Fertilisation Part Two Flashcards
What do follicles grow from prophase 1 to metaphase 2 of meiosis in response to?
Follicles grow in response to FSH, completing meiosis 1 and stopping at metaphase 2 of meiosis.
Describe oocyte maturation.
During oogenesis the arrested prophase 1 oocytes have prominent morphological properties. They are known as germinal vesicles.
Follicles grow in response to FSH, completing meiosis 1 and stopping at metaphase 2 of meiosis.
The germinal vesicle (with P1 oocyte) moves from a central position in the ooblast to a more peripheral situation at the time of the LH surge and just prior to the germinal vesicle break down which is the first step of meiosis resumption.
Oocyte maturation which alters the prophase 1 arrested oocyte into a meiotic metaphase 2 cell consists of 3 phases which require between 24 and 36 hours to complete in total:
1) . The resumption of the meiotic cell cycle including germinal vesicle breakdown, chromosome condensation and spindle formation.
2) . The transition between meiosis 1 and meiosis 2 occurs without an S phase.
3) . The egg now arrests at metaphase 2.
Describe how metaphase 2 is characterised in the oocyte.
Metaphase 2 is characterised by the extrusion of the first polar body into the perivitelline space, the space between the cell membrane and the zona pellucida. It is a small structure of 15um in diameter and contains 23 chromosomes which are emitted at the completion of telophase 1.
What happens to the remaining tissue in the follicle after ovulation?
After ovulation the remaining tissue in the follicle reorganises to form the corpus luteum. The egg which is ovulated will not complete meiosis unless it is fertilised/activated.
Briefly describe the overall process of oogenesis.
1) . Germinal vesicle with oocyte in prophase 1 of first meiosis.
2) . LH surge resumes maturation.
3) . Germinal vesicle swells and degenerates.
4) . Enters meiosis 2 with first polar body extrusion.
5) . Mature egg at metaphase 2 ovulated into Fallopian tube.
What are the three main phases of oogenesis?
1) . Recruitment.
2) . Selection.
3) . Dominance.
What determines the side of ovulation?
In humans it is assumed that about equal numbers ovulation a occur from each of the ovaries. Whether ovulation occurs ipsilaterally or contralaterally in subsequent cycles seems to correspond to follicular cycle length. Contralateral ovulation seems to correlate with shorter cycles.
Are ipsilaterally or contralaterally ovulated follicles usually healthier?
Dominant follicles in contralateral ovulation are usually healthier than those of ipsilateral ovulation. This is due to local intraovarian factors from the corpus luteum which may negatively affect the health of the dormant follicles and the enclosed oocyte. Therefore contralateral selection of the dominant follicle in the succeeding cycle may favour pre embryo development.
Describe how the hypothalamus can stimulate ovulation.
1) . Hypothalamus releases GnRH.
2) . GnRH stimulates the anterior pituitary to release FSH and LH. Both are gonadotrophic factors.
3) . FSH stimulates follicle growth and LH stimulates follicles to produce oestrogen. Theca cells in the growing follicles start producing oestrogen.
4) . When levels of oestrogen begin to rise steeply the levels of FSH and LH rise too. This is because high concentrations of oestrogen stimulate the hypothalamus to produce more GnRH.
5) . Once the oestrogen levels reach a threshold value the LH surge occurs. The maturing follicle releases the secondary oocyte in response to the LH surge. This is called ovulation.
6) . LH stimulates the follicular tissue left behind in the ovary to transform into the corpus luteum. Under continued stimulation by LH the corpus luteum secretes oestrogen and progesterone.
7) . As progesterone and oestrogen levels rise the combination of these hormones inhibits the secretion of GnRH from the hypothalamus thus reducing the secretion of LH and FSH to low levels.
8) . The low levels of LH and FSH cause the corpus luteum to disintegrate leading to a sharp decline in oestrogen and progesterone levels.
9) . Decrease in oestrogen and progesterone stops the inhibition of the hypothalamus to producing GnRH and the pituitary can begin to secrete enough FSH to stimulate the growth of new follicles in the ovary thus initiating the next ovarian cycle.
10) . However, if fertilisation occurs then the HTG which is produced by the blastocyst will take over the role of LH to maintain the corpus luteum and thus it will not regenerate.
What kind of cycles do most mammals have rather than a menstrual cycle?
An estrous cycle.
What is the main difference between the estrous cycle and the menstrual cycle?
In both types of cycle there is a phase of follicular development and a phase of corpus luteum dominance.
The main difference is that the follicular and luteal phase are of equal length in the menstrual cycle whilst the follicular phase is significantly shorter in the estrous cycle.
Why is the luteal phase significantly longer than the follicular phase in the estrous cycle (they are of equal length in the menstrual cycle)?
The uterine endometrium develops prior to implantation to a much greater degree in the menstrual cycle than in the estrous cycle. Therefore then estrous cycle requires a longer proportion of cycle time to develop the endometrium in the luteal phase as more development of the endometrium has to take place during the luteal phase in the estrous cycle than the menstrual cycle (as the menstrual cycle does more of it before the luteal phase so doesn’t need the luteal phase to be as long).
What animals are menstrous? What physical and behavioural differences can be seen between estrous and menstrous animals?
Humans and some primates are menstrual. The uterine lining is shed if the egg is not fertilised. Other mammals are estrous and the endometrium is mostly reabsorbed into body. Menstrual cycles are restricted to humans and old world monkeys.
In contrast to humans animals do show clear changes in the behaviour at the time of ovulation in estrous and a period of increased sexual receptivity.
The length and frequency of the estrous cycle varies from species to species.
What is the basis for menstruation?
The basis for menstruation is apparently due to the physical shape of the arterioles within the uterine endometrium. In primates including humans and old world monkeys the arterioles at the base of the uterine endometrium are present as spirals. When oestrogen and progesterone levels decline due to disintegration of the corpus luteum construction of the arterioles occurs and this results in necrosis of the endometrium and damage to the walls of the blood vessels in the endometrium. When the arterioles relax again bleeding occurs.
What animals does menopause occur in? What is reproductive ageing thought to be a result of?
Reproductive ageing is a natural progression. Menopause is very unique to humans and it might have evolved to allow the mother to provide better care for her children and grandchildren.
Reproductive ageing in women is thought to be due to the progressive decline in the quality and quantity of oocytes in primordial, intermediate and primal follicles.
