Lecture 5: Fertilisation and Implantation

Question 1

Describe the transition of the oocyte back into meiosis following its arrest:

Answer 1

• Oocyte was arrested in meiosis 1 in the primary follicle.
• Following the LH surge, meiosis is resumed, with the first reduction division, resulting in formation of a polar body, but, stops again at metaphase of meiosis 2.
• At ovulation, the egg consists of the oocyte and a small polar body surrounded by the zone pellucida.
• Meiosis is resumed, for the second time, upon fetilisation.

Question 2

Are meiotic divisions equal?

Answer 2

Meiotic divisions in the oocyte are unequal

First meiotic division (reduction division) in response to LH surge. (n)

Primordial follicle (Arrested in prophase of meiosis 1) (2n)

n = first polar body, Polar bodies are retained within the zona pellucida

Question 3

Describe what happens in ejaculation:

Answer 3

• Sperm leaves the male reproductive tract during ejaculation bathed in seminal plasma
• As the prostatic and seminal vesicle fluids mix (in the vagina) a coagulum is formed that holds the ejaculate together and adheres the ejaculate to the female reproductive tract.

Question 4

Insert diagram of the uterine ligaments

Answer 4

slide 6

Question 5

Describe the deposit of sperm in the female reproductive tract:

Answer 5

• Vaginal insemination
• Within 15 minutes of ejaculation the coagulum is dispersed allowing individual sperm to begin their quest for an egg
• The breakdown of the coagulum is called liquefaction.

Question 6

How does vagina pH impact sperm?

Answer 6

• pH of the vagina is low (3.5-7.4) and this pH is inhibitory to sperm motility and survival
• Seminal plasma buffers the vaginal pH to around 7.2 at this pH sperm can become motile.

Question 7

Describe sperm transport in the female reproductive tract:

Answer 7

• Fertilisation occurs in the oviduct (fallopian tube) and therefore sperm must migrate (deposit->along tube)
• First barrier is cervix. Cervix secretes mucous which is under hormonal control throughout menstrual cycle.
• Cervical mucous is hostile to sperm transport apart from short window during ovulation.
• During periovulation window cervical mucus is produced in large amounts and it is thin. Fertile mucus aligns its fibres and allows passage of sperm in columns through the cervix.

Question 8

How does cervical mucus filter sperm:

Answer 8

• The tails of normal sperm cause the fibers of the cervical mucus to vibrate in rhythm
• Failure of abnormal sperm to induce this vibration in the cervical mucus may retard the progress of abnormal sperm.

Question 9

How much of the sperm enters the uterus and where are they stored?

Answer 9

• up to 200m sperm may be contained in human ejaculate of these only 1m will enter uterus
• Not all sperm traverse the cervix immediately and some sperm are stored in the crypts of the cervix from which they may later move into the uterus.
• After traversing the cervix sperm move to the utero-tubal junction in about 30 mins.

Question 10

How long do sperm survive?

Answer 10

~48hrs

Clinical advice is ~5 days. 99% of spermatozoa do not enter cervix

Question 11

What influences the sperm transport speed:

Answer 11

~2-7hrs to reach the ampulla of the fallopian tube

Swimming speed ~3mm/min

Speed is related to fertility
Speed decreases with time
Speed decreases with freezing and thawing

Stimulation of the cervix by the penis causes release of oxytocin from the pituitary, resulting in rhythmic uterine contractions aiding sperm movement.

Question 12

Describe how sperm produce an innate immune response:

Answer 12

• Sperm arriving in the uterus induce an infiltration of leukocytes (leukocytosis)
• These phagocytose dead or dying sperm
• Exposure of the maternal immune response to paternal antigens. (unprotected sex with partner for year are less at risk of developing preeclampsia)
• Half of sperm arrive at wrong utero-tubal junction.

Question 13

Describe movement of sperm in the fallopian tube:

Answer 13

• Sperm then enter the fallopian tube and their movement is slow.
• Only a few hundred will reach the distal end of the fallopian tube.
• If sperm enter the fallopian tube prior to ovulation their rate of tail beating slows and they wait in the narrow isthmus of the tube.
• At ovulation sperm move up to the egg and stored sperm may also be released from the crypts of cervix.

Question 14

Insert slide 18

Answer 14

now

Question 15

What is capacitation:

Answer 15

• Freshly ejaculated sperm are not capable of fertilising an ovum. Capacitation occurs in the female reproductive tract
• Capacitated sperm show hyperactivation in which the tail movement changes with an increased bending of the tail and swinging of the head.
• Hyperactivation may facilitate movement of sperm through the tortuous folds of the fallopian tube.
• The length of time for capacitation of human sperm is variable but only capacitated sperm can undergo the acrosome reaction.

Question 16

Describe the acrosome reaction:

Answer 16

Acrosome: Membranous pouch containing proteinases and other enzymes such as hyaluronidase.

• > During the acrosome reaction, acrosomal + plasma membranes of sperm head fuse. = Acrosomal contents released.
• > Reaction triggered by influx of Ca
• > Several factors can trigger this i.e Progesterone, ZP3 protein
• > Ca ionophore can trigger this in vivo
• > Must occur close to egg to fertilise naturally. (10% sperm undergo spontaneously distal to oocyte)

Question 17

What is step one of fertilisation:

Answer 17

The sperm penetrates the layer of cumulus cells that surround the egg.

This is assisted by the hyaluronidase secreted from the acrosome which digests the basement membrane allowing the sperm to squeeze between the cumulus cells.

This secretion of hyaluronidase may be only partial release of the acrosomal products stimulated by progesterone contained in the fluid surrounding the oocyte.

Question 18

What is step two of fertilisation:

Answer 18

Next the sperm digests its way past the corona radiata (the innermostlayer of tightly packed cumulus cells) contacts the zona pelucida.

Contact with the zona pelucida triggers the acrosome reaction.

Question 19

Describe the zona pellucida:

Answer 19

• The zona pelucida is a protein coat surrounding the ovum.
• There are three major zona pelucida proteins called ZP1,2,3
• ZP3 is the primary sperm receptor and contact between ZP-3 and the sperm ligand for ZP-3 induces the acrosome reaction completely.

The sperm ligand for ZP-3 is unknown. There are also many other receptor/ligand pairs that have been implicated in the binding of sperm to ovum during implantation.

Question 20

What is step 3 of fertilisation:

Answer 20

Aided by the enzymes of the acrosome the sperm then penetrates through the zona pelucida to the perivitelline space

• This process takes about ten mins
• The sperm then attaches to the vitelline membrane and the two membranes fuse allowing the sperm nucleus to enter the ovum

Question 21

What is the cortical reaction:

Answer 21

• Entry of the sperm into the vitelline membrane causes a release of intracellular calcium. This is followed by regular spikes of calcium in the oocyte.
• Granules are released by the ovum (cortical granules) into the perivitelline space that prevent further sperm penetrating the zona.
• Release of enzymes (proteases and b-hexosaminidase) cleave ZP2 digest ZP3 cross-link the zona (involves ZP1)
• Polyspermy is blocked.

Question 22

What stimulates the second meiotic division?

Answer 22

Ca influx at fertilisation activates the oocyte nucleous.

The activated oocyte nucleous completes the second meiotic division in response to fertilisation.

This is unequal, meaning that a second polar body is formed of excess cytoplasm (retained within zona pellucida)

[first meiotic division occurs in response to LH surge and is also unequal, forming a first polar body]

Question 23

Describe at what point the fertilised eggs becomes a zygote:

Answer 23

When the two membranes of the pronuclei break down and mitotic division begins.

Question 24

Describe zygote transport:

Answer 24

• Zygote begins cleave in the tube and takes ~5 days to transverse tube into uterus
• Symmetric division occurs during this passage
• This gives rise to morula when 8-16 cell stage is present.
• Morula becomes tightly cohesive and compacted.
• Blastocyst is formed when ~64 cells are present (96hrs)

Question 25

Describe implantation:

Answer 25

The blastocyst

• Two differentiated cell populations trophectoderm (forms placenta) and inner cell mass (embryo proper)
• Remains free in the uterine cavity for 2-3 days before implanting in the uterine wall.

Question 26

What is nidation:

Answer 26

• The hatched blastocyst comes into physical contact with the receptive decidua
• The blastocyst attaches to the endometrial/decidual epithelium via specific adhesion molecules. This is called nidation.