Menstrual Cycle Flashcards
What
The menstrual cycle consists of two phases: the follicular phase and the luteal phase. The follicular phase is from the start of menstruation to the moment of ovulation (the first 14 days in a 28-day cycle). The luteal phase is from the moment of ovulation to the start of menstruation (the final 14 days of the cycle).
Follicular Phase
From puberty, the ovaries have a finite number of cells that have the potential to develop into eggs. These cells are called oocytes. Granulosa cells surround the oocytes, forming structures called follicles.
Follicles go through four key stages of development in the ovaries:
Primordial follicles
Primary follicles
Secondary follicles
Antral follicles (also known as Graafian follicles)
The process of primordial follicles maturing into primary and secondary follicles is always occurring, independent of the menstrual cycle. Once the follicles reach the secondary follicle stage, they develop the receptors for follicle stimulating hormone (FSH). Further development after the secondary follicle stage requires stimulation from FSH.
At the start of the menstrual cycle, FSH stimulates further development of the secondary follicles. As the follicles grow, the granulosa cells that surround them secrete increasing amounts of oestradiol (oestrogen). The oestradiol has a negative feedback effect on the pituitary gland, reducing the quantity of LH and FSH produced. The rising oestrogen also causes the cervical mucus to become more permeable, allowing sperm to penetrate the cervix around the time of ovulation.
One of the follicles will develop further than the others and become the dominant follicle. Luteinising hormone (LH) spikes just before ovulation, causing the dominant follicle to release the ovum (an unfertilised egg) from the ovary. Ovulation happens 14 days before the end of the menstrual cycle, for example, day 14 of a 28-day cycle, or day 16 of a 30-day cycle.
Luteal Phase
After ovulation, the follicle that released the ovum collapses and becomes the corpus luteum. The corpus luteum secretes high levels of progesterone, which maintains the endometrial lining. This progesterone also causes the cervical mucus to become thick and no longer penetrable. The corpus luteum also secretes a small amount of oestrogen.
When fertilisation occurs, the syncytiotrophoblast of the embryo secretes human chorionic gonadotrophin (HCG). HCG maintains the corpus luteum. Without hCG, the corpus luteum degenerates. Pregnancy tests check for hCG to confirm a pregnancy.
When there is no fertilisation of the ovum, and no production of hCG, the corpus luteum degenerates and stops producing oestrogen and progesterone. This fall in oestrogen and progesterone causes the endometrium to break down and menstruation to occur. Additionally, the stromal cells of the endometrium release prostaglandins. Prostaglandins encourage the endometrium to break down and the uterus to contract. Menstruation starts on day 1 of the menstrual cycle. The negative feedback from oestrogen and progesterone on the hypothalamus and pituitary gland ceases, allowing the levels of LH and FSH to begin to rise, and the cycle to restart.
Placenta Functions
Respiration
The placenta is the only source of oxygen for the fetus. Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin. The fetal haemoglobin is more attractive to oxygen molecules than the maternal haemoglobin. As a result, when maternal blood and fetal blood are nearby in the placenta, oxygen is drawn off the maternal haemoglobin, across the placental membrane, onto the fetal haemoglobin. Carbon dioxide, hydrogen ions, bicarbonate and lactic acid are also exchanged in the placenta, allowing the fetus to maintain a healthy acid-base balance.
Nutrition
All of the nutrition for the fetus comes from the mother. This nutrition is mostly in the form of glucose, which is used for energy and growth. The placenta can also transfer vitamins and minerals to the fetus, as well as potentially harmful substances if the mother is consuming medications, alcohol, caffeine or cigarette smoke.
Excretion
The placenta performs a similar function to kidneys in a child or adult, filtering waste products from the fetus. These waste products include urea and creatinine.
Endocrine
Human Chorionic Gonadotrophin
The syncytiotrophoblast produces hCG. hCG levels increase in early pregnancy, plateau at around ten weeks gestation, then start to fall. HCG helps to maintain the corpus luteum until the placenta can take over the production of oestrogen and progesterone. HCG can cause symptoms of nausea and vomiting in early pregnancy. Higher levels of hCG occur with multiple pregnancy (e.g. twins) and molar pregnancy. Pregnancy tests look for hCG as a marker of pregnancy.
Oestrogen
The placenta produces oestrogen, which helps to soften tissues and make them more flexible. Oestrogen allows the muscles and ligaments of the uterus and pelvis to expand, and the cervix to become soft and ready for birth. It also enlarges and prepares the breasts and nipples for breastfeeding.
Progesterone
The placenta mostly takes over the production of progesterone by five weeks gestation. The role of progesterone is to maintain the pregnancy. It causes relaxation of the uterine muscles (preventing contraction and labour) and maintains the endometrium. It causes side effects by relaxing other muscles, such as the lower oesophageal sphincter (causing heartburn), the bowel (causing constipation) and the blood vessels (causing hypotension, headaches and skin flushing). It also raises the body temperature between 0.5 and 1 degree Celsius.
Immunity
The mother’s antibodies can transfer across the placenta to the fetus during pregnancy. These antibodies allow the fetus to benefit from the long term immunity of the mother during the pregnancy and shortly after birth. An example of this is with recurrent genital herpes, where the mother’s antibodies to the herpes virus cross the placenta and protect the baby during labour and delivery, preventing infection during birth. This protection does not occur during an initial episode of genital herpes, as the mother has not yet started producing sufficient antibodies against the herpes virus to offer the fetus protection.
