Explains the ovarian cycle's follicular, ovulatory and luteal phases, with hormonal control by GnRH, FSH, LH, estrogen, progesterone and inhibin. Then covers the menstrual cycle, describing the uterine layers, endometrial zones and menstrual, proliferative and secretory phases. Ends with ovulation, including oocyte maturation through meiosis, the role of cortical granules and where fertilisation usually occurs.
Topic 74 of 127 in the MU Sofia syllabus for Cytology (Dental Medicine, Year 1). The free AceMed note covers it in 4 sections, about 1,075 words (8 min of reading), with 15 flashcards.
In this note
Ovarian Cycle, Menstrual Cycle, and Ovulation,
The Ovarian Cycle
The Menstrual Cycle
Ovulation
The Ovarian Cycle
Definition: The ovarian cycle describes the changes that occur in the follicles of the ovary over the course of a month. Its average duration is 28 days.
Purpose: The ovaries produce both oocytes (female gametes) and sex hormones.
Hormonal Control: The cycle is initiated by Gonadotropin-Releasing Hormone (GnRH) from the hypothalamus, which stimulates the anterior pituitary to secrete Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH).
Phases of the Ovarian Cycle:
1. Follicular Phase:
This is the first part of the ovarian cycle where ovarian follicles mature and prepare to release an egg.
Through a rise in FSH during the initial days, a small group of ovarian follicles (present since birth but arrested in development) are stimulated to grow.
Typically, only one dominant follicle (also called a tertiary or vesicular follicle) continues to develop to maturity, containing the ovum.
Estrogen, secreted by the growing follicles, plays a crucial role in the development of this vesicular follicle.
Growing primary follicles differentiate into theca interna (steroid-secreting, producing progesterone and estrogen precursors) and theca externa (fibrous). Granulosa cells convert these precursors into estrogen.
2. Ovulation:
This stage involves the release of the tertiary (dominant) follicle, containing the ovum, from the ovary into the oviduct.
It normally occurs around the 14th day of a typical 28-day cycle.
A surge in LH (triggered by high estrogen levels from the mature follicle stimulating the hypothalamus and anterior pituitary) around day 12 of the cycle is the primary trigger.
The LH surge matures the egg and weakens the wall of the follicle. This weakening involves plasminogen degrading collagen in the tunica albuginea and surface epithelium, combined with smooth muscle contractions in the theca externa (triggered by prostaglandins).
The oocyte, along with surrounding granulosa cells (forming the corona radiata) and follicular fluid, is expelled.
3. Luteal Phase:
After ovulation, the remaining granulosa cells and theca interna of the ruptured follicle reorganise to form a temporary endocrine gland called the corpus luteum (L., yellowish body).
Under the influence of LH, these cells become specialised to produce large amounts of progesterone and some estrogens.
Progesterone and estrogen, along with the polypeptide hormone inhibin (produced by granulosa and luteal cells), provide negative feedback to the hypothalamus and anterior pituitary, suppressing FSH and LH production.
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