Term 2 Lecture 14: The reproductive system and oogenesis Flashcards
Female reproductive tract
(See diagrams middle of notebook 3)
- ovaries - gonads, produce eggs
- oviducts - transport eggs
- uterus - where a baby develops
- vagina - where sperm enter/ where baby is born from
- vulva - collective term for female external genitalia
Menstrual cycle
- female cycle is more complicated than in males
- menstrual cycles begin at puberty and end at menopause
- outward sign is periodic vaginal bleeding
- internal cyclic changes occur in: ovaries, uterus, cervix/vagina, breasts due to hormones which also affect behaviour.
Non-primates do not have a monthly cycle but do have a breeding cycle which is similar but less regular.
Changes in the pH of the vagina during the menstrual cycle allows different bacteria to grow and mucous will also change depending on whether you are in the steroid led part of the cycle or not
Female sex hormones
Oestrogen:
- ova maturation
- female secondary sexual characteristics
- effects on endometrium (womb lining) and myometrium (womb muscles)
- feedback on pituitary gonadotrophins
Progesterone:
- effects on endometrium and myometrium
- feedback on pituitary gonadotrophins
- thermogenic - causes body temp to rise by 0.5 degrees c in the second half of the menstrual cycle.
cholesterol differentiation
Theca internal cells convert cholesterol to androstenedione to oestradiol which is released into circulation
Granulosa cells convert androstenedione to oestradiol and release it into the antrum
(Oestradiol is a specific form of oestrogen)
Oogenesis - production of eggs
- undifferentiated primordial germ cells are formed in the fetal ovary.
- during the last part of fetal life they begin the first meiotic division to become “primary oocytes”
- these remain in meiotic arrest until puberty
- at the start of each menstrual cycle a few primary oocytes begin to develop but usually only one matures and is ovulated successfully per cycle
After 24 weeks of gestation a decline in oocytes occurs in the fetus due to apoptosis. From birth onward oocyte decline occurs due to atresia (primary oocytes dying before they can develop into ova. Apoptosis occurs with each ovulation too until all viable eggs are used up and menopause occurs.
At birth a baby girl has 10^6 ova stored as primary oocytes in her ovaries. By the time she reaches puberty she has just 10^% but only about 400 ova will be ovulated within a womans lifespan
Oogenesis process
See diagram mid notebook 3
1 oocyte develops into one ovum.
An oogonium proliferates mitotically into 16 primary oocytes one of which develops into an enlarged primary oocyte it then divides by meiosis and one of the two becomes a secondary oocyte and divides again to produce one mature ovum and one secondary polar body. The other secondary oocyte is referred to as a first polar body it splits into two other secondary polar bodies and these polar bodies degenerate.
Ovarian cycle
Each menstrual cycle consists of ovarian and uterine events.
We call the first day of bleeding the first day of the menstrual cycle as it is easier to monitor than hormone levels.
A primary follicle develops into a mature antral follicle, binds to the ovary wall and ruptures it to release a mature ovum.
The ovum moves out and up a fallopian tube in towards the uterus. The rest of the follicle becomes mature corpus luteum (yellow body) if fertilisation doesn’t occur the corpus luteum dies and becomes corpus albicans (white body)
More than one follicle will start to mature each cycle but these usually die (do not mature successfully) and these are referred to as atretic follicles.
Phases of the ovarian cycle
Follicular phase ( first half)
- primary follicles start to enlarge
- day 6 - one dominant antral follicle remains and some atretic follicles
-mechanism of dominance?
It is thought that the successful follicle is the one that secretes the most oestrogen
- the oestrogen is secreted into circulation from the theca cells and into the follicular fluid from the granulosa cells
- ovulation occurs
Luteal phase (second half)
- proliferation of granulosa and thecal cells whilst the rest of the follicle becomes corpus luteum
- corpus luteum secretes oestrogen and progesterone to be able to support a pregnancy
- if fertilisation does not occur it degenerates into corpus albicans and the cycle begins again
Uterine events
Days:
1-5 menstruation
5-14 proliferative phase, endometrium, little secretion
14-28 secretory phase, increased vascularisation, coiled glands and secretion of fluids (lining becomes spongy)
-FSH and LH levels peak around ovulation
- oestrogen rises in follicular phase
- once developed the corpus luteum secretes a lot of progesterone and oestrogen
- when the corpus luteum dies ( if fertilisation doesn’t occur) levels of progesterone and oestrogen fall - lining of the endometrium cannot be maintained and menstruation begins again
Clinical tests for ovulation
-Endometrial biopsy - taking a sample of uterine lining and studying it - very invasive
- Collecting cervical mucus to test for stretchiness or ferning
ferning: smear on a glass slide if mucus is watery it will form drops if it is sterioid heavy it will spread across the slide and as it dries you will be able to observe a ‘ferning’ pattern like veins on a leaf.
Spinnbarkeit test - tests how long the mucus strretches without snapping and an be measured by a ruler.
-Measure body temperature - progesterone causes a 0.5 degrees C increase in body temperature in the ovulating (luteal) phase of the menstrual cycle. Therefore it is a good indication of ovulation. To test this you would need to record body temperature daily to observe when it rises for a few days in a row suggesting ovulation has occurred.
- Hormone levels
LH kits - e.g. urine sticks
Mid-luteal progesterone test - blood test
Hormone control summary
- The hypothalamus releases gonadotrophin releasing hormone (GnRH) and this is carried in the blood stream to the anterior pituitary gland
>causing release of LH (luteinising hormone)
> and release of FSH (follicle stimulating hormone)
-Levels of LH and FSH both peak just before ovulation
- normally as LH and FSH levels rise negative feedback occurs so that levels don’t rise too much
- However just before ovulation there is a switch from negative feedback to positive feedback so levels of oestrogen rise and instead of lowering LH levels they enhance them (and FSH to a lesser extent)
- this LH surge triggers ovulation
- after ovulation rising progesterone levels prevent further positive feedback from oestrogen on LH - preventing ovulation until the next cycle
Summary of the menstrual cycle
- development of follicles (due to LH and FSH)
- midcycle increase to oestrogen from developing follicles
- augmented pituitary response to GnRH
- switch from neg to pos feedback as oestrogen increases LH
- LH surge triggers ovulation
- Corpus luteum formation
- increase in progesterone and oestrogen secretion
- CL degenerates if no fertilisation occurs and it becomes corpus albicans
- loss of hormonal support causes uterus lining to break down - menstruation and new cycle begins
Ways to boost fertility
Lifestyle changes
- smoking damages sperm and decreases fertility
- increased adiposity (body fat) can lead to insulin resistance and result in polycystic ovary syndrome and diabetes which can effect ovulation and can even cause loss of ovulation
Assisted reproductive technologies (ART)
if a male has a low sperm count or low sperm quality (e.g. sperm are poor swimmers) then the sperm can be concentrated and introduced to a female mechanically directly into the fallopian tubes.
Or eggs can be harvested and sperm mixed with them (IVF)
Or a step further, intracytoplasmic sperm injection can be used to inject the sperm directly into the cytoplasm of the egg or even into the nucleus.
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An egg has specialised features
- stored nutrients and factors needed for early embryonic development ( for before the placenta develops)
- egg coat - vitelline coat (zona pellucida) blocks polyspermy - preventing more sperm from entering the egg once it has been fertilised by hardening
- cells grouped around the egg cell called corrola radiata provide another obstacle to sperm before they can enter the egg cell.
How do sperm know where to go?
Sperm are attracted to the egg by chemotaxis (chemicals that the egg releases attract the sperm)
e.g. Arabacia (sea urchins) eggs release the protein resact which attracts sperm to swim towards them
