What is the Difference Between Luteal Phase and Follicular Phase

The main difference between luteal phase and follicular phase is that the luteal phase occurs after ovulation, whereas the follicular phase occurs before ovulation. 

Luteal and follicular phases are two phases of the ovarian cycle. They occur in the ovary.    

Key Areas Covered

1. What is Luteal Phase
– Definition, Facts, Importance
2. What is Follicular Phase
– Definition, Facts, Importance
3. Similarities Between Luteal Phase and Follicular Phase
– Outline of Common Features
4. Difference Between Luteal Phase and Follicular Phase
– Comparison of Key Differences

Key Terms 

Follicular Phase, Luteal Phase

What is Luteal Phase

The luteal phase is the second ovarian cycle phase that occurs after ovulation. Importantly, the luteal phase does not change its duration depending on individuals. It is approximately 14 days. A high concentration of progesterone levels and low estrogen, FSH, and LH levels characterize it. Especially in the luteal phase, the Graafian follicle differentiates into the corpus luteum, which secretes progesterone. Also, during this phase, the endometrial layer is promoted for implantation.

Figure 1: Luteal Phase

Furthermore, the proliferation of the epithelial layer of the endometrium occurs in response to high estrogen levels secreted by the developing follicle. It occurs in the proliferative phase of the uterine cycle. In the ovarian cycle, the endometrial layer is in the secretory phase in response to the high progesterone levels secreted by the corpus luteum in the luteal phase. At the end of the ovarian cycle, corpus luteum atrophies and progesterone levels also drop. 

What is Follicular Phase

The follicular phase is the main phase of the ovarian cycle in which the maturation of the Graafian follicle occurs. Follicles in the ovary mature to become Graafian follicles during this phase. The follicular phase ends up with ovulation. Other names of the follicular phase include the proliferative phase and preovulatory phase. Also, gonadotrophin-releasing hormones are the primary hormone secreted in the follicular phase. They include follicular stimulating hormone (FSH) and luteinizing hormone (LH). The duration of the follicular phase depends on the length of the menstrual cycle.

Figure 2: Luteal Phase

Moreover, in the follicular phase, increased FSH levels stimulate the secretion of inhibin B protein from granulosa cells. LH surge occurs at the highest levels of inhibin B. At the end of the follicular phase, inhibin B blunt the secretion of FSH. Meanwhile, increased FSH levels recruit 5-6 ovarian follicles in the tertiary stage and proliferate the granulosa layer of the developing follicles, inducing the secretion of estrogen. Also, LH stimulates the proliferation of the thecal cells of follicles.  

Similarities Between Luteal Phase and Follicular Phase

• The Luteal and follicular phases are two phases of the ovarian cycle.
• They are responsible for the release of an ovum from the ovary.

Difference Between Luteal Phase and Follicular Phase

Definition

The luteal phase is the second stage of the ovarian cycle that occurs after ovulation. In contrast, the follicular phase is the main stage of the ovarian cycle in which the maturation of ovarian follicles occurs to prepare one of them for release during ovulation.

Ovulation

The luteal phase occurs after ovulation, while the follicular phase occurs before ovulation. 

Function

The ruptured Graafian follicle transforms into the corpus luteum in the luteal phase, while in the follicular phase, the primary follicle grows to become a mature follicle.

Occurrence

The luteal phase occurs 14 days after ovulation, while the follicular phase occurs in the first 10-12 days of the ovarian cycle. 

Hormones

The luteal phase produces progesterone, while many estrogen, FSH, and LH occur in the follicular phase. 

Conclusion

In brief, the luteal and follicular phases are two ovarian cycle phases. The luteal phase occurs after ovulation. A ruptured Graafian follicle develops into the corpus luteum in the luteal phase. It occurs in the 14 days after ovulation. Also, the luteal phase produces progesterone in high concentrations. In comparison, the follicular phase is the phase that occurs before ovulation. It occurs during the 10-12 days of the ovarian cycle. FSH, LH, and estrogen also occur in high concentrations during the follicular phase. Therefore, the main difference between luteal phase and follicular phase is their occurrence.  

References:

1. Luteal phase of the menstrual cycle: Symptoms & length. Cleveland Clinic. 
2. Follicular phase of menstrual cycle: Hormone levels & length. Cleveland Clinic. 

Image Courtesy:

1. “MenstrualCycle2 en.” By Isometrik – Own work (CC-BY SA 3.0) via Commons Wikimedia
2. “Follicular phase diagram – early and late.” By Procedureready – Own Work (CC-BY SA 3.0) via Commons Wikimedia