From the Guidelines
The ovulation process is intricately linked with the endometrial cycle, comprising distinct phases that ensure optimal preparation of the endometrium for potential implantation of a fertilized egg. The phases of ovulation in relation to the endometrial cycle can be understood by examining the hormonal and physiological changes that occur throughout the menstrual cycle. During the follicular phase, which corresponds to the proliferative phase of the endometrial cycle, estrogen levels increase as follicles mature in the ovary, stimulating endometrial thickening and development 1. This phase is crucial for preparing the endometrium for a potential pregnancy. Key features of this phase include:
- Rising estrogen levels
- Follicular development in the ovary
- Endometrial thickening and development of glands and blood vessels Ovulation, triggered by a surge of luteinizing hormone (LH), typically occurs mid-cycle and marks the transition to the luteal phase, corresponding to the secretory phase of the endometrial cycle 1. The luteal phase is characterized by:
- Production of progesterone by the corpus luteum
- Increased vascularity and secretory activity of the endometrium
- Preparation of the endometrium for potential implantation of a fertilized egg Understanding these phases and their hormonal underpinnings is essential for managing conditions like polycystic ovary syndrome (PCOS), which can disrupt normal ovulation and endometrial cycling 1. In clinical practice, recognizing the interplay between ovulation and the endometrial cycle informs the diagnosis and treatment of reproductive disorders, ultimately impacting morbidity, mortality, and quality of life for affected individuals.
From the Research
Phases of Ovulation
The phases of ovulation in relation to the endometrial cycle can be understood by examining the roles of follicle stimulating hormone (FSH) and luteinizing hormone (LH) in folliculogenesis 2. The process involves several stages, including:
- Follicular growth to the stage of antrum formation, which is independent of gonadotrophic stimulation
- Further growth to the stage at which follicles become potentially able to begin preovulatory development, requiring tonic stimulation by FSH
- Selection of one follicle to ovulate, while the remainder become atretic, due to increases in responsiveness to FSH and LH
Follicular Phase
During the follicular phase, LH acts through specific receptors on thecal cells to stimulate androgen production, which is positively involved in folliculogenesis 3. The minimal amount of endogenous LH required for optimal production of oestradiol is likely low, with studies suggesting a threshold of < 1.5 IU/L of plasma LH level 3.
Ovulation and Luteal Phase
Ovulation is triggered by the mid-cycle LH surge, and the dominant follicle releases an oocyte, which then travels to the uterus 2. The empty follicle in the ovary produces progesterone, which prepares the uterine lining for a potential pregnancy 4. The corpus luteum function is determined by factors in both the follicular and luteal phases, including adequate FSH and oestrogen stimulation, as well as LH pulse frequency and amplitude 4, 5.
Hormonal Regulation
The regulation of inhibin and progesterone secretion by the human corpus luteum is dependent on LH, with studies showing a close correlation between serum inhibin and progesterone levels 6. The use of GnRH antagonists has helped to elucidate the role of LH in controlling CL function, with results suggesting that LH is essential for maintaining progesterone and inhibin secretion 6.
Key Hormones
Key hormones involved in the phases of ovulation include:
- FSH, which stimulates follicular growth and development
- LH, which triggers ovulation and maintains corpus luteum function
- Oestradiol, which is produced by the dominant follicle and prepares the uterine lining for pregnancy
- Progesterone, which is produced by the corpus luteum and maintains the uterine lining during the luteal phase
- Inhibin, which is produced by the corpus luteum and regulates FSH secretion 2, 3, 4, 5, 6