W5- Fertilisation and Luteal Phase Flashcards
How does ejaculated sperm work?
Ejaculated semen is coagulated
Prostatic and seminal vesicle secretions comprise seminal fluid which coagulates - prevents loss, later liquefies.
Movement through cervical mucus removes seminal fluid,
abnormally morphological sperm and cellular debris.
Sperm passes into cervix
Cervical mucus is less viscous in the absence of progesterone allowing sperm to pass.
Sperm can inhabit cervical crypts which may form a reservoir. Some evidence of thermotaxis, but mechanism not yet elucidated.
Fertilisation typically within 24-48 hours but sperm have been recovered alive after 5 days.
What is the journey to the egg like?
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.
What is sperm capacitation?
Capacitation is partly achieved by removing the sperm
from the seminal fluid, also uterine or tubal fluid may
contain factors which promote capacitation.
In IVF if you put freshly ejaculated sperm with eggs, they do not fertilise, this is because:
Biochemical rearrangement of the surface glycoprotein
and changes in membrane composition must occur
before the acrosome reaction can occur. This can generally take between 4 and 18 hours. This process is called capacitation.
What is acrosome reaction?
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.
In order for the acrosomal reaction to occur, the sperm must first have been capacitated.
What is ovulation?
LH spike causes resumption of meiosis
and ovulation. Converts the primary
oocyte to secondary oocyte plus 1st
polar body.
Basement membrane breaks so blood
pours into the middle.
Oocyte cumulus complex extruded out
and caught by fimbrae of uterine tube.
Theca and granulosa become mixed
and the empty follicle is known as the
corpus luteum. It produces
progesterone in the luteal phase.
At the same time as this LH spike, there is an inflammatory reaction.
What is the luteal/secretory phase?
Progesterone, released by the corpus luteum. makes the endometrium secretory and receptive to implantation.
Supresses cilia in uterine tubes once oocyte has already passed. Makes cervical mucus viscous again to prevent further sperm penetration.
Estrogen
Helps to maintain endometrium in luteal phase (causes proliferation in follicular phase).
In the second phase, there is an increase in differentiation of the endometrium. There are then secretions from the endometrial glands, and we get an increase in the small arterioles that makes the endometrium secretory and receptive to endometrium.
Increase in progesterone starts to supress the cillia in the uterine tubes, but this takes about a week or so, so the oocyte would already have passed. Towards the end of the cycle, the increased progesterone would make the cervical mucus viscous again to prevent further sperm penetration.
How does the Demise of the Corpus Luteum work?
If fertilisation does not occur, CL has an inbuilt finite lifespan of 14 days.
Regression of CL essential to initiate new cycle.
Fall in CL-derived steroids causes inter-cycle rise in FSH.
Cell death occurs, vasculature breakdown, CL shrinks. Over time it becomes a corpus albicans.
CL rescued in pregnancy by hCG from embryo binding to its LH receptors. CL continues to produce progesterone and maintain 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 by
completing meiosis I and beginning meiosis II.
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 corpus luteum progesterone.
Initiating a new cycle if fertilisation does not occur.
What are the features of the oocyte at ovulation?
Cumulus oophorus - Derived from granulosa cells. Looks a bit like a cloud and protects egg.
Corona radiata - Granulosa cells closely in contact with the zona pellucida. Innermost layer of cumulus cells in contact with the ZP. Formed by granylosa cells adhering to the oocytes before it leaves the ovarian follicle.
Zona Pellucida - Glycoprotein layer secreted by the egg. Becomes impenetrable after fertilisation. They also have adhesion molecules specific to the sperm.
1st Polar body - That is the result of the uneven meiotic division.
Cortical granules are involved in the zona pellucida that makes it become impenetrable. Between the zona pellucida and the cytoplasm, there is the plasma membrane. And between the zona pellucida and the plasma membrane, there is a space called the perivitelline space. This is part of the reaction that seals the zona pellucida and prevents polyspermy.
What is the process of the sperm binding and penetrating?
- 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 (increases Ca) activated by basal Ca2+ inside egg. PIP2 → DAG + IP3 Causes release of intracellular Ca2+ leading to large Ca2+ spike.
- Cortical reaction as wave of Ca2+ sweeps around egg…release of proteases, peroxides and hyaline prevents polyspermy.
What is 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.
What are the steps of early embryonic development?
The fertilized egg has 2 pronuclei.
This is the first sign of fertilization.
The developing embryo contains 6-8 cells 3 days after
fertilization.
Five days after fertilization it is called a blastocyst and differentiates into inner cell mass, blastocoel and trophoblast.
