Human Reproductive Biology 정리노트 Ch03 The Menstrual Cycle [월경주기]

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1. Reproductive Cycle in Mammals

Seasonal breeders는 낮의 길이 변화를 감지하여 배란하는 동물(말, 양 등)이며, 인간은 그렇지 않다. Estrous cycle(발정주기)은 발정행동을 보이는 동물의 암컷 생식주기이며, Menstrual cycle(생리주기)은 자궁출혈을 동반한 동물의 암컷 생식주기이다. Induced ovulation(배란 유도)은 교미 또는 다른 남성의 성적 행동 후에만 배란이 일어나는 것이며, Spontaneous ovulation(자발적 배란)은 남성의 성행위에 영향을 받지 않고 배란이 일어나는 것이다.
2. Menstrual Cycle

월경주기는 약 25-30일 정도이며, 때로는 35일까지 지속될 수 있다. 월경주기는 월경기, 난포기, 황체기로 구분된다. 월경기에는 자궁내막의 허혈로 인해 자궁내막이 파괴되고 월경 분비물이 배출된다. 난포기에는 FSH와 에스트라디올의 작용으로 난포가 성장하고 자궁내막이 증식한다. 황체기에는 프로게스테론과 에스트라디올이 분비되어 자궁내막을 분비기로 준비시킨다.
3. Ovaries in Menstrual Cycle

월경주기 동안 난소에서는 다음과 같은 변화가 일어난다. 이전 주기의 황체는 1일차까지 완전히 퇴화하며, 에스트라디올과 프로게스테론 수치가 크게 감소한다. 1일차에는 작은 삼차 난포와 퇴화 난포만 존재하지만, FSH의 자극으로 인해 이들이 성장하기 시작한다.
4. Hormonal Changes during Follicular Phase

난포기 동안 FSH와 에스트라디올의 상호작용으로 인해 난포가 급속도로 성장한다. 에스트라디올은 자궁내막을 증식시키고 자궁의 혈관 발달과 자궁근층 수축을 유도한다. 에스트라디올 농도가 증가하면 LH 급증을 유발한다. LH 급증 직전에는 프로게스테론 수치도 약간 상승한다.
5. Hormonal Changes during Ovulation

LH 급증은 GnRH에 대한 에스트라디올의 긍정적 피드백 효과에 의해 일어난다. LH 급증은 FSH 급증보다 더 크게 나타나는데, 이는 난소에서 분비되는 추가적인 호르몬(inhibin, follistatin)이 FSH 분비를 억제하기 때문이다.
6. Hormonal Changes during Luteal Phase

황체기에는 에스트로겐과 프로게스테론이 분비되어 자궁내막을 분비기 상태로 유지한다. 프로게스테론은 GnRH에 대한 부정적 피드백을 일으켜 FSH와 LH 분비를 억제한다. 황체 퇴화 4일 전부터 프로게스테론 수치가 감소하면서 새로운 주기가 시작된다.
7. Corpus Luteum Formation and Maintenance

LH에 의해 황체세포에서 프로게스테론이 분비되며, 다른 호르몬들(prolactin, growth hormone, IGF-I, oxytocin 등)도 황체 유지에 관여한다. 황체 퇴화 기전은 아직 명확히 밝혀지지 않았다.

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1. Reproductive Cycle in Mammals

The reproductive cycle in mammals is a complex and fascinating process that involves the coordination of various hormonal and physiological changes within the body. This cycle is essential for the continuation of the species and plays a crucial role in the development and growth of offspring. Understanding the intricacies of the reproductive cycle in mammals is crucial for various fields, including biology, medicine, and animal husbandry. By studying this process, we can gain insights into the mechanisms that govern fertility, pregnancy, and the overall health and well-being of mammals. Additionally, this knowledge can be applied to address issues related to reproductive health, such as infertility, contraception, and the management of reproductive disorders. Ultimately, the study of the reproductive cycle in mammals contributes to our understanding of the fundamental principles of life and the evolution of species.
2. Menstrual Cycle

The menstrual cycle is a remarkable physiological process that occurs in the female reproductive system. It is a cyclical pattern of hormonal changes that prepares the uterus for the potential implantation of a fertilized egg. This cycle is a testament to the incredible complexity and adaptability of the human body. Understanding the various stages of the menstrual cycle, including the follicular phase, ovulation, and the luteal phase, is crucial for maintaining reproductive health, managing menstrual-related issues, and planning for pregnancy. Additionally, the study of the menstrual cycle has broader implications, as it provides insights into the interplay between hormones, the endocrine system, and the overall well-being of women. By exploring the nuances of the menstrual cycle, we can develop more effective strategies for addressing reproductive health concerns, empowering women to take control of their bodies and make informed decisions about their reproductive choices.
3. Ovaries in Menstrual Cycle

The ovaries play a central role in the menstrual cycle, serving as the primary site of hormone production and the release of mature eggs. During the follicular phase, the ovaries undergo a series of complex changes, including the development of follicles and the secretion of estrogen. This hormonal interplay prepares the uterus for potential implantation and sets the stage for ovulation, the release of a mature egg. The ovaries also produce progesterone during the luteal phase, which helps to maintain the uterine lining and support early pregnancy if fertilization occurs. Understanding the intricate workings of the ovaries and their role in the menstrual cycle is crucial for addressing a wide range of reproductive health issues, from infertility to polycystic ovarian syndrome. By delving deeper into the ovarian function, we can develop more targeted and effective interventions to support women's reproductive health and well-being.
4. Hormonal Changes during Follicular Phase

The follicular phase of the menstrual cycle is characterized by a complex interplay of hormonal changes that prepare the body for ovulation and potential fertilization. During this phase, the pituitary gland secretes follicle-stimulating hormone (FSH), which stimulates the growth and development of follicles in the ovaries. These follicles, in turn, produce increasing amounts of estrogen, which has a profound impact on the uterine lining, cervical mucus, and other reproductive organs. The rise in estrogen levels also triggers a surge in luteinizing hormone (LH) from the pituitary gland, which ultimately leads to ovulation. Understanding the intricate hormonal changes during the follicular phase is crucial for understanding fertility, menstrual irregularities, and the overall reproductive health of women. By delving deeper into this phase, we can develop more effective strategies for managing reproductive disorders, supporting fertility, and empowering women to make informed decisions about their reproductive choices.
5. Hormonal Changes during Ovulation

Ovulation is a critical event in the menstrual cycle, marking the release of a mature egg from the ovary. This process is driven by a surge in luteinizing hormone (LH) from the pituitary gland, which triggers the final maturation and release of the egg. During ovulation, the ovarian follicle that contained the egg ruptures, allowing the egg to be released and travel down the fallopian tube, where it may be fertilized by a sperm. Alongside the LH surge, there are also significant changes in the levels of other hormones, such as estrogen and progesterone, which play crucial roles in preparing the uterine lining for potential implantation. Understanding the intricate hormonal changes that occur during ovulation is essential for addressing issues related to fertility, contraception, and the management of reproductive disorders. By delving deeper into this critical phase of the menstrual cycle, we can develop more effective strategies to support women's reproductive health and empower them to make informed choices about their fertility.
6. Hormonal Changes during Luteal Phase

The luteal phase of the menstrual cycle is characterized by a series of hormonal changes that prepare the uterus for potential implantation and support early pregnancy. After ovulation, the empty follicle that released the egg transforms into the corpus luteum, which begins to secrete progesterone. This surge in progesterone helps to thicken and stabilize the uterine lining, making it more receptive to implantation. Alongside the rise in progesterone, there are also changes in the levels of other hormones, such as estrogen and luteinizing hormone (LH), which play important roles in regulating the menstrual cycle. Understanding the complex hormonal dynamics during the luteal phase is crucial for addressing issues related to infertility, menstrual irregularities, and the management of reproductive disorders. By delving deeper into this phase, we can develop more effective strategies to support women's reproductive health and empower them to make informed choices about their fertility.
7. Corpus Luteum Formation and Maintenance

The corpus luteum is a remarkable structure that forms in the ovary after ovulation and plays a crucial role in the menstrual cycle and early pregnancy. Following the release of the mature egg, the empty follicle transforms into the corpus luteum, which begins to secrete progesterone. This surge in progesterone helps to prepare the uterine lining for potential implantation and supports early pregnancy if fertilization occurs. The maintenance of the corpus luteum is essential for the continuation of the pregnancy, as it provides the necessary hormonal support until the placenta takes over this function. Understanding the formation and maintenance of the corpus luteum is crucial for addressing issues related to infertility, miscarriage, and the management of reproductive disorders. By exploring the intricacies of this process, we can develop more effective strategies to support women's reproductive health and improve outcomes for those facing challenges in their fertility journey.