Fertilisation and the luteal phase Flashcards
Where is ejaculated semen deposited?
What is this a method of?
Ejaculated semen is coagulated so it is able to be deposited close to the cervix, in cervical crypts.
Thermotaxis
What liquefies the semen?
Prostatic and seminal vesicle secretions comprise seminal fluid which coagulates - prevents loss, later liquefies the semen.
What does movement of the sperm through the cervix cause?
Movement through cervical mucus removes seminal fluid, abnormally morphological sperm and cellular debris.
What property of the cervix allows the sperm to pass through?
Cervical mucus is less viscous in the absence of progesterone allowing sperm to pass.
When does fertilisation occur and how long can sperm remain in the cervix?
24-48 hours and 5 days
What assists in the movement of the sperm towards the egg?
- Passage through uterus not well understood, currents set up by uterine or tubal cilia may have a role.
- Chemo-attractants released from the oocyte cumulus complex may attract the sperm.
- Sperm become hyperactivactivated. Forceful tail beats with increased frequency and amplitude mediated by Ca2+ influx via CatSper channels.
How is capitation achieved for sperm?
- Capacitation is partly achieved by removing the sperm from the seminal fluid, also uterine or tubal fluid may contain factors which promote capacitation (between 4 -18 hours).
- Biochemical rearrangement of the surface glycoprotein and changes in membrane composition must occur before the acrosome reaction can occur.
- This delay allows the sperm ti the region of the egg, increasing the chances of fertilisation
What happens in terms of the acromosome when the egg and sperm meet?
What digests the zona pellucida?
- Acrosome reaction occurs in contact with the zona –cumulus complex; the acrosomal membrane on the sperm head fuses releasing enzymes that cut through the complex.
- Acrosin bound to the inner acrosomal membrane digests the zona pellucida so the sperm can enter.
What is the function of the:
- Cumulus Oophorus
- Corona Radiata
- ZP
- 1st polar body
- Cumulus Oophorus - protect the egg, derived from granulosa cells
- Corona Radiata - Innermost layer of cumulus cells in contact with the zona pellucida. Formed by granulosa cells adhering to the oocyte before it leaves the ovarian follicle. The corona radiate are granulosa cells in close contract with the zona pellucida
- ZP - Secreted by the egg. Becomes impenetrable after fertilisation
- 1st polar body - 1st meiotic division completed
What happens in the Sperm Binding & Penetration phase?
- The acrosome reaction occurs in contact with the zona-cumulus complex. Sperm penetrate cumulus and bind to ZP.
- Sperm enzymes cut through ZP and sperm fuses with plasma membrane.
- Sperm taken in by phagocytosis. Phospholipase Zeta activated by basal Ca2+ inside egg. PIP2 → DAG + IP3 Causes release of intracellular Ca2+ leading to large Ca2+ spike. This is found inside the sperm membrane, when it gets into the egg you add it to the egg membrane.
- . Cortical reaction as wave of Ca2+ sweeps around egg…release of proteases, peroxides and hyaline prevents polyspermy.
Describe Syngamy
- After meiosis I the oocyte has 23X chromosomes, but 2 copies of each chromosome arranged as sister chromatids.
- Entry of the sperm causes an increase in Ca2+ via phospholipase Zeta from sperm.
- Ca2+ causes the completion of meiosis II expelling the second polar body & cortical reaction.
- The sperm nuclear membrane breaks down, the chromatin decondenses and chromosomes separate.
- 4-7 hours after sperm penetration the two sets of haploid chromosomes become surrounded by distinct membranes forming two pronuclei. These haploid structures synthesise DNA in preparation for the first mitotic division.
- The pronuclei fuse and the mitotic metaphase spindle forms with the chromosomes assuming their position at its equator.
- Mitosis is completed and the one cell zygote becomes a two cell embryo.
Give a summary of the fertilisation stage
On image
