Lecture 9; Fertilisation and Implantation

Question 1

Describe ejaculate composition

Answer 1

• Sperm leaves the male reproductive tract bathed in seminal plasma. (unmixed)

Question 2

What happens when seminal plasma mixes?

Answer 2

As prostatic and seminal fluids mix in the vagina, a coagulum forms that holds ejaculate together and adheres the ejaculate to the female reproductive tract

(~1minute) (vaginal insemination)

Question 3

What happens after vaginal insemination and coagulate formation?

Answer 3

Within 15 mins of ejaculation the coagulum is dispersed (lots of proteases in seminal fluid) allowing individual sperm to begin their quest for an egg

(breakdown of coagulum = liquefaction) Allows sperm motility

Question 4

How does the vaginal environment make it difficult for sperm?

Answer 4

pH of the vagina is ~4.2pH and this pH is inhibitory to sperm motility and survival.

Question 5

How does ejaculate counter vaginal pH?

Answer 5

Seminal plasma buffers the vaginal pH to ~7.2 at this pH sperm can become motile.

Question 6

Describe where sperm must migrate to and from in the female reproductive tract;

Answer 6

Sperm must migrate from the site they are deposited to the oviduct for fertilisation to occur.

Question 7

What is the first barrier in sperm transport?

Answer 7

The first barrier in sperm transport is the cervix.

The cervix secrete a mucous whose characteristics are under hormonal control and change throughout the menstrual cycle

Question 8

How does timing matter with cervical mucous and sperm movement?

Answer 8

There is a short window (peri-ovulation) around the time of ovulation where sperm can pass through the mucous. Otherwise cervical mucous is hostile to sperm transport.

Question 9

Describe cervical mucous during peri-ovulation

Answer 9

During peri-ovulation cervical mucous is produced in large amounts and it is thin.

Fertile mucous aligns its fibres and allows the passage of sperm through columns through the cervix (normally cross fibres = barrier)

Question 10

What speed to sperm move through the cervix?

Answer 10

Sperm move through the cervix at a speed of around 1.2-3mm/minute

Question 11

How does cervical mucous filter poor sperm?

Answer 11

The tails of normal sperm cause the fibres of the cervical mucus to vibrate in rhythm and breakdown.

Failure of abnormal sperm to induce this vibration in the cervical mucous may retard the progress of abnormal sperm

Question 12

How many sperm are in ejaculate and how many reach the uterus?

Answer 12

• Up to 20million sperm in ejaculate and 1million enter the uterus.

Question 13

Do all sperm immediately traverse the cervix?

Answer 13

• Not all sperm traverse the cervix immediately, some are stored in the crypts of the cervix which from they may later move into the uterus

Question 14

What happens to sperm once they have traversed the cervix?

Answer 14

After traversing the cervix sperm move to the utero-tubal junction in about 30 minutes.

Question 15

Describe the rate of sperm movement and the implications of discrepancies;

Answer 15

Sperm advance at 3mm/minute because of tail beating. This does not account for the distance covered in 30 minutes.

Therefore muscular contractions must be important in trans-uterine transport

Question 16

What may cause uterine contractions to aid sperm movement?

Answer 16

Stimulation of the cervix by the penis may cause the release of oxytocin from the pituitary which then causes rhythmic uterine contractions aiding sperm.

Question 17

What does sperm arriving in the uterus cause?

Answer 17

Sperm produce an innate immune response

Question 18

Describe the innate immune response;

Answer 18

Sperm arriving in the uterus induce an infiltration of leukocytes (leukocytosis)

These phagocytose dead or dying sperm

Question 19

Why else may an innate immune response be important?

Answer 19

Exposure of the maternal immune system may be important in dampening down or tolerising maternal immune response to paternal antigens

e.g women who have unprotected sex with a single partner for more than a year before becoming pregnant are at less risk of developing pre-eclampsia

Question 20

Whats the implication of producing only one egg during ovulation?

Answer 20

Half the sperm will arrive at the utero-tubal junction on the wrong side!!!

They will continue their fruitless journey along a fallopian tube that has no egg

Question 21

Describe sperm movement in the fallopian tube and numbers;

Answer 21

Sperm enter the follopian tube and movement is very slow

Only a few hundred will reach the distal end of the follopian tube

Question 22

What happens if sperm enters the follopian tube prior to ovulation?

Answer 22

If sperm enter the follopian tube prior to ovulation their rate of tail beating slows and they wait in the narrow isthmus of the tube.

Question 23

What happens to sperm during ovulation?

Answer 23

They move up the egg and stored sperm may also be released from the crypts of the cervix.

Question 24

Where does fertilisation occur?

Answer 24

The ovum is usually fertilised close to the junction of the isthmus and the ampulla of the fallopian tube.

If unfertilised the oocyte can last 24hrs or less

Question 25

What happens when sperm miss their target?

Answer 25

The continue up the fallopian tubes and enter the abdominal cavity

Question 26

How long can sperm survive in the female reproductive tract?

Answer 26

Sperm usually only survive in the female reproductive tract only for a few ays but there are rare case reports of much longer survival times (up to a few months)

Question 27

what is the first reaction of the sperm in the female reproductive tract?

Answer 27

Capacitation Occurs a few seconds after ejaculation

Question 28

What is capacitation?

Answer 28

The process whereby sperm in the female reproductive tract can the ability to fertilise the ovum

Question 29

Describe the cellular events that happen in capactitation;

Answer 29

Rapid capacitation events; HCO3- and Ca2+ activate: - Soluble Adenyl Cyclase (SACY) SACY activates PKA, increasing membrane solubility to Ca and thus ICF Ca concentrations Slow Capacitation events; Removal of Cholesterol in the membrane. Allowing sperm to maintain high levels of HCO3.

Question 30

What do capacitated sperm show?

Answer 30

Capacitated sperm show hyperactivation in which the tail movements changes with an increased bending of the tail and swinging of the head.

Question 31

What may hyperactivation facilitate?

Answer 31

Hyperactivation may facilitate movement of sperm through the tortuous folds of the follopian tubes.

Question 32

What is the length of time for capacitation?

Answer 32

The length of time for capacitation of human sperm is variable but only capacitated sperm can undergo the acrosome reaction.

Question 33

Describe ovulation;

Answer 33

Ovum release is a gradual gentle process that takes 30 seconds. Not an explosion!

Question 34

What is the function of the cumulus cells?

Answer 34

Cumulus cells act like velcro for fibria to have grip and sweep the ovum up. If they are not present it is unlikely that the ovum will be swept up

Question 35

How does the follopian tubes move the ovum?

Answer 35

Combination of fibria beating and peristaltic contractions

Question 36

What is the first step in fertilisation?

Answer 36

The sperm penetrates the surrounding cumulus cells. Assisted by hyaluronidase secreted by the acrosome. This digests the basement membrane allowing sperm to squeeze b/w cumulus cells

Question 37

What stimulates the hyaluronidase release in step one?

Answer 37

Stimulated by progesterone in the fluid surrounding the oocyte. Only partial release of acrosome products

Question 38

What is step two in fertilisation?

Answer 38

Sperm digests through the corona radiata and contacts the zona pelucida. This contact triggers the acrosome reaction

Question 39

Describe the zone pelucide proteins that are involved in initiating the acrosome reaction;

Answer 39

Zona pelucida = protein coat Major proteins= ZP 1 , 2 , 3 ZP 3 = primary sperm receptor (contact with sperm ligand initiates the acrosome reaction completely) Sperm ligand is unknown (other receptor/ligand pairs may be involved)

Question 40

What is the acrosome?

Answer 40

The acrosome is a membranous pouch containing proteinases and other enzymes such as hyaluronidase

Question 41

Describe the acrosome reaction;

Answer 41

- Acrosomal and plasma membranes of sperm head fuse. Thus allowing release of the acrosomal contents to the environment around the sperm - Acrosome reaction triggered by Ca infux - Progesterone or ZP3 can trigger this Ca influx

Question 42

In vitro how can the acrosome reaction be triggered?

Answer 42

Using a Ca ionophore

Question 43

Whats a requirement for fertilisation and acrosome reaction?

Answer 43

The acrosome reaction must occur in close proximity to the oocyte for a sperm to be able to fertilise it naturally About 10% of sperm spontaneously undergo the acrosome reaction distal to the oocyte and these sperm cannot fertilise the egg.

Question 44

What is step three in the acrosome reaction?

Answer 44

Aided by enzymes of the acrosome the sperm then penetrates through the zona pellucida to the pervitelline space - ~10mins

Question 45

Once sperm enters the vitelline space what happens? (part of step 3)

Answer 45

- Sperm then attatches to the vetilline membrane and the two membranes fuse allowing the sperm nucleus to enter the ovum

Question 46

What else happens as the sperm enters the vitelline space?

Answer 46

As sperm enters the vitelline membrane granules are released by the ovum (cortical granules) that prevent further sperm penertrating the zona may include anti-acrosin

Question 47

Describe fertilisation beyond the third step;

Answer 47

- Entry of sperm into the oocyte causes a release of intracellular Ca, this is followed by regular spikes of Ca in the oocytes. - These Ca changes alter protein phosphorylation patterns in the oocyte and the oocyte nucleus becomes activated - oocyte nucleolus completes the second mitotic division - oocyte forms a pronucleus as does the sperm - Two membranes of the pronuclei then break down and mitotic division begins. The fertilised egg is now a zygote.

Question 48

Describe movement of the zygote from the follopian isthmus

Answer 48

The zygote begins cleavage in the tube and takes 5 days to transverse and enter the uterus. (during the passage, 2 then 4 cell products of conception form) Further doubling gives rise to the morula at the 8-16 cell stage. The cells of the morula become tightly cohesive and compact

Question 49

What is gestation age?

Answer 49

Age of foetus from ovulation onwards

Question 50

What is a 64 cell product?

Answer 50

Blastocyst (comes after morula)

Question 51

Describe the blastocyst

Answer 51

Blastocyst; - Two differentiated cell populations: trophectoderm (forms placenta) and inner cell mass with will form the embryo proper Blastocyst remains free in the uterine cavity 2-3 days before implanting

Question 52

What is decidualisation?

Answer 52

Endometrial cells differentiate to facilitate pregnancy; - stromal cells become plump and glycogen rich - Recruitment of leukocytes This starts during the menstural cycle. (not all animals are decidual)

Question 53

Describe when implantation normally occurs

Answer 53

Implantation usually occurs 7-10 days post ovulation The blastocyst remains encompassed by the zona pelucida while free floating in the uterine cavity

Question 54

Describe the blastocyst adhering to the uterine wall (first part of implantation)

Answer 54

The hatched blastocyst comes into contact with the receptive decidua The blastocyst sticks to the decidual epithelium via specific adhesion molecules . This is called nidation

Question 55

Following nidation what happens

Answer 55

The embryo erodes the uterine wall and becomes completely embedded the epithelium heals over the implantation site. The trophoectoderm differentiates into two populations; - mononuclear cytotrophoblasts - multinucleated syncyotiotrophoblasts

Question 56

Describe the placenta;

Answer 56

- The placenta has a branching villous structure - The placenta is made of specialised cells called trophoblasts - 3 main populations of trophoblasts - Cytotrophoblasts - Syncytiotrophoblast - Extravillous trophoblast (EVTs)

Question 57

Describe cyotrophoblast function;

Answer 57

Cytotrophoblasts are the pre-cursors for both syncytiotrophoblast and EVTs

Question 58

Describe syncytiotrophoblast;

Answer 58

The syncytiotrophoblast is a continuous single (multinucleated) cell formed by fusion from underlying cytotrophoblasts

Question 59

Describe EVTs

Answer 59

IN villous tips cytotrophoblasts differentiate into EVTs that break through the overlying syncytiotrophoblast and grow out from the placenta by invading into the decidual stroma.

Question 60

Describe EVT's movement and implantation

Answer 60

EVTs invade the decidua in columns and also spread outwards from the nidation site invading into the decidua

Question 61

What us the function of EVTs invading the decidua?

Answer 61

1) Physical attatchment of the placenta to the decidua | 2) Adaptation of blood vessels in the uterus called the spiral arteries to maintain pregnancy

Question 62

Describe EVT movement with regard to the spiral arteries;

Answer 62

EVTs migrate down the spiral arteries and replace the endothelial cells that line these arteries. They also digest the muscular/elastic wall of these vessels and transform them into - non-vasoactive - flaccid - wide bore tubes, no muscular walls, lined by trophoblasts Traditionally called the physiological changes of pregnancy more commonly referred to as spiral artery remodelling

Question 63

What does spiral artery remodelling allow and what sort of process is it?

Answer 63

This means the placenta and fetus can get a constant and increased supply of blood as they grow. Constant perfusion These processes are highly regulated both spatially and temporally

Question 64

What are trophoblast plugs and their function?

Answer 64

During the first trimester EVTs form plugs in the spiral arteries so that little maternal blood reaches the placenta. The trophoblast plugs act as a pressure reducing valves that stop the implanting placenta being exposed to the full force of the maternal blood pressure. (early exposure increases risk of miscarriage)

Question 65

Describe early embyronic nutrition;

Answer 65

In the absence of maternal blood the embryo is nourished by; 1) Glandular secretions (stored in yolk sac) 2) Yolk sac