Fertilization Flashcards
What is required for fertilisation to occur?
For fertilization to occur, proper gametes are require:
- Binucleated oocytes dies mostly before reaching maturity, and it is caused by microtubule dynamics that affect chromosome segregation.
- Abnormal sperm has a higher frequency, due various causes (infections, toxins, varicocele, fever, lifestyle, drugs, testicular abnormalities)
Where does fertilisation occur? And how? And when?
Fertilisation occur between the ovaries and the uterus, specifically near the ampulla-isthmus, at c.a. ovulation: the ovum is transferred from the ovaries to the uterus by ciliary movement and peristaltic contractions and it will take 3-4 days to reach the uterus.
Fertilisation is the process in which one spermatozoa out of hundreds that reach the ampulla, penetrates the ovum, causing the fusion of the nuclei:
As spermatozoa reach the ovum, they need to:
1. penetrate the cell layer of granulosa cells, literally like in a maze.
2. penetrate the extracellular matrix, zona pellucida, by hyaluronidase and acrosin enzymes that spermatozoa had in the acrosome.
3. cortical granule reaction: as one spermatozoa reaches the oocyte cytoplasm, it reacts with cortical granules and react with the. The function of this reaction is to:
- inactivate sperm receptors (only one can enter)
- harden zona pellucida (to make it more ifficult for other spermatozoa
to enter)
As one spermatozoa enters the oocyte, fusion of the nuclei start with the moment of “conception”
The moment of fertilization is very important: the uterus needs to be ready to welcome a baby, thus it must happen at the secretory phase of the menstrual cycle (the uterus has been reconstructed after the menses, and the secretion rate of pregnancy hormones is at its peak) and at the luteal phase of the ovarian cycle (the oocyte has been released from the follicle and it’s making its way down to the uterus).
Why is calcium important in fertilisation?
Calcium ions are essential for the process of oocyte activation:
1. First calcium wave:
- exocytosis of cortical granules (prevents polyspermy)
- completion of meiosis II (second polar body is created and the ovum
has reached maturity)
- activation of enzymes
2. Second calcium wave:
- formation of pronucleus (the nucleus that will fuse with the
spermatozoa’s nucleus)
On the other hand, the M-phase promoting factor regulate the process of fertilization, and Ca2+ induce cyclin degradation, which cause the inactivation of MPF and the completion of meiosis II.
What happen to the haploid nuclei of the ovum and the spermatozoa?
The two haploid pronuclei will fuse together (conception) and form a diploid zygote.
What happen right after the fusion of the two nuclei?
After the conception (fusion of two nuclei and formation of the zygote) a series of cell division begin. This process is called “cleavage”, and form, first, blastomeres (identical cells) which is then converted into morula (16+ cells). This cellular division ends with a pre-embryo (multicellular blastocysts) that enters in contact with the uterine wall (implantation).
As the zygote becomes a blastocyst, the process of cavitation occur: a cavity (blastocoele) full of fluid is formed, while the cells that surround the cavity, the trophoblasts, will adhere with each other, thanks to E-cadherin, in a process called “compaction”. Inside the cavity, the Inner Cell Mass is formed, which are cells that will develop into the embryo.
As the cavity is formed, the blastocyst will try to exit the zona pellucida that remained from the oocyte. In the process of hatching, the trophoblasts release hyaluronidase enzyme that erode the zona pellucida allowing the exit of the blastocyst.
Why is calcium important in conception?
Calcium ions, once again, regulate the whole process of conception. It gives the signal to start cleavage.
