruation, and the hormonal regulation of the female reproductive system.
The Ovarian Cycle
Primordial follicles are the “sleeping” follicles that make up a female’s lifetime ovarian reserve. During the reproductive years, a small number of these primordial follicles are continuously activated (recruited) to begin developing into primary follicles. Most of the recruited follicles do not make it very far and instead die off through atresia. Only a small cohort continues developing into secondary follicles and then into tertiary follicles, and typically just one becomes the dominant tertiary (Graafian) follicle that will ovulate in a given cycle by releasing its egg in a process called ovulation. This pattern repeats month after month until the ovarian reserve is depleted, which ultimately leads to menopause.
After ovulation, the emptied follicle transforms into a temporary hormone-making structure called the corpus luteum, which releases hormones that support the uterus in case pregnancy occurs. If fertilization does not happen, the corpus luteum breaks down, hormone levels fall, and the next cycle begins.
The ovarian cycle has two main phases: the follicular phase and the luteal phase. The follicular phase begins on day 1 of the cycle (the first day of menstrual bleeding) and lasts until ovulation, when an oocyte is released. During this time, several follicles begin to grow, but usually only one becomes the dominant follicle. As it matures, it produces rising levels of estrogen, which supports follicle development and helps rebuild and thicken the uterine lining. After ovulation, the luteal phase begins. The emptied follicle transforms into the corpus luteum, a temporary hormone-secreting structure that releases mostly progesterone (and some estrogen) to stabilize and further prepare the uterine lining for possible implantation. If fertilization and implantation do not occur, the corpus luteum breaks down, hormone levels fall, and the next menstrual period begins.
The Uterine (Endometrial) Cycle
The uterine cycle, also called the endometrial cycle, describes the predictable monthly changes that occur in the endometrium, the innermost glandular lining of the uterus. These changes are driven by fluctuations in ovarian hormones and prepare the uterus for a possible pregnancy. The cycle is counted from the first day of menstrual bleeding, and a typical cycle lasts about 28 days, although normal cycles vary in length.
During each cycle, the endometrium follows a repeating pattern of shedding, regrowth, and specialization.
After menstruation, rising estrogen levels stimulate the endometrium to rebuild and thicken, increasing glandular tissue and blood vessel supply. Following ovulation, progesterone further modifies the lining, making it highly secretory and capable of supporting an implanted embryo. If fertilization and implantation do not occur, ovarian hormone levels fall, the functional layer of the endometrium is shed, and a new cycle begins.
Like the ovarian cycle, the uterine cycle has two main phases: the proliferative phase and the secretory phase. The proliferative phase begins on day 1 with menstrual bleeding, during which the superficial functional layer of the endometrium is shed. As estrogen levels rise during the follicular phase of the ovarian cycle, the endometrium regenerates and thickens in preparation for possible pregnancy. After ovulation, the secretory phase begins under the influence of progesterone released by the corpus luteum. During this phase, the endometrium becomes highly vascular and glandular and secretes nutrients that support potential implantation. If implantation does not occur, progesterone and estrogen levels decline, triggering breakdown of the endometrial lining and the onset of the next menstrual period.
Each Cycle Has Two Phases
As you have seen above, the ovarian and uterine cycles are closely linked and follow a predictable pattern each month. Also, in both cycles, events are organized into two main phases. Here is a little closer look at the 4 phases:
The Follicular Phase (Ovary) and the Proliferative Phase (Uterus)
The first half of a typical ovarian cycle is called the follicular phase, while the equivalent time frame in the uterus is called the proliferative phase.
In the ovary, several primordial follicles (each containing an oocyte surrounded by supporting follicle cells) begin to grow. FSH from the anterior pituitary stimulates follicle growth by causing granulosa cells to multiply. As the follicles enlarge, they produce increasing amounts of estradiol (a major estrogen). Typically, one follicle becomes the dominant follicle and continues to mature, while the others stop developing and undergo atresia (they break down).
The uterus — as estradiol levels rise — responds by rebuilding and thickening the endometrium (that is why this time period in the uterus is called the proliferative phase). Estradiol also makes cervical mucus thinner and more slippery, which helps sperm survive and move through the cervix. Near the end of this phase, high estradiol triggers a surge of LH (luteinizing hormone). This LH surge is the signal that leads to ovulation, which typically occurs about 30 ± 2 hours after the LH surge begins.
Ovulation
Ovulation is the part of the ovarian cycle when a mature tertiary (Graafian) follicle breaks open and releases an ovum (also called an oocyte, female gamete, or egg.
The Luteal Phase (Ovary) and the Secretory Phase (Uterus)
In the ovaries, the luteal phase begins immediately after ovulation as FSH and LH stimulate the remnants of the dominant follicle to transform into the corpus luteum. The corpus luteum grows for a short time and secretes large amounts of progesterone and smaller amounts of estrogen. Progesterone prepares the endometrium for implantation by thickening it and stimulating glandular secretions that support early embryonic development. Progesterone also causes a measurable rise in basal body temperature.
The luteal phase ends in one of two ways: pregnancy or luteolysis.
If pregnancy occurs, the implanted embryo produces human chorionic gonadotropin (hCG), a hormone similar to luteinizing hormone (LH). hCG maintains the corpus luteum, allowing continued secretion of progesterone. Progesterone preserves the endometrium and sustains the elevated basal body temperature characteristic of the luteal phase. After approximately eight to twelve weeks, the placenta takes over progesterone production. Because hCG is unique to pregnancy, most pregnancy tests detect this hormone.
If pregnancy does not occur, luteolysis takes place. The corpus luteum degenerates, causing progesterone and estrogen levels to fall. This decline in ovarian hormones triggers an increase in follicle-stimulating hormone (FSH), which initiates follicle recruitment for the next ovarian cycle.
