Menstrual Cycle (5) Flashcards
In human women the length of a menstrual cycle varies greatly (ranging from 21 to 35 days), with 28 days as the average length. It includes the ___ and ____ cycles. These cycles occur from puberty until menopause.
uterine
ovarian
The ovarian cycle consists of the: (3)
- Development of Graafian follicle
- Ovulation
- Formation of the corpus luteum
What does the follicular phase entail?
The follicular phase is the first part of the ovarian cycle. During this phase, the primary follicles mature and get ready to release an egg cell. Under the influence of follicle stimulating hormone (FSH), from the pituitary gland, a primary follicle develops into a fluid-filled Graafian follicle. During this process the primary oocyte in the follicle undergoes meiosis and develops into haploid egg cell (ovum). Usually only one Graafian follicle and, therefore, one egg cell is formed
What does ovulation entail?
The Graafian follicle contains the egg cell (ovum). As the follicle matures and enlarges it moves to the surface of the ovary. Approximately mid-cycle, a second hormone luteinizing hormone (LH), also released from the pituitary gland causes the Graafian follicle to rupture releasing an egg cell in an event called ovulation.
How does ovulation occur?
This takes place approximately on day 14 of the cycle.
How does the corpus luteum form?
After ovulation the remains of the Graafian follicle, under the influence of FSH and LH changes into glandular tissue called the corpus luteum (yellow body). This structure produces large amounts of progesterone as well as oestrogen.
What happens to the corpus luteum?
If pregnancy does not occur, the corpus luteum will begin to disintegrate and disappear 10 – 14 day after ovulation. If pregnancy does occur, it remains as it is needed to sustain a healthy pregnancy.
What makes human ovulation unique? (2)
- Human females can be sexually active at any time in their cycle, even when they are not about to ovulate. Other mammal females are only sexually active in the oestrus phase of their cycle, i.e. when ‘in heat’ and are about to ovulate.
- There are no obvious external signs or sexual excitement to signal ovulation. It is pure chance as to whether fertilisation occurs, unlike in other animals.
What is the uterine cycle?
The uterine cycle is a series of changes in the endometrium and is regulated by hormones. It may be divided into two main phases.
How does the endometrium change in the uterine cycle? (2)
- The hormone oestrogen, secreted from the maturing follicles starts the repair of the endometrium. Days 6 – 14 of menstrual cycle.
- Later under the influence of the two hormones (oestrogen and progesterone) from the corpus luteum the endometrium continues to thicken, becoming more glandular (secretes mucus and nutrients) and vascular (has blood vessels).It is now suitable for the implantation of a fertilised egg (blastocyst).
How does menstruation occur? (3)
- The high levels of oestrogen and progesterone exert a negative feedback on the pituitary inhibiting the release of the hormones FSH and LH. As a result the corpus luteum degenerates and progesterone secretion is reduces and then stops.
- With no hormone to maintain the endometrium the lining breaks down and is shed in a process termed menstruation.Days 1 to 5 of the menstrual cycle.
- The detached tissue and blood pass out through the vagina as menstrual flow for 3 to 7 days.
The secretion of ___ (FSH and LH) and _____ (oestrogen and progesterone) hormones is controlled by a simple negative feedback mechanism. The signalling mechanism is the increased concentration of each hormone in the blood. An increased level of one will ____ affect or stop the release of another.
pituitary
ovarian
negatively
How does the hormonal control of menstrual cycle? (6)
- Under the influence of FSH a primary follicle develops into a Graafian follicle and causes the ovaries to produce oestrogen.
- Oestrogen then stimulates the pituitary gland to produce LH.
- LH causes the Graafian follicle to form the corpus luteum, which secretes progesterone.
- The increased progesterone and oestrogen concentrations activate a negative feedback mechanism, which stops the pituitary glands secreting FSH and LH.
- As there is no FSH a Graafian follicle will not develop and no oestrogen, and later progesterone, will be released by the ovary.
- Because of the low levels of oestrogen and progesterone the secretion of FSH, and later LH, from the pituitary gland no longer is suppressed and the whole cycle starts again.
What is menopause?
At about the age of 46 to 54 the ovaries stop producing oestrogen and progesterone. As a result, the ovaries stop releasing eggs and menstruation no longer occurs. At that point women are said to have reached menopause.
Menopause causes many changes in a woman, such as hot flushes and mood changes. Post-menopausal conditions include: (3)
- atrophy (degeneration) of the reproductive organs
- bone mass loss
- increasing risk of cardiac disease.
