Fertilisation, Implantation & ARTs Flashcards
What is meant by ovulation?
What changes occur in the vesicular follicle and primary oocyte prior to ovulation?
the eruption of a follicle / oocyte from the ovary and into the uterine tube
- the vesicular follicle grows rapidly under the influence of FSH and LH to become a mature Graafian follicle
- an abrupt increase in LH causes the primary oocyte to complete meiosis I
- meiosis II is initiated but the oocyte is arrested in metaphase II around 3 hours prior to ovulation
What hormone is responsible for the physical action of ovulation?
- the surge in LH leads to an increase in prostaglandin levels
- prostaglandins cause local muscular contractions in the ovarian wall which extrude the oocyte
- the oocyte and surrounding granulosa cells break free and float out of the ovary
How is the oocyte transported from the ovary into the uterine tube?
- shortly before fertilisation, fimbriae of the uterine tube sweep over the surface of the ovary
- the uterine tube begins to contract rhythmically
- the oocyte and surrounding granulosa cells are swept into the tube by the sweeping movements of the fimbriae and motion of cilia on the epithelial lining
- once within the tube, peristaltic muscular contractions of the tube aid in transport of the oocyte
Where does fertilisation usually take place?
the ampulla of the uterine tube
this is the widest part of the tube and is close to the ovary
What happens to the zygote as it travels towards the uterine cavity?
Why does it need to implant into the uterine lining?
- as it travels to the uterine cavity, it undergoes cleavage (rapid cell division)
- initially it obtains all its nutrients through diffusion from the surroundings
- at the blastocyst stage of development, a denser nutrient supply is required
- the blastocyst implants into the uterine lining and develops a uteroplacental blood supply
How do sperm travel from the cervix to the uterine tube?
How long does this journey take?
- only 1% of sperm deposited in the vagina will enter the cervix
- they can survive in the cervix for many hours
- the sperm move from the cervix into the uterine tube by muscular contractions of the uterus and uterine tube
- the propulsion produced by the sperm themselves has very little influence
- this journey from cervix to tube can take from 30 minutes to 6 days and fertilisation can occur at any point
What happens when the sperm reach the isthmus?
- the sperm become less motile when they reach the isthmus and cease their migration
- they regain motility at the point of ovulation and continue to move towards the ampulla
- the female reproductive tract becomes more receptive to sperm at ovulation
- this is mainly through changes in composition of the secretions
What 2 changes need to occur within the spermatozoa that give them the capability to fertilise the oocyte?
- capacitation
- acrosome reaction
What is involved in capacitation of sperm and where does it occur?
How long does it take?
- this is “conditioning” of the sperm that occurs in the uterine tube that lasts around 7 hours
- it involves interaction between the sperm and uterine tube that modifies the chemical composition of proteins in the head of the sperm
- plasma proteins are removed from the plasma membrane overlying the acrosome region of the sperm (which contains degradative enzymes)
- only capacitated sperm can penetrate the corona radiata around the oocyte and begin the acrosome reaction
What happens during the acrosome reaction?
- the capacitated sperm that have passed through the corona radiata bind to the zona pellucida
- zona proteins induce the acrosome reaction
- the acrosome of the sperm contains degradative enzymes that are released to penetrate the zona pellucida
- this allows the head of the sperm to penetrate the oocyte
What are the 3 phases of fertilisation?
- penetration of the corona radiata
- penetration of the zona pellucida
- fusion of the cell membranes of the sperm and oocyte
The oocyte immediately completes meiosis II after sperm entry
What are the 3 outcomes of fertilisation?
- restoration of the diploid number of chromosomes
- half come from the mother and half from the father
- initiation of cleavage
- without fertilisation, the oocyte degenerates 24 hours after ovulation
-
chromosomal sex is established
- a Y carrying sperm produces a male (XY) embryo
- a X carrying sperm produces a female (XX) embryo
What is meant by cleavage and when does it occur?
- once the zygote has reached the 2-cell stage it undergoes a series of rapid mitotic cell divisions to increase the number of cells
- the cells (blastomeres) become smaller with each cleavage division
When is the morula formed?
- the morula is a ball of 16 cells that is formed around 3 days after fertilisation
- the inner cells of the morula constitute the ICM that will give rise to the embryo proper
- the surrounding cells constitute the OCM that will give rise to the trophoblast, which contributes to the placenta
What processes must occur within the morula for it to become a blastocyst?
- when the morula enters the uterine cavity, fluid begins to penetrate through the zona pellucida and into the intercellular spaces of the ICM
- the intercellular spaces become confluent and form a single cavity - the blastocele
- the embryo becomes a blastocyst following formation of the blastocoele
What process begins following blastocyst formation?
What cell layer initiates this process?
- the cells of the ICM are the embryoblast
- the cells of the OCM are the trophoblast
- the zona pellucida disappears, allowing implantation to begin
- it is the trophoblastic cells that initiate implantation, needed to form a more robust nutrient supply to the blastocyst
When and where does implantation normally occur?
- normal implantation occurs on the anterior or posterior wall of the body of the uterus
- this occurs 6-7 days post-fertilisation
- the trophoblastic cells penetrate between the epithelial cells of the uterine mucosa around day 6
What is meant by “capture” during implantation?
- this refers to the initial binding of the blastocyst to the uterine wall
- the trophoblasts anchor onto the uterine lining and initiate implantation
- trophoblastic invasion into the uterine mucosa occurs around day 6 post-fertilisation
What is meant by signalling & adhesion during implantation?
Which cell type is particularly important for implantation?
- this describes the attachment and migration of trophoblast cells
- whilst this is occurring, the trophoblast differentiates into cytotrophoblasts and syncitiotrophoblasts
- it is the syncitiotrophoblasts that are particularly involved in burrowing into the uterine lining
What are the 2 major changes that occur in the uterus that allow for implantation to occur?
- the endodermal stroma at the site of implantation changes to make it more receptive to the burrowing blastocyst
- there is closure of the defect in the uterine epithelium created by penetration of the blastocyst through fibrous plug formation
Why does the mother’s immune system need to change following implantation?
- changes in the maternal immune system prevent rejection of the blastocyst
- half of the genetic material comes from the father so the blastocyst appears foreign to maternal WBCs
What are the 4 main complications that can arise as a result of suboptimal implantation of the blastocyst?
- pre-eclampsia / eclampsia
- intrauterine growth restriction
- preterm birth
- intrauterine foetal death
What condition can result from the blastocyst implanting near the cervix?
placenta praevia
- the placenta is positioned above the endocervical os
- the placenta may cover the cervix entirely or only cover a portion of it
- the pressure of the embryo on the placenta can lead to placental rupture
What are fibroids?
How can they influence implantation?
- fibroids are structural growths within the reproductive tract
- they can change the tissue type within the uterus to make it less receptive to the blastocyst
- or they can physically obstruct the process of implantation
What is septate uterus?
How can it influence the development of an embryo?
- the uterus develops from 2 ducts that come towards the midline and fuse
- in this congenital abnormality, the septation does not disintegrate at the point of fusion
- this can lead to problems with the growth of the embryo and attachment of the placenta to the endometrium
What happens in endometriosis?
What is the most common associated symptom?
- there are endometrial cells that are usually confined to the uterus growing outside of the uterus
- this abnormal tissue will break apart and bleed at the end of each menstrual cycle
- as the blood has nowhere to go, surrounding areas can become inflamed and swollen
- it is associated with scarring and cysts, particularly around the ovaries
- the cysts contain blood which can rupture and release “chocolate-coloured contents”
What are the most common abnormal implantation sites?
- uterine tube (95%)
- cervix
- ovary
- abdominal cavity - peritoneum & omentum
- extra-uterine pregnancies occur in 2% of all pregnancies but account for 9% of all pregnancy related maternal deaths
What usually happens if the blastocyst implants in the abdominal cavity?
- usually the embryo reaches a certain point in development and is then broken down
- there is not a sufficient blood supply within the abdomen to support the growing embryo
- lithopaedion can occur if the embryo becomes too large to be reabsorbed by the body
- it calcifies on the outside as part of a foreign body reaction, producing a “stone baby”
What are the main categories of causes of infertility?
- male factors
- female factors
- combination of male and female factors
- unexplained
- others
- cases of infertility are equally due to male and female factors
What are the 6 main causes of female infertility?
- endocrine disorders (e.g. PCOS)
- uterine tube abnormalities (e.g. PID caused by an STD such as chlamydia)
- abnormalities of the uterus (e.g. fibroids, septate uterus)
- endometriosis
- antisperm antibodies in the cervix or other factors leading to the female reproductive tract becoming hostile to sperm (e.g. pH)
- advancing age (health of oocytes arrested in prophase I deteriorates)
How does clomiphere citrate work?
- it is an oestrogen antagonist
- it suppresses the normal negative feedback mechanism of oestrogens on the hypothalamus and pituitary gland
- this “tricks” the pituitary gland into increasing FSH secretion, which induces ovulation
In general, what are the 5 stages involved in IVF?
IVF is an umbrella term for multiple techniques, but in general:
- Superovulation - female is given a hormone treatment that stimulates production of mature oocytes
- Obtaining gametes - the mature oocytes are surgically removed from the ovaries
- IVF - mature oocytes mixed with sperm cells allowing fertilisation to occur
- Culture - fertilised embryos are incubated for around 48 hours until they reach the 8-cell stage
- Transfer - embryos are implanted into the uterus or frozen for future implantation
What type of IVF method can be used in severe male infertility?
intracytoplasmic sperm injection (ICSI)
- this involves injection of a single sperm into an egg’s cytoplasm to produce fertilisation
- this single sperm can be obtained from any point in the male reproductive tract
- it carries an increased risk of foetuses with Y chromosome deletions and birth defects
During IVF, how is gamete production initiated?
this stage involves controlled ovarian hyperstimulation (“superovulation”)
- a GnRH agonist is administered which overrides the normal secretion of GnRH to decrease FSH & LH secretion
- this prevents spontaneous ovulation to allow time for multiple oocytes to mature, which can be harvested
- FSH is then administered daily and follicular development is monitored using USS
- when the follicles reach 18-20mm diameter, hCG is administered
What syndrome can potentially result as a consequence of superovulation?
ovarian hyperstimulation syndrome (OHSS)
- this can occur when ovaries primed with gonadotropins are then exposed to hCG, which stimulates ovulation
- the presence of multiple follicles results in high levels of oestrogen, which increases vascular permeability
- there is excessive movement of fluid from capillaries into tissues to produce oedema
What are some of the vascular and extravascular symptoms associated with OHSS?
increased capillary permeability leads to a fluid shift from the vascular to extravascular compartment
Vascular:
- thrombosis
- shock
- oliguria
Extravascular:
- increased weight
- cysts
- swollen abdomen
- dyspnoea
When is oocyte retrieval performed?
What intervention is used to acheive this?
- it is performed around 43 hours following hCG administration
- it must be performed before the follicles rupture
- it is performed via transvaginal USS
- a probe is used as a guide with an aspiration needle attached
- the ovary is visualised using USS and the needle is advanced through the vaginal wall and into a mature follicle
- follicular fluid plus an oocyte is aspirated into a collecting tube and sent to a lab to be placed into an incubator and assessed
What are the 2 surgical methods that can be used for sperm retrieval?
Percutaneous epididymal sperm aspiration (PESA):
- a needle is inserted into the epididymis to extract the sperm
- this is less invasive and suitable for obstructive cases of azoospermia
Testicular sperm extraction (TESE):
- this is more invasive and requires biopsies of the testes
- the tissue sample is incubated to allow sperm to mature prior to fertilisation
- this is suitable for low sperm counts and problems with development of mature sperm
What happens during the in vitro fertilisation stage of IVF?
Why is a long incubation period contraindicated?
- 500,000 to 5 million sperm are added to the culture dish containing the oocytes
- the sperm are washed, which allows for some degree of capacitation
- they are co-incubated together for 15-20 hours
- a long incubation period can be detrimental as reactive oxygen species are produced by the sperm
What happens during the embryo transfer stage of IVF?
How is the number of embryos transferred limited and why?
- embryos are implanted into the uterus using a catheter under USS guidance
- 2 embryos are transferred for women < 40
- 3 embryos are transferred for women > 40
- this increases the chance of implantation occurring in older women
- to increase the chances of implantation, progesterone is administered daily
How can surplus embryos be stored for future use?
What must be added to prevent cell damage and how well do the cells survive?
- surplus embryos can be cryopreserved for use in future cycles
- cryoprotectants are added to prevent ice crystal formation, which would otherwise damage the cells
- the embryos are slowly cooled to -196oC
- survival rates after thawing are 80%
