Lecture 7: Fertilisation and Implantation

Question 1

Describe the Process of Fertilisation

Answer 1

• In the ampulla, the oocyte meets the sperm
  
  • Unfertilised oocyte can survive for 24 hours (or less)
    
    • If fertilisation in vivo, requires firstly that the sperm penetrates the layer of cumulus cells that surround the egg
• This is assisted by hyalurodinase 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 p_artial release of the acrosomal products_ stimualted by progesterone contained in the fluid surrounding the oocyte
• Next, the sperm encounters the corona radiata, a layer of tightly packed follicle cells and digests its way past tehse to encounter the zona pellucida
• The zona pellucida is a protein coat surrounding the ovum
  
  • There are 3 major pellucida porteins called ZP-1, 2 and 3
  • ZP-3 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.
• One sperm attaches to the oolemmal membrane (oolemma) membrane and the 2 membranes fuse allowing the s_perm nnucleus to enter the ovum_).
• C_ortical reaction_ occurs.

Question 2

What are blastomeres?

Answer 2

blastomere is a type of cell produced by cleavage (cell division) of the zygote after fertilization and is an essential part of blastula formation.

Question 3

Describe the innate immune response caused by sperm

Answer 3

• Sperm arriving in the uterus induce an infiltration of leucocytes (leucocytosis).
• These invading leucocytes phagocytose dead or dying sperm
• This immune exposure to potential paternal antigens may help with tolerance

Question 4

What is the word for “sperm learns the ability to swim”?

Answer 4

Capacitation - occurs in the female reproductive tract

Question 5

Describe the Meitotic divisions in the follicle

-describe what they are in response to

Answer 5

First meiotic divison (reduction division)- in response to LH surge

Second meiotic division - in response to fertilization

Question 6

The acrosome is a membranous pouch containing ______ and _____

Answer 6

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

Question 7

Fertilisation usually occurs in the fallopian tube at the _______

Answer 7

Near the isthus/ampulla junction

Question 8

How does the Cervical mucus filter poor sperm?

Answer 8

• The tails of normal sperm cause the fibres of the c_ervical mucus to vibrate in rhythm_
• Failure of abnormal sperm to i_nduce this vibration_ in the cervical mucus may r_etard the progress of abnormal sperm._

Question 9

Only capacitated sperm can undergo the ________

Answer 9

Only capacitated sperm can undergo the acrosome reaction.

Question 10

Describe Meiosis in the female reproduction

Answer 10

• Initially, there is a primordial follicle (in early stage of meiosis in arrested phase)
• Following the _LH surg_e, meiosis is resumed at the first division is completed. The process stops again, however at second metaphase
• Thus at ovulation, the egg consists of the oocyte and a small polar body surrounded by the zona pellucida
• Meiosis is resumed, for the second time on fertilization.

Question 11

Describe the Cortical Reaction

Answer 11

The cortical reaction is a process initiated during fertilization by the release of cortical granules from the egg, which prevents polyspermy, the fusion of multiple sperm with one egg.

Entry of a sperm into the oocyte causes a release of intracellular calcium

This is followed by regular spikes of calcium in the oocyte.

Induce the r_esumption of meiosis_

C_ortical granules released into perivitelline space_ Release of enzymes (proteases and B-hexosaminidase) cleave ZP2 digest ZP3 cross-link the zona (involves ZP1)

Block to polyspermy

Question 12

Describe the process of the Acrosome reaction

Answer 12

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

• During the acrosome reaction, the acrosomal and p_lasma membranes_ of the sperm head fuse.
• This allows the release of acrosomal contents to the environement around the sperm

2) The acrosome reaction is triggered by an influx of calcium into the cell

• The calcium influx can be triggered by several factors including progesterone or a protein of the zona eplucida, ZP3
• The acrosome reaction must occur in close proximity to the oocyte for a sperm to be able to ferilise and egg naturally.
• In a normal ejaculate, about 10% of sperm spontaneously undergo acrosome reaction distal to the oocyte and these sperm can not fertilise the egg.

Question 13

What is a blastocyst?

Answer 13

• The blastocyst is a structure formed in the early development of mammals. It possesses an inner cell mass (ICM) which subsequently forms the embryo
• Has 2 differentiated cell populations
  
  • trophectoderm (forms the placental trophoblat)
  • _Inner cell mas_s will form the embryo proper
• Remains free-floating in the uterine cavity for 2-3 days before attaching to the decidualendometrial epithelium

In the 2nd photo, you can see at the early hatching blastocyst (at 5 o’clock)

Question 14

Speed of swimming is related to _____

Speed ____ with time

Speed _____ with freezing and thawing

Answer 14

Speed is related to fertility

Speed decreases with time

Speed decreases with freezing and thawing

Question 15

What is Nidation?

Answer 15

The hatched blastocyst comes into physical contact with the receptive decidua

The blastocyst attaches to the endometrial/decidua epithelium via specific adhesion molecules.

This is called nidation

Question 16

What are the 3 features of Capacitation of sperm?

Answer 16

• Capacitation sperm becomes hyperactivated
  
  • Tail movement changes with an increased bending of the tail and winging of the head
  • Hyperactivation may facilitate movement of sperm through the tortuous folds of the fallopian tube.
  • Only capacitated sperm can undergo the acrosome reaction.

Question 17

Clinical advise to patients- indicate that the survial time of sperm in the female repdocutvei tract is…

Answer 17

5 days

Question 18

What are the 2 consequences of sperm after capacitation

Answer 18

1) Capacitation sperm becomes hyperactivated

Tail movement changes with an increased bending of the tail and winging of the head

Hyperactivation may facilitate movement of sperm through the tortuous folds of the fallopian tube.

2) Only capacitated sperm can undergo the acrosome reaction.

Question 19

The acrosome reaction is triggered by ______

The calcium influx can be triggered by several factors including progesterone or a protein of the zona eplucida, ZP3

Answer 19

The acrosome reaction is triggered by an influx of calcium into the cell

Question 20

What is the perivitelline space?

Answer 20

Perivitelline space is the space between the zona pellucida and the _cell membran_e of an oocyte or fertilized ovum.

In the slow block to polyspermy, the cortical granules released from the ovum are deposited in the perivitellinespace.

Question 21

Describe the pathway of the sperm after it has been ejaculated until the sperm becomes motile

Answer 21

• The sperm leaves the male reproductive tract during ejaculation bathed in seminal plama
• As the prostatic and seminal vesicle fluds mix (in the vagina), a coagulum is formed that holds the ejaculate together and adheres the ejaculate to the female productive tract
  
  • The ejacualte is deposited into the vagina (vaginal insemination)
• Within 2-15 minutes, the ejaculation of the coagulum is dispersed, allowing the individual sperm to begin their quest for an egg
  
  • The breakdown of the coagulum is called liquefaction__​
• The pH of the vagina is low, therefore pH is inhibitory to sperm motility and survival. Therefore the smeinal plasma buffers the vaginal pH to around 7.2 so the sperm can become motile.

Question 22

______s the primary sperm receptor and contact between_____ and the sperm ligand induces _______

Answer 22

ZP-3 (Zona pellucida-3) is the primary sperm receptor and contact between ZP-3 and the sperm ligand for ZP-3 induces the acrosome reaction completely.

Question 23

What are the stages of Gamete transport, fertilisation and implantation?

Answer 23

• Resumption of Meiosis
• Fallopian tube transport
• Capacitation and hyperactivation
• Acrosome reaction
• Zona penetration
• Cortical reaction
• Early development of embryo
• Nidation

Question 24

What happens immediately after fertilisation?

Answer 24

Cortical reaction

Question 25

The ______is a process initiated during fertilization by the release of cortical granules from the egg, which prevents polyspermy, the fusion of multiple sperm with one egg.

Answer 25

The cortical reaction is a process initiated during fertilization by the release of cortical granules from the egg, which prevents polyspermy, the fusion of multiple sperm with one egg.

Question 26

Describe the process of capacitation

Answer 26

• Freshly ejaculated sperm are not capable of fertilising an ovum
• Capacitation occurs in the female reproductive tract
  
  • Capacitation involves the r_emoval of capcitation inhibitory substances_ from the sperm
  • During capacitation, there is a loss of cholesterol from the sperm membrane
  • Capacitation sperm becomes hyperactivated
    
    • Tail movement changes with an increased bending of the tail and winging of the head
    • Hyperactivation may facilitate movement of sperm through the tortuous folds of the fallopian tube.
  • Only capacitated sperm can undergo the acrosome reaction.

Question 27

What is Zygote Transport?

Answer 27

• blastomere is a type of cell produced by cleavage (cell division) of the zygote after fertilization and is an essential part of blastula formation.
• The zygote begins cleavage in the tube and takes approx 5 days to traverse the tube and enter the uterus
• The blastomeres divide symmetrically
• Continued doubling gives rise to the morula at the 8-16 cells tage. The cells of the morula become tightly cohesive and compacted

Question 28

What is another word for “Implantation”

Answer 28

nidation

Question 29

Describe the pathway of the sperm from the time the sperm becomes motile until fertilisation

Answer 29

• The sperm must migrate from the vagina through the uterus and into the fallopian tube
• The first barrier to sperm transport is the cervix
• Apart from a s_hort window of opportunity,_ which occurs around the time of ovulation, the cervical mucus is hostile to sperm transport.
• During the periovulation window, _cervical mucu_s is produced in large amounts and it is thin. Fertile mucus aligns its fibres and allows the passage of sperm in columns through the cervix.
  
  • The cervical mucus filtres poor sperm
• Sperm move through the cervix at a speed of 1.2-3mm/minute
• Some sperm may stay in the crypts of the cervix from which they later move into the uterus

​

• If sperm enter the fallopian tube prior to ovulation, their rate of tail beating slow 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 the cervix

Question 30

Describe the process of Resumption of Meiosis 2(?)

Answer 30

• Dependent on the Ca rise
• Destabilizes a protein complex that held the chromosomes in metaphase
• Loss of complex allows chormosomes to migrate to the poles and for meiosis to resume
• Second polar boy is shed and the first may also divide (but not usually)