What is the Difference Between Proliferative Phase and Secretory Phase

The main difference between proliferative phase and secretory phase is that in the proliferative phase, endometrial cells proliferate, increasing the thickness of the lining, whereas, in the secretory phase, uterine glands secrete watery secretions.

The proliferative phase and secretory phase are two phases of the uterine cycle. Their function is to prepare the uterus for implantation.  

Key Areas Covered

1. What is Proliferative Phase
– Definition, Structure, Function
2. What is Secretory Phase
– Definition, Structure, Function
3. Similarities Between Proliferative Phase and Secretory Phase
– Outline of Common Features
4. Difference Between Proliferative Phase and Secretory Phase
– Comparison of Key Differences

Key Terms 

Follicular Phase, Proliferative Phase, Secretory Phase 

What is Proliferative Phase 

The proliferative or the follicular phase is the first phase of the uterine cycle, occurring on the first to 14th day. Estrogen is the primary hormone secreted by the ovary during the proliferative phase. Upregulation of the FSH receptors within the follicle increases the estrogen level. Also, the endometrial layer’s growth is the proliferative phase’s primary function. Estrogen is responsible for the growth of the endometrial layer. It increases the stroma and glands in the endometrial layer. Additionally, it increases the depth of spiral arteries that provide blood to the endometrium.

Figure 1: The Uterine Cycle

Furthermore, estrogen creates an environment that is helpful to the incoming sperm. It forms channels within the cervix, allowing the entry of the sperm. Here, cervical mucous undergoes much watery and elasticity changes. However, at the end of the proliferative phase, high estrogen levels produce a negative feedback effect on the anterior pituitary. Additionally, the primary follicle develops into the Graafian follicle during the proliferative phase. The Graafian follicle becomes mature by the degeneration of the surrounding follicles, setting up ovulation on the 14th day of the uterine cycle. 

What is Secretory Phase

The secretory or luteal phase is the second phase of the uterine cycle that occurs from the 14th day to the 28th day. In this phase, LH stimulates progesterone production by the corpus luteum. The Corpus luteum is the structure formed at the site of the mature follicle rupture. However, progesterone is the main hormone in the secretory phase, preparing the endometrium for the implantation of the fertilized ovum. In the secretory phase, the endometrium increases the vascular supply while increasing the mucus secretions. Also, progesterone decreases the rate of endometrial proliferation and the thickness of the lining of the endometrium. Further, it develops more complex glands, accumulates glycogen as the energy source, and provides more surface area to the spiral arteries.

Figure 2: Endometrium in the Secretory Phase

Moreover, progesterone decreases the thickness of the cervical mucous and makes it non-elastic. It increases the temperature of the body during the secretory phase. Corpus luteum persists in producing progesterone if the pregnancy occurs. If not, it regresses, decreasing the serum levels of progesterone. 

Similarities Between Proliferative Phase and Secretory Phase

• The proliferative and secretory phases are two phases of the uterine cycle.
• They are important for the preparation of the uterus for implantation.
• They occur in the uterus.

Difference Between Proliferative Phase and Secretory Phase

Definition

The proliferative phase is when the endometrial cells multiply and spread during the menstrual cycle. In contrast, the secretory phase is the second half of the menstrual cycle after ovulation. 

Phase

The proliferative phase is the first phase of the uterine cycle, while the secretory phase is the second phase of the uterine cycle. 

Occurrence

The proliferative phase occurs from day one to day 14 of the uterine cycle, while the secretory phase occurs from day 14 to day 28.  

Follicle 

The primary follicle develops into the Graafian follicle in the proliferative phase, while the empty Graafian follicle develops into the corpus luteum in the secretory phase.

Hormones

The ovary secretes estrogen in the proliferative phase, while the ovary secretes progesterone in the secretory phase. 

Endometrium Thickness

The endometrium is 2-3 mm thick in the proliferative phase, while the endometrium is 5 mm thick in the secretory phase.

Secretions

The endometrium is not secretory in the proliferative phase, while the endometrium secretes watery secretions in the secretory phase.  

Conclusion

In brief, the proliferative and secretory phases are two phases of the uterine cycle. The proliferative phase is the first phase of the uterine cycle, occurring from the first to the 14th day of the uterine cycle. Simultaneously, the ovary develops the primary follicle into the Graafian follicle and secrete estrogens in the proliferative phase. Also, the endometrium is 2-3 mm thick in this phase. In comparison, the secretory phase is the second phase of the uterine cycle, which occurs from the 14th day to the 28th day. In the ovary, the empty Graafian follicle develops into the corpus luteum and secretes progesterone during the secretory phase. However, the endometrium of the secretory phase is 5 mm thick, and it secretes watery secretions. Therefore, the difference between proliferative and secretory phases is the endometrial secretions. 

References:

1. Thiyagarajan DK, Basit H, Jeanmonod R. Physiology, Menstrual Cycle. [Updated 2022 Oct 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-.

Image Courtesy:

1. “Menstrual Cycle bottom” By CFCF – Own work (CC-BY SA 3.0) via Commons Wikimedia
2. “Secretory phase endometrium” By KGH – Own Work (CC-BY SA 3.0) via Commons Wikimedia