Fertilisation and Luteal Phase Flashcards
Describe the sperm capacitation and acrosome reaction Understand the physiological changes in the luteal phase of the menstrual cycle Explain the role of the corpus luteum in pregnancy or menstruation Review rescue of the corpus luteum by hCG Describe the structure of the oocyte Describe the molecular events during fertilisation Explain syngamy and early embryo development
How does the sperm get coagulated and how is that reverse when they enter the cervix?
Prostatic and seminal vesicle secretions comprise of the seminal fluid which coagulates
- Prevents loss and later liquefies after 15 minutes
A short while after the semen liquefies - sperm can then move out close to the cervix
Movement through the cervical mucus removes seminal fluid, abnormally morphological sperm and cellular debris allowing sperm to swim free to fertilise.
Describe sperm’s journey to the egg
Cervical mucus is less viscous in the absence of progesterone allowing sperm to pass
Sperm becomes hyper activated - have forceful tail beat with increased frequency and amplitude - mediated by Ca2+ influx via CatSper channels
Voltage gated Ca2+ channels are on sperm surface which open allowing hyper activation of the sperm
Fertilisations occurs within24-48 hours
We don’t know:
- Passage through uterus not well understood
- Currents set by uterine or tidal cilia may have a role
- Chemo-attractants released from oocyte cumulus complex may attract the sperm
Some evidence of thermotaxis but not clarified.
What is capacitation?
Molecular changes in sperm allowing acrosomal reaction
“Capacity” = sperm reaching full capacity
Destabilises the sperm membrane to
How is capacitation done?
- Removing sperm from the seminal fluid
- Uterine or tubal fluid may contain factors that promote capacitation
- Sperm undergo biochemical rearrangement of the surface glycoprotein
- Changes in membrane composition must occur before acrosomal reaction
- Takes between 4-18 hours for this to happen
What is acrosome reaction?
Sperm approaching the egg and releasing enzymes to penetrate the egg outer shell.
How does the acrosome reaction happen?
When acrosin bound to the inner acrosomal membrane digest the zona pellucida so sperm can enter the egg.
Occurs when sperm comes in contact with the zona-cumulus complex
The acrosomal membrane on the sperm fuses releasing enzymes that cut through the complex,
What are 7 main acheivements of the menstrual cycle?
- Selection of a single follicle and oocyte
- Regular spontaneous ovulation
- Correct haploid number of chromosomes in the oocyte by completing meiosis 1 and start of meiosis 2.
- Cyclical changes in the cervix and uterine tubes - enable egg transport and sperm access
- Preparation of the endometrium - to receive fertilised egg
- Support of the implanting embryo and endometrium - by progesterone from the corpus luteum
- Initiating new cycle if fertilisation does not happen
Describe the role of progesterone in the luteal phase
- Released by corpus luteum
- Makes the endometrium secretory and receptive to implantation
- Suppresses cilia in the uterine tube
- Makes the cervical mucus viscous again to prevent further sperm penetration
Describe the role of oestrogen in the luteal phase
- Helps to maintain the endometrium in the luteal phase
- But MAIN function is in the proliferative phase (first half)
Describe the syngamy post fertilisation
- After meiosis 1, oocyte has 23x chromosomes but 2 chromosomes are arranged as sister chromatids
- Entry of the sperm causes increase in Ca2+ via PZeta from the sperm
- Ca2+ causes completion of meiosis 2 expelling the second polar body and the cortical reaction
- Sperm nuclear membrane breaks down, chromatin decondenses and chromosomes separate
-4-7hours after the fusion, 2 sets of haploid chromosomes become surrounded by distinct membrane forming pronuclei - one containing sperm chromsomes and one containing egg chromosomes - These 2 haploid strucure syntehsise DNA in preparation for first mitotic division
- Pronuclei fuse and mitotic metaphase spindle forms
- Mitosis is completed - oonce xell zygote becomes a two cell embryo
Describe the demise of the corpus luteum
If fertilisation does not occur:
- Corpus luteum has an inbuilt finite lifespan of 14 days
- Regression of the corpus luteum is essential to initiate new cycle
- Fall in corpus luteum derived steroids causes an inter-cycle rise in FSH
- Cell death occurs, vasculature is broken down and corpus luteum shrinks - becoming corpus albicans
If fertilisation did occur:
- Corpus luteum still required
- Embryo produces hormone called hCG (very similar to LH only beta subunit is different)
-hCG binds to LH receptors on the CL and does not die - Progesterone remains high and maintains the endometrium
What is cumulus oophorus?
Derived from ganulosa cells and protects the egg
What is the corona radiata?
Inner most layer of cumulus cells in contact with ZP
Formed by granulosa cells adhering to the oocyte before it leaves the ovarian follicle
What is the zona pellucida?
Secreted by the egg
- Becomes impenetrable after cortical reaction
Describe sperm binding and penetration
- Acrosome reaction occurs in contact with zona cumulus complex
Sperm penetrates the cumulus and binds to the ZP via adhesion molecules and receptors, ZP3. - The sperm enzymes cut through the ZP and sperm fuses with the plasma membrane.
- Sperm is taken by phagocytosis. Phospholipase Zeta (PZeta) is activated by the basal Ca2+ inside the egg and causes further release of intracelullar Ca2+, leading to a large Ca2+ spike.
- The cortical reaction occurs as a wave of Ca2+ sweeps around the egg. This reaction involves the release of proteases, peroxides and hyaline which prevents polyspermy.
