REPRO: Fertilisation And The Luteal Phase Flashcards
How does the sperm get coagulated, and how is that reversed when they enter the cervix?
Prostatic and seminal vesicle secretions comprise of the seminal fluid, which coagulates the sperm - prevents loss, later liquifies (after about 20mins). The coagulated semen allows it to be deposited close the cervix where it will then liquify.
Movement through the cervical mucus removes seminal fluid, abnormally morphological sperm and cellular debris. This allows the sperm to swim free and fertilise the egg.
Describe the sperm’s passing through the cervix.
Cervical mucus is less viscous in the absence of progesterone, allowing sperm to pass.
Sperm can inhabit the cervical crypts, which may form a reservoir of sperm. This allows for slow release of sperm - useful for if they haven’t reached ovulation. There is some evidence of thermotaxis, but the mechanism is not yet elucidated (clarified).
Fertilisation typically occurs within 24-48 hours, but sperm has been recovered alive after 5 days.
Describe the sperm’s passage from the cervix to the fallopian tube.
Its passage through the uterus is not well understood. Currents set up by uterine or tubal cilia may play a role.
Chemoattractants released from the oocyte cumulus complex may attract the sperm.
The sperm becomes hyperactivated. The forceful tail beats with increased frequency and amplitude, mediated by Ca2+ influx into the sperm via CatSper channels.
What is capacitation?
It is the biochemical arrangement of the surface glycoproteins and changes in membrane composition which must occur before the acrosome reaction can occur. This takes between 4-18 hours.
This is partly achieved by removing the sperm from the seminal fluid; also, the uterine and tubal fluid may contain factors which promote capacitation.
What is the acrosome reaction?
It is when the acrosin bound to the inner acrosomal membrane digests the zona pellucida so that the sperm can enter the egg.
It occurs when the sperm comes in contact with the zona-cumulus complex. The acrosomal membrane on the sperm head fuses, releasing enzymes that cut through the complex.
Briefly, describe ovulation of the egg as it is released from the ovary.
The LH spike causes the resumption of meiosis and ovulation. It converts primary oocytes to secondary oocytes, plus the first polar body
The basement membrane breaks so that blood pours into the middle.
The oocyte cumulus complex is extruded out and caught by the fimbriae of the uterine tube.
The theca and granulosa cells become mixed and the empty follicle is known as the corpus luteum. It produces progesterone in the luteal phase.
Describe the role of progesterone in the menstrual cycle (luteal phase).
- makes the endometrium secretory and receptive to implantation
- suppresses cilia in the uterine tubes once the oocyte has already passed
- makes cervical mucus viscous again to prevent further sperm penetration
Describe the role of oestrogen in the menstrual cycle (luteal phase).
- helps to maintain the endometrium in the luteal phase (causes proliferation in the follicular phase)
Describe the demise of the corpus luteum (CL).
If fertilisation does not occur, the corpus luteum has an inbuilt finite lifespan of 14 days. The regression of the corpus luteum is essential to initiate a new cycle. Progesterone negatively feedbacks to the hypothalamus and pituitary, thus inhibiting the production of more progesterone.
So the fall in corpus luteum-derived steroids causes an inter-cycle rise in FSH. Cell death occurs, the vasculature is broken down and the corpus luteum shrinks. Over time, it becomes a corpus albicans.
The corpus luteum would be rescued in pregnancy by hCG (from the embryo) binding to its LH receptors. The corpus luteum then continues to produce progesterone and maintains the endometrium.
What does the menstrual cycle achieve?
- selection of a single follicle and oocyte
- regular spontaneous ovulation
- 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 endometrium by the corpus luteum progesterone
- initiating a new cycle if fertilisation does not occur
What is the cumulus oophorus?
It is derived from granulosa cells, and it protects the egg.
What is the corona radiata?
It is the innermost layer of cumulus cells in contact with the zona pellucida.
It is formed by granulosa cells adhering to the oocyte before it leaves the ovarian follicle.
What is the zona pellucida?
It is secreted by the egg.
It becomes impenetrable after the fertilisation.
Step-by-step, describe sperm binding and penetration.
1) The acrosome reaction occurs in contact with the zona-cumulus complex. The sperm penetrates the cumulus and binds to the zona pellucida via adhesion molecules and receptors, ZP3.
2) The sperm enzymes cut through the zona pellucida and the sperm fuses with the plasma membrane.
3) The sperm is taken in by phagocytosis. Phospholipase Zeta (PZeta) is activated (in the sperm membrane) by the basal Ca2+ inside the egg, and causes further release of intracellular Ca2+, leading to a large Ca2+ spike.
4) 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. - makes the zona pellucida impenetrable.
Describe the syngamy post-fertilisation.
After meiosis 1, the oocyte has 23x chromosomes, but 2 chromosomes are arranged as sister chromatids.
Entry of the sperm causes an increase in Ca2+ via phospholipase Zeta from the sperm. The Ca2+ causes the completion of meiosis 2, expelling the second polar body, and the cortical reaction.
The sperm nuclear membrane breaks down, the chromatin decondenses and the chromosomes separate.
4-7 hours after the fusion, the two sets of haploid chromosomes become surrounded by distinct membranes, forming pronuclei. These haploid structures synthesis 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.
