Fertilisation, Implantation & ARTs Flashcards
(62 cards)
1
Q
What is meant by ovulation?
What changes occur in the vesicular follicle and primary oocyte prior to ovulation?
A
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
2
Q
What hormone is responsible for the physical action of ovulation?
A
- 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
3
Q
How is the oocyte transported from the ovary into the uterine tube?
A
- 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
4
Q
Where does fertilisation usually take place?
A
the ampulla of the uterine tube
this is the widest part of the tube and is close to the ovary
5
Q
A
6
Q
What happens to the zygote as it travels towards the uterine cavity?
Why does it need to implant into the uterine lining?
A
- 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
7
Q
How do sperm travel from the cervix to the uterine tube?
How long does this journey take?
A
- 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
8
Q
What happens when the sperm reach the isthmus?
A
- 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
9
Q
What 2 changes need to occur within the spermatozoa that give them the capability to fertilise the oocyte?
A
- capacitation
- acrosome reaction
10
Q
What is involved in capacitation of sperm and where does it occur?
How long does it take?
A
- 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
11
Q
What happens during the acrosome reaction?
A
- 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
12
Q
What are the 3 phases of fertilisation?
A
- 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
13
Q
What are the 3 outcomes of fertilisation?
A
- 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
14
Q
What is meant by cleavage and when does it occur?
A
- 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
15
Q
When is the morula formed?
A
- 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
16
Q
What processes must occur within the morula for it to become a blastocyst?
A
- 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
17
Q
What process begins following blastocyst formation?
What cell layer initiates this process?
A
- 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
18
Q
When and where does implantation normally occur?
A
- 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
19
Q
What is meant by “capture” during implantation?
A
- 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
20
Q
What is meant by signalling & adhesion during implantation?
Which cell type is particularly important for implantation?
A
- 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
21
Q
What are the 2 major changes that occur in the uterus that allow for implantation to occur?
A
- 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
22
Q
Why does the mother’s immune system need to change following implantation?
A
- 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
23
Q
What are the 4 main complications that can arise as a result of suboptimal implantation of the blastocyst?
A
- pre-eclampsia / eclampsia
- intrauterine growth restriction
- preterm birth
- intrauterine foetal death
24
Q
What condition can result from the blastocyst implanting near the cervix?
A
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
25
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
26
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
27
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"**
28
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
29
What usually happens if the blastocyst implants in the uterine tube?
* the uterine tube is not specialised to deal with a developing embryo
* the blastocyst may implant and begin to develop but usually **_spontaneously aborts_** and is **expelled with the next menstrual cycle**
* if this does not happen, it can lead to **_rupture of the uterine tube_**
30
How does abdominal implantation occur?
* the ovary and uterine tube are **_not_ connected together**
* the **_fimbriae_** sweep the oocyte into the uterine tube, but if they fail to do this, the oocyte can **fall into the peritoneal cavity**
31
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"
32
What is meant by the term "ectopic pregnancy"?
ectopic pregnancy refers to implantation **_OUTSIDE_ of the uterine cavity**
most of the time this abnormal implantation leads to **_spontaneous abortion_** as other sites are **not specialised** to deal with the developing embryo
33
What is the definition of infertility?
the failure to conceive within 1 year of trying
34
What is the difference between infertility and subfertility?
Which group is suitable for ARTs?
* infertility refers to an individual who is **_completely incapable_ of conceiving**
* subfertility refers to an individual who is **_biologically capable_ of conceiving**, but some mechanisms make the process **less efficient**
* ARTs can aid couples who have difficulty conceiving, but are not suitable for total infertility
35
What are the main categories of causes of infertility?
1. male factors
2. female factors
3. combination of male and female factors
4. unexplained
5. others
* cases of infertility are equally due to male and female factors
36
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)
37
What characteristics of sperm influence fertility in males?
male infertility may result from **_insufficient numbers of sperm_** and/or **_poor motility_**
* in some cases, **viable sperm are still produced** (e.g. obstruction, impotence)
* in some cases, the **quality of sperm is reduced** (e.g. varicocele, endocrine disorders)
* in any of these cases, **_some normal sperm may still be produced_** that are capable of fertilisation
38
What are the 8 main causes of infertility in males?
* **endocrine disorders** (abuse of anabolic steroids)
* **testicular abnormalities** (e.g. cryptorchidism)
* **congenital absence of the vas deferens** (e.g. in CF)
* **obstruction** of the reproductive tract
* **varicocele** (engorgement of venous structures surrounding the testes)
* **retrograde ejaculation** (sperm ejected into bladder rather than penile urethra)
* **impotence** (neurological, psychological, endocrine, vascular)
* **autoimmune conditions**
39
What is the most common situation that results in permanent infertility in males?
* sperm can **_remain as spermatids_** and not go through the process of meiosis
* the spermatids are still **present within the ejaculate**, but are **_incapable of fertilisation_**
40
What are the 3 main methods used to induce ovulation in women with endocrine disorders?
What risk is associated with all of these methods?
1. clomiphene citrate
2. gonadotrophin releasing hormone
3. gonadotrophins (FSH & LH)
* these all carry the risk of **stimulating more than one follicle**, resulting in **_multiple pregnancy_**
41
How does induction of ovulation using methods targeting endocrine regulation work?
* it works by **_stopping ovulation_**, which gives the **oocytes longer to develop**
* this allows **more oocytes to mature and be released**
* the more oocytes that are released, the **greater chance of fertilisation**
42
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_**
43
Who might clomiphene citrate be given to?
What are the 2 main drawbacks of this method?
* it is given early in the menstrual cycle to **induce ovulation** in women who **_do not ovulate_** or **_ovulate irregularly_**
* there is a **10-25% risk of multiple pregnancies** due to release of multiple follicles
* 15-40% of women are **clomiphene citrate resistant**
44
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
45
46
Who is suitable for IVF treatment?
* it is suitable for most types of infertility, including:
* tubal disease
* endometriosis
* unexplained infertility
* mild male fertility
* it is **_not suitable for severe male infertility_** where the ejaculate contains **few viable sperm**
47
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
48
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_**
49
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_**
50
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
51
How many stimulated cycles are affected by OHSS?
Who is at a higher risk?
* it affects around **1-2%** of stimulated cycles
* the risk is increased to **20% in high risk cases**:
* low body weight
* PCOS
* high doses of gonadotropins
* rapidly rising levels of oestrogen
52
Before oocyte retrieval begins, why is USS used?
* USS is used to determine whether the **follicles are mature** before they are removed
* this is determined both through **timing** and **follicular diameter**
53
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
54
How is sperm retrieval usually accomplished?
* a fresh semen sample is obtained at the same time as oocyte retrieval
* surgical retrieval is used if a sample is unable to be obtained
55
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
56
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
57
What happens during the embryo culture stage of IVF?
* the zygote is transferred to a culture dish under strict control conditions
* *e.g. pH, temperature, light levels, air quality*
* **cleavage is monitored for 2-3 days** until the embryo reaches the **_8-cell stage_**
58
What is meant by embryo grading during the embryo culture stage?
* this ensures there are:
* even, regular-sized blastomeres
* no fragmentation
* a single nucleus in each cell
* smooth intact zona pellucida
59
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_**
60
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%
61
Why do sperm survive cryopreservation better than oocytes?
What has improved survival rates of oocytes?
* sperm survive cryopreservation well as they have a **small volume**
* oocytes are the largest human cells and have a **high water content**
* **_vitrification_** (flash-freezing with a high concentration of cryoprotectants) has improved survival rates of oocytes
62
When might use of donated gametes be suitable?
* congenital absence of gametes
* cancer treatment
* premature menopause
* inherited genetic disorders
* single women
* same-sex partners
