Fertilisation and luteal phase Flashcards
1
Q
Why is ejaculated semen coagulated?
A
- Prevents loss
- later liquifies so that sperm can swim out of the seminal fluid through cervical mucus into the uterus.
2
Q
What parts of the semen does the cervical mucus remove?
A
Seminal fluid, abnormal sperm and cellular debris
3
Q
What happens when sperm passes through the cervix?
A
- cervical mucus is less viscous in the absence of progesterone allowing sperm to pass
- sperm inhabits cervical crypts which may form a reservoir
- some evidence of thermotaxis
- survive in female for around 24-48 hrs
4
Q
How do sperm make their way to the egg?
A
- currents set up by uterine and tubal cilia
- chemo-attractants released from oocyte cumulus complex - thermotaxis
- sperm become hyperativated. Forceful tail beats with increased frequency and amplitude mediated by Ca2+ influx via CatSper channels
5
Q
What is sperm capacitation?
A
- Biochemical rearrangement of the surface glycoprotein and changes in membrane composition
- partly achieved by removing the sperm from the seminal fluid
- uterine and tubal fluid may contain factors promoting it
- has to occur to allow acrosome reaction
6
Q
What is the acrosome reaction?
A
- occurs in contact with the cells surrounding the egg
- acrosomal membrane on the sperm head fuses releasing enzymes
- these cut through the outer layers of cumulus surrounding the egg
- acrosin bound to the inner acrosomal membrane binds and digests the zona pellucida so that the sperm can enter
7
Q
How does ovulation get triggered?
A
- Consistently high levels of oestrogen flips the negative feedback on the pituitary into positive feedback
- causes the LH surge - a huge release of LH
- This triggers and inflammation reaction, producing cytokines, VEGF etc
- Follicular fluid is exuded with the oocyte inside
- Basement membrane breaks and so blood pours into the middle
- oocyte cumulus complex is extruded out and caught by the fimbrae of uterine tube
- theca and granulosa become mixed
8
Q
What happens in meiosis I of the oocyte?
A
- Ovulated eggs are released in meiosis I
- Converts the primary oocyte into a secondary oocyte plus the first polar body
- undergoes uneven cell division so that the oocyte keeps all of the cytoplasm and cellular machinery
9
Q
What does the progesterone produced by the corpus luteum do?
A
- differentiation of the endometrium - glands start to secrete glucose, growth factors, adherence molecules, which all make the endometrium more receptive, easily implanted into and highly vascularised with a large blood supply
- as progesterone rises later in the luteal phase, it suppresses the cilia in uterine tubes, as the fertilised oocyte will have already passed
- makes the cervical mucous viscous again (criss-crossing glycoproteins) to prevent further sperm penetration
10
Q
What does oestradiol do in the luteal phase vs follicular phase?
A
L = Helps to maintain the endometrium F = causes proliferation
11
Q
What happens if there is no fertilisation?
A
- After 14 days, the CL dies and becomes a corpus albicans
- Progesterone levels drop, and without it, the endometrum cannot survive. The endometrium starts to produce PGs that constrict the blood vessels, causing the tissue to become hypoxic and necrotic
- The fall in CL-derived steroids causes an inter-cycle rise in FSH, causing break down and bleeding.
12
Q
What happens if there is fertilisation?
A
- Human chorionic gonadotrophin (hCG) is released by the embryo
hCG binds to FSH receptor in CL, keeping it alive and producing progeterone, maintaining the endometrium - It inhibits FSH and LH and stops the menstrual cycle, allowing pregnancy
13
Q
Name 5 things the menstrual cycle achieves
A
- selection of a single follicle and oocyte
- regular spontaneous ovulation - one good egg a month
- correct haploid number of chromosomes in the oocyte
- cyclical changes in the cervix and uterine tubes to enable egg transport and sperm access
- preparation of the endometrium of the uterus to receive the fertilised egg
- support of the implanting embryo and enometrium by the CL progesterone
- initiating a new cycle if fertilisation does not occur
14
Q
What does the oocyte look like at ovulation?
A
- can see the first polar body but no second (undergone meiosis I)
- remnants of the granulosa cells that were surrounding the oocyte in the follicle
- on the inside of the granulosa cells there is the zona pellucida, a glycoprotein layer secreted by the egg around projections. Binds sperm at ZP3 receptors. At LH surge, projections are withdrawn
- Cumulus cells - protect the egg, secrete mucus, matrix projections into the plasma membrane
- corona radiata is the innermost layer
- inside of the oocyte membrane there are cortical granules which will release contents at the point of fertilisation, preventing polyspermy
15
Q
How does the sperm bind to and penetrate the egg?
A
- identifies a sensory molecule, likely mediated by CatSper channels
- Ca rushes into the sperm hyper-activating them, increasing the frequency and amplitude of the tail beating
- move the granulosa cells out of the way
- receptor molecules on the head of the sperm bind to proteins on the ZP, such as ZP3
- when binding occurs, the acrosome reaction occurs and enzymes flood out, cutting through the ZP, and the sperm fuses with the plasma membrane
- it is taken up by phagocytosis and the tail and mitochondria are left behind or destroyed
- Phospholipase Z in the sperm membrane causes Ca release inside the egg when it meets it
- Wave of Ca2+ sweeps around the egg, causin a cortical granule reaction, releasing proteases, peroxides and hyaluronic acid hardening the oocyte coat, preventing polysermy
16
Q
What does the cortical granule reaction achieve?
A
- PL Z in the sperm membrane causes a release of Ca in the egg when they meet.
- This causes a wave of Ca sweeping around the egg, causing the cortical granule reaction
- proteases sever anchors between ZP and the egg, getting rid of any other proteins that another sperm can bind to such as ZP3
- peroxides cross-link and make it harden and prevent polyspermy
- hyaluronic acid released into perivitelline space. Has a lot of sulphate groups that absorb water, causing it to swell up and move away from the ZP
17
Q
What is syngamy?
A
The fusion of two gametes in reproduction
18
Q
What are the steps in syngamy?
A
- oocyte has undergone meiosis I, so has 23 chromosomes, but 2 copies of eah arranged as sister chromatids
- the sperm binds to the ZP before penetrating it and dusing with the oocyte membrane - causes an increasing in Ca2+, triggering the completion of meiosis II, expelling the second polar body
- sperm’s nuclear membrane breaks down, the chromatin decondenses and chromosomes separate
- 4-7 hours later, the two sets of haploid chromosomes beome surrounded by distinct membrane forming pronuclei
- inside the pronuclei, both sets of haploid chromosomes get duplicated
- pronuclear membranes break down and the mitotic metaphase spindles forms with the chromosomes assuming their position at its equator
- mitosis is completed and the one cell zygote becomes a two cell embryo
19
Q
Fertilisation summary
A
- Sperm enters oocyte causing Ca2+ waves
This causes:
- Cortical granules to fuse with ZP to block polyspermy
- nucleus of sperm to transform to male pronucleus
- completion of Meiosis II and expulsion of 2nd polar body –> formation of female pronucleus
- the 2 pronuclei come together, the membranes break down and there is alignment of chromosomes on the spindle = mitosis
20
Q
How long after fertilisation does implantation occur?
A
- fertilisation occurs very close to the ampulla
- day 2 = 2 cells
- day 3 = 4 cells
- day 4 = morula
- inner cell mass is visible inside blastocyst
- spends around 4/ days in the uterine tube and then implants into the endometrium around day 7
