Maternal Recognition of Pregnancy

Question 1

What is fertilization? where does it occur?

Answer 1

• The fusion of haploid gametes to form a diploid zygote
• Occurs in the ampulla of the fallopian tube (aka oviduct)

Question 2

What has to happen to the egg and sperm before fertilization can occur?

Answer 2

• Transport:
  
  • Egg wiped off the ovary and into the funnel shaped end of the oviduct
    
    • Cumulus-oocyte complex released
      
      • Most oocytes are 2° (1°in bitch)
    • Cilia move the egg to the site of fertilization
    • Block at ampula/isthmus junction or tubo-uteral junction
    • Uterus is hostile to embryo until progesterone levels increase
  • Sperm -
    
    • deposited in vagina (cattle, dog) or uterus (horse, pig)
    • Uterine contractions move sperm to oviduct
    • Only a few hundred to a few thousand sperm reach the fertilization site (<1%)
    • Many barriers to reach the uterine tube
      
      • Vagina - hostile to sperm
      • Cervix - mucus-filled crypts (easier to pass if estrogen is high)
      • Uterus - sperm are rapidly phagocytosed in some species
    • Motility is important for final approach to egg
      
      • Moves by rotating its tail in a spiral motion, induces waves of force backwards
      • if the sperm its a hard surface (like an egg) the spiral motion causes the entire sperm to rotate
• Maturation of sperm:
  
  • Sperm need to mature in the epididyus
    
    • Develop ability for sustained progressive motility
    • Modify membrane
    • Increase stability of chromatin in sperm head
  • Capacitation in female tract
    
    • Chemical s in female tract needed to change so it can fertilize the egg
      
      • Develop hyperactive motility
      • Prepare membrane for acrosome reaction
    • If doing in vitro fertilization - must artificially capacitate the sperm

Question 3

What is the Acrosome Reaction?

Answer 3

• Components in the fluid surrounding the egg promotes the acrosome reaction
• True reaction only takes place on the zone pellucida - initiated by ZP receptors
  
  • sperm that are acrosome reacted before they reach the ZP are unable to fertilize the egg
• Sperm with an intact acrosome cannot penetrate the egg
• Enzymes in the acrosomes are:
  
  • Hyaluronidase - breaks down extracellular matrix
  • Acrosin - trypsin-like enzyme that digests the ZP
  • Corona penetrating enzyme - aids in penetrating the layers of corona cells surrounding the egg

Question 4

How do sperm bind to the egg?

Answer 4

• Binding site is on the mid-region of the sperm head
• Sperm binding to the ZP is a Ca⁺⁺ dependent event
• Sperm binding to the ZP receptor triggers the acrosome reaction
• After the sperm penetrates the ZP it contracts the egg membrane
• Fusion of the sperm and egg membranes trigger:
  
  • Release of cortical granules from the egg that act on the ZP to block penetration by another sperm
  • Decondensation of the sperm nucleus
  • Egg cortex engulfs the front of the sperm head
  • Formation of the 2nd polar body

Question 5

How is polyspermic fertilization prevented?

Answer 5

• Zona Block:
  
  • Removal of receptors for sperm from the ZP surface and exocytosis of cortical granules form the oocyte making the ZP impermeable to sperm cells
  • Triggered by the fusion of the sperm head with the vitelline surface of the egg
• Vitelline Block:
  
  • Changes the electrostatic character of the vitelline membrane - prevents binding of sperm
  • Triggered by the fusion of the sperm head with the vitelline surface of the egg

Question 6

Following fusion of the sperm and oocyte plasma membranes what happens?

Answer 6

• sperm head is incorporated into the egg
• Sperm chromatin loosens from its tightly-packed state (decondensation)
• The chromatin from both the sperm and egg are encapsulated in a nuclear membrane
• The two haploid genomes migrate toward each other and condense into chromosomes
• Creation of a diploid organism

Question 7

summarize how the sperm enters the oocyte and creates a haploid cell?

Answer 7

Question 8

How does a female tolerate a foreign animal in her body and alter her hormonal cycles so that the environment is hospitable to the embryo/fetus?

Answer 8

• Immunologic response suppression
  
  • Early pregnancy factor - immunosuppressive factor produced by early embryo
• Endocrine alteration
  
  • CL regression must be prevented
  • Need to maintain P₄ levels so that uterus provides viable environment

Question 9

How is the regression of the CL prevented?

Answer 9

• Antiluteolytic Stimulus
  
  • Sheep/Cow:
    
    • Conceptus produces trophoblastin (Type I interferon - INFtau) which reduces PGF_2a release by uterus
  • Mare:
    
    • Conceptus migrates between horns, endometrial contact inhibits PGF_2a production
    • Conceptus increases in size and trophoblast cells invade endometrium - endometrial cups form and produce eCG
• Placental Luteotrophin (Luteotrophic)
  
  • Human:
  • Blastocyst produces hCG which acts at the CL to override luteolysis
• Redirection of PGF release
  
  • Pig:
  • Conceptus Produces estrogen which is luteotrophic and redirects PGF_2a release to the uterine lumen (rather than systemic circulation)

Question 10

Why does the dog not need a similar mechanism for maternal recognition of pregnancy like mares/cows/etc?

Answer 10

• Monoestrus - does not cycle like cows, mares, etc. do not need to stop the next cycle
  
  • CL has a gestation-long life span

Question 11

Why does the cat not need a similar mechanism for maternal recognition of pregnancy like mares/cows/etc?

Answer 11

• Induced ovulator
• CL has a gestation-long life-span

Question 12

Why do camelids not need a similar mechanism for maternal recognition of pregnancy like mares/cows/etc?

Answer 12

• Induced ovulator
• CL will lyse in late diestrus if not “rescued”