[#3] OVÁRIOS: CICLO MENSTRUAL (CICLO UTERINO) | MK Fisiologia

Understanding the Hormonal Regulation of the Uterine Cycle

Introduction to Uterine Cycle

• The video introduces the focus on hormonal regulation of the uterine cycle, following a previous discussion on ovarian hormonal regulation.

• It emphasizes that understanding the uterine cycle requires knowledge of its structure, specifically noting that the uterus has three layers: endometrium (inner mucosal layer), myometrium (muscular layer), and perimetrium (outer connective tissue).

Phases of the Uterine Cycle

• The endometrium undergoes significant cyclical changes, leading to its classification as part of the menstrual cycle, which can be divided into three phases: menstrual phase, proliferative phase, and secretory phase.

• These phases are influenced by events in both the follicular and luteal phases of the ovarian cycle discussed previously.

Menstrual Phase

• Near the end of the luteal phase, if fertilization does not occur, regression of the corpus luteum leads to a sharp decline in progesterone and estrogen levels.

• This drop causes shedding of the functional layer of the endometrium—marking the start of menstruation—which lasts about 3 to 5 days in a typical 28-day cycle.

Proliferative Phase

• During this phase, as ovarian follicles develop and estrogen levels rise, it stimulates proliferation in non-shed cells within the basal layer of endometrium.

• Restoration occurs in stromal cells and uterine glands lost during menstruation due to estrogen's proliferative effects.

Secretory Phase

• The secretory phase begins post-ovulation with increased progesterone from corpus luteum; this hormone prepares endometrial cells for potential embryo implantation.

• Progesterone promotes glandular secretion within an enriched vascular environment created by spiraled arteries supplying nutrients essential for embryo support.

Conclusion on Endometrial Preparation

• Overall actions during this phase ensure that if fertilization occurs, conditions are optimal for implantation through enhanced nutrient supply and vascularization.

Understanding the Uterine Cycle

The Role of Hormones in the Uterine Cycle

• If the ovum is not fertilized, the corpus luteum regresses, leading to a sharp decline in progesterone and estrogen levels. This drop triggers changes in the endometrium.

• The decrease in hormone levels stimulates the secretion of enzymes and inflammatory substances by the endometrium, which degrade extracellular matrix and cause vasoconstriction of spiral arteries.

• Vasoconstriction results in ischemia, reducing blood flow to the functional zone of the endometrium, causing cell death due to hypoxia and leading to menstruation.

• Following vasoconstriction, there is a phase of vasodilation that can lead to minor hemorrhage from ruptured spiral arteries; women typically lose about 30 ml of blood during menstruation.

• Prostaglandins also induce uterine contractions necessary for expelling blood and tissue; these contractions are often responsible for menstrual cramps.

Phases of the Uterine Cycle

• The uterine cycle consists of three phases: menstrual phase (beginning with shedding), proliferative phase (regeneration post-menstruation), and secretory phase (preparation for potential implantation).

• During the proliferative phase, estrogen from developing follicles promotes regeneration of the functional zone until ovulation occurs.

• In the secretory phase, high levels of progesterone prepare the endometrium for embryo implantation if fertilization occurs; if not, hormone levels drop again.

Cyclical Changes Beyond Endometrium

• Other reproductive structures like fallopian tubes, myometrium, and cervix also undergo cyclical changes influenced by estrogen and progesterone throughout menstrual cycles.

• Estrogen facilitates fertilization by promoting nutrient-rich mucus secretion in fallopian tubes and aiding sperm movement towards it while relaxing cervical conditions for sperm entry.

Actions Post-Ovulation

• After ovulation, progesterone takes precedence over estrogen; its role shifts towards supporting embryo implantation rather than facilitating fertilization processes.

• Progesterone reduces ciliary movement within fallopian tubes while promoting relaxation in myometrial muscles to support pregnancy maintenance.

Conclusion & Next Steps

• Understanding hormonal actions across various reproductive organs highlights their broader physiological roles beyond just reproduction.

• Future discussions will delve deeper into non-reproductive effects of estrogen and progesterone on other body systems.