Topic 93- Fertilization, embryogenesis; maternal recognition of pregnancy

Question 1

Words to include in fertilization

Answer 1

• Fertilization in mammals and non-mammals:​
  
  • Final maturation phase for sperm and eggs
  • Penetration by sperm
  • Species-specific binding of sperm to egg
  • Fusion of sperm and egg
  • Activation of eggs
• Basic molecular events:
  
  • Receptor-ligand interaction
  • Signaling cascade
  • Specific proteolysis
  • Nuclear transformation

Spermatozona

• Ascend the cervix
• Transported to oviduct
• Capaitation
• Bind to oocyte
• Acrosome reaction
• Penetrate the zonal pellcida
• Fusion with oocyte plasma membrane
• Spermatozoon enters oocyte cytoplasm
• Nucleus de-condenses
• Pronucleus is formed
• Sperm eter ampulla for fertilization
  
  • Copulation reflex → increas motility of uterus → oxytocin secretion

Capacitation

• ​Sperm-cells are exposed to reversibele transformation = capacitation
• Maturation period of spermium
  
  • Capacitation + acrosome reaction
• Sperm capable of penetrating into egg → enzyme release

Acrosome reaction

• Sperm passes through zona pellucida
  
  • Plasma membrane + acrosomal content lost
  • Inner acrosomal membrane denuded
• Sperm loosing acrosomes before encountering oocyte → ø binding to zona pellucida

Transport of oocyte (egg)

• Ovulated egg picked up and transported into oviduct trough ostium
• Ciliary motion
  
  • Oviduct → ampulla
• Ampulla
  
  • Site of fertilization

Cortical granules of the oocyte

• Oogenesis
  
  • Oocyte devlopes membrane-bound granules
    
    • Acumulate in corical cytoplasm
• Cortical granules
  
  • ​Digest zona pellucida

Egg activaiton

• Quiescent state of egg
  
  • Prior to fertilization
  • Arrested in metaphase of 2^nd​ meiotic division
• Sperm binding
  
  • Metabolic and physological changes
    
    • IC concentration of Ca²⁺ ↑
    • Completion of 2^nd meiotic division
    • Cortical reaction

Sperm-oocyte binding and penetration

• Receptor-ligand interaction
  
  • Sperm binding to zona pellucida
• Carbohydrate groups on zona pellucida
  
  • Glycoproteins
  • Sperm receptors

Zona reaction

• Excocytosis of cortical granules (cortical reaction
  
  • Enzymes digesting zona pellucida with proteases
• Zona pellucida hardnes
• Runner-up sperm
• Sperm receptors in zona pellucida destoryed

Summary of fertilization

• Hyaluronidase enzyme / Corona Penetrating Enzyme (CPE)
  
  • Dissolving of corona radiata
• Acrosin
  
  • Trypsin-like enzyme
  • Penetration of zona pellucida
• Perivitellines pace
  
  • Apical part (sperm head) adheres cytoplasmic body of egg
  • Plasma membranes fuse → cytoplasm fusion of egg and sperm
• Thrust of spermium
  
  • Activation of ovum
  • Embryonic life
• Polyspermy defense
  
  • Permability of zona pellucida ↓
• Cortical granules / vitellus
  
  • Released from cytoplasm
• Zona reaction
  
  • Cortical granules
    
    • Spermium-accepting capability of ovum ↓

Post-fertilization events

• Fertilizing sperm + oocyte fusion
  
  • Egg cytoplasm
• Nuclear envelope of sperm (ø)
  
  • De-condensation
    
    • Chrmatin loosens

Question 2

Words to inculde in embryogenesis

Answer 2

Development of pre-implatation embryo

• Early stages: embryo encased in zona pellucida
• Stages:
  
  • ​Zygote
    
    • One-cell embryo
    • Diploid complement of chromosomes
  • Clevage divisions
    
    • 2-4-8-16-32 cells tage
    • Blastomers
      
      • Cells in clevage stage
  • Morula
    
    • 16 or more cells
    • Tight junctions between blastomers
      
      • Compaction (embryo shape change)
  • Blastocyst (blastula)
    
    • Blastogenesis
      
      • Junctional complexes between blastomers
      • Membrane transport molecules
        
        • Na⁺ pumps
      • Blastocyst accumulate blastocelic fluid
        
        • Expanded blastocyst
      • Pluripotent cells
  • Hatching
    
    • Stretched zona pellucida
      
      • Blastocyst cracking
      • Empty zona pellucida
      • Zona-free/hatched blastocyst
        
        • Blastocyst in lumen of uterus
• Implantation
  
  • Blastocyst stage
  • Embryo settles down in hypertrophied mucosa of uterus
• Nidation
  
  • Blastocyst penetrated deeper into tissue embedding it
  • Embryonogenis starts around nidation

​Embryonic membranes

• Yolk sac
  
  • Nutrient supply of early embryo
• Amino
  
  • ​From trophoblast
  • Protection from injury
  • Lubrication for parturition
  • Reservior for urine & wastes
• Allatois
  
  • Chorio-allantoic placenta
    
    • ​Fuse with chorion
  • Carries blood vessels of placenta
  • Reservior for nutrients & waste
  • Umbilical cord attaches fetus → allantois
• Chorion
  
  • Embryonic part of placenta
    
    • ​Chorionic villi
  • Uterinal endometrium attachment

Placenta

• Division:
  
  • ​Embryonic part
    
    • Chorionic villi together
  • Maternal part
    
    • Embryonic placenta is embedded in maternal endometrium
• Role:
  
  • ​Fetal nutrition
    
    • Water and electrolytes
    • Minerals
    • Glucose → fructose
    • Fatty acids, amino acids transferred across placenta
    • Oxygen
      
      • Fetal hemoglobin
  • Immunological role
    
    • Eq, Ru: no antibodies cross placenta
      
      • Colostrum
  • Blood gas exchange
    
    • Placenta is lungs of fetus
    • Diffusion through epithelial cells of chorionic villy
    • Fetal hemoglobin (HbF)
      
      • 2,3 DPG (HbF binds weaker than HbM)
    • Arterial blood passing towards fetus with v. umbilicalis
      
      • Oxygen saturation
        
        • 85-90%

Question 3

Words to include in maternal recognition of pregnancy

Answer 3

• Maintainance of luteal phase
  
  • Preventing luteal regression
  • Persisting of corpus luteum
    
    • Secrete P4
• Cyclic activity suspended
  
  • PGF_2α
    
    • Ø production from pregnant endometrium (ø Ca)
    • Luteolysis
      
      • P4 inhibits luteolysis
• Corpus luteum → corpus luteum graviditatis
  
  • P4 production sufficient
  • Car: special situation

Primates

• Human chorionic gonadotropin (hCG)
  
  • Blastocyst secrete
  • Protein hormone
  • Binds to LH-receptors in corpus luteum
    
    • P4 secretion stimulated

Horse

• Inhibitor of PGF_2α synthesis
  
  • Early Pregnancy Factor (EPF)
  • Estradiol

Ruminants

• Interferon tau
  
  • Early ruminant embryo secrete
  • OTP1, BTP1: Bovine Trophoblast protein 1
    
    • Endometrium exposure to OTP1 inhibits PGF_2α synthesis
      
      • Luteolysis signal blocked
    • Corpus luteum survives
    • P4 levels maintained

Dogs

• Following ovulation, P4 pattern secretion same non-pregnant or pregnant
  
  • Ø maternal recognition

Swine

• Embyo migrate before implantation
  
  • Estrogen secretion
    
    • PGF_2α secretion → exocrine secretion
• < 4 embryo
  
  • Less E2 →luteolysis → abortion

Question 4

topics to include in the esay

Answer 4

1. Fertilization
   
   1. Common features between mammals and non-mammals
   2. Basic molecular events
   3. Spermatozoa
   4. Capacitation
   5. Acrosome reaction
   6. Transport of oocyte (egg)
   7. Cortical granules of the oocyte
   8. Egg activation
   9. Sperm-oocyte binding and penetration
   10. Zona reaction
   11. Summary of fertilization
   12. Post-fertilization events
2. Embryonogenesis
   
   1. Development of pre-implantation embryo
      
      • Stages
   2. Embryonic membranes
   3. Placenta
3. Maternal recognition of pregnancy
   
   1. General
   2. Primates
   3. Horse
   4. Ruminants
   5. Dogs
   6. Swine

Question 5

Fertlization

Give the common features in fertilization of mammals and non-mammals

Answer 5

1. Final maturation phase for sperm and eggs
2. Penetration by sperm of one or more EC coats surrounding the eggs
3. Species-specific binding of sperm to eggs
4. Fusion of sperm and eggs
5. Activation of eggs

Question 6

Fertlization

Give the basic molecular events

Answer 6

• Receptor-ligand interactions
• Signaling cascades
• Specific proteolysis
• Nuclear transformations

Question 7

Fertilization

Name the main stages of fertilization

Answer 7

1. Spermatozoa
2. Capacitation
3. Acrosome reaction
4. Transport of oocyte (egg)
5. Cortical granules of the oocyte
6. Egg activation
7. Sperm-oocyte binding and penetration
8. Zona reaction
9. Post-fertilization events

Question 8

Fertilization

1. Spermatozoa

Answer 8

1. Ascend the cervix
2. Transported to the oviduct
3. Undergo capacitation
4. Bind to the oocyte
5. Undergo acrosome reaction
6. Penetrate the zona pellucida
7. Fuse with the oocyte plasma membrane
8. Spermatozoon enters the oocyte cytoplasm
9. Nucleus de-condenses
10. The male pronucleus is formed, signalig successful fertilization

• Sperm cells must enter the ampulla in order to fertilize the egg
  
  • Oxytocin secretion, elicited by the copulation reflex, increases motility of the uterus

Question 9

Fertilization

2. Capacitation

Answer 9

• Capacitation of the spermium:
  
  • During their travelling along female reproductive ducts, sperm-cells are exposed to a reversible transformation
• Maturation period of the spermium:
  
  • Capacitation + “acrosome reaction”
• Sperm becomes capable of penetrating into the egg

Question 10

Fertilization

3. Acrosome reaction

Answer 10

• As the sperm passes through the zona pellucida, plasma membrane and acrosomal content are lost
• By the time the sperm traverses the zona pellucida, the entire anterior surface of its head, down to the inner acrosomal membrane, is denuded
• Sperm that lose their acrosomes before encountering the oocyte are unable to bind to the zona pellucida and thereby unable to fertilize

Question 11

Fertilization

4. Transport of oocyte

Answer 11

• The ovulated egg is picked up and transported into oviduct through an opening called the ostium
• Once an oocyte enters the oviduct, it is propelled by ciliary motion down into the ampulla, where fertilization takes place (site of fertilization)
• Site of fertlization: Ampulla of fallopian tube​

Question 12

Fertlization

5. Cortical granules of the oocyte

Answer 12

• During oogenesis, the oocyte develops membrane-bound granules that accumulate in the cortical cytoplasm
• The final structural feature of the egg that serves a critical function during fertilization is a set of cortical granules
  
  • They help “digesting” the zona pellucida from inside

Question 13

Fertilization

6. Egg activation

Answer 13

• Prior to fertilization, the egg is in a quiescent state, arrested in metaphase of the 2^nd meiotic division
• Upon binding of a sperm, the egg rapidly undergoes a number of metabolic and physical changes:
  
  • IC concentration of Ca²⁺ ↑
  • Completion of the 2^nd meiotic division
  • Cortical reaction

Question 14

Fertilization

7. Sperm-oocyte and penetration

Answer 14

• Binding of sperm to the zona pellucida is a receptor-ligand interaction with a high degree of species specificity
  
  • The carbohydrate groups on the zona pellucida glycoproteins function as sperm receptors

Question 15

Fertilization

8. Zona reaction

Answer 15

1. Exocytosis of cortical granules (cortical reaction)
   
   • Enzymes digesting the zona pellucida with proteases
2. Zona pellucida hardens
3. Runner-up sperm that have not finished traversing the zona pellucida by the time the hardening occurs are stopped in their tracks
4. Sperm receptors in the zona pellucida are destroyed

Question 16

Fertilization

9. Post-fertilization events

Answer 16

• Following fusion of the fertilizing sperm with the oocyte, the sperm head is incorporated into the egg cytoplasm
• De-condensation:
  
  • ​The nuclear envelope of the sperm disperses, and the chromatin rapidly loosens from its tightly packed state
• Chromatin from both the sperm and egg are soon encapsulated in a nuclear membrane, forming pronuclei

Question 17

Fertlilzation

Fertilization is executes in several consecutive steps

Answer 17

1. Dissolving of corona radiata by hyalornidase enzyme / CPE (Corona Penetrating Enzyme)
   
   • ​Penetration occurs
2. Acrosin (trypsin-like enzyme) which promotes pepentration of zona pellucida
3. The sperm cells enters perivitellines space, sperm head adhere to the cytoplasmic body of the egg → plasma membranes of the two cells fuse → fusiog of the two cytoplasms
4. Activation of the ovum
   
   • ​Beginning of embryonic life
5. Defense mechanisms against polyspermy by decreasing permability of zona pellucida
   
   • ​Cortical granules are released from cytoplasm, vitellus
   • Zona reaction: substances of cortical granulse decrease spermium-accepting capability of ovum

Question 18

Embryogenesis

Development of pre-implantation embryo

Answer 18

• In all early stages, the embryo is encased in its zona pellucida

Stages:

1. Zygote
   
   • ​One-cell embryowith adiploid complementofchromosomes
2. Cleavage division
   
   • _​_2-4-8-16-32 cell stage
   • Blastomers: the cells in clevage stage
3. Morula
   
   • _​_16+ cells
   • The blastomers begin to form thight junctions
     
     • ​Compaction: embryo shape change
4. Blastocyst
   
   • _​_Blastogenesis (creation of blastocyst)
   • Cells of the blastocyts are pluripotent
5. Hatching
   
   • ​Stretched zona pellucida cracks the blastocst which escapes
   • This leaves an empty zona pellucida and zona free or hatched blastocyst lying in the lumen of the uterus

• Implantation:
  
  • ​In blastocyst stage, the embryo settles down in the hypertrophied mucosa of the uterus
• Nidation:
  
  • ​The blastocyst penetrates deeper into the tissue embedding it, for better supply of nutrients
  • Embryogenesis starta around nidation
    
    • Forming:
      
      • Primary germ layers
      • Linings of organs
      • Fundaments of organs
      • Embryonic membranes

Question 19

Embryogenesis

Embryonic membranes

Answer 19

• Yolk sac
  
  • Early nutrition suplly of early embryo
• Amnion
  
  • ​Formed from the trophoblast
  • Task:
    
    • Protects fetus from injury
    • Provides lubrication for parturition
    • Reservior for urine and wastes
• Allantois
  
  • ​Fuses with cohroin (chorio-allantoic placenta)
  • Carries blood vesses of placenta
  • Reservior for nutrients and wastes
  • Umbilical cord attaches fetus → allantois
• Choroin
  
  • ​The chorionic villi form the embryonic part of the placenta
  • Attaches to uterinal endometrium (maternal part of placenta)
    
    • Absorbs nutrients from uterus
    • Allows maternal/fetal gas exchange
    • Produces hormones
  • Precursor of placenta

Question 20

Embryogenesis

Placenta

Division and role

Answer 20

• Divided into two parts
  
  1. Embryonic part
     
     • ​All the chorionic villi together
  2. Maternal part
     
     • ​Embryonic placenta is embedded in the maternal endometrium

Role of placenta:

• Fetal nutrition:​
  
  • ​Water and electrolytes
  • Minerals
  • Converts glucose → fructose
  • Fatty acids, amino acids transferred across the placenta
    
    • Fetal fat and protein synthesis
  • Oxygen
    
    • ​Fetal hemoglobin
    • HbF binds 2,3-DPG weaker than HbM
• Immunological role:
  
  • ​Eq, Ru: antibodies does not cross the placenta
    
    • Colostrum

Question 21

Maternal recognition of pregnancy

Function

Answer 21

• Maintainance of luteal phase
  
  • ​Certain chemicals have contact with endometrium and prevents luteal regression, allowing corpus luteum to persist and continue with P4 secretion
• Cyclic activity of the ovary is suspended during pregnancy
• No pregnant endometrium produces PGF_2α (luteolysis)
  
  • ​Ø primates, Ca
• During pregnancy P4 inhibits luteolysis
• In most of the species corpus luteum does not persist during pregnancy
  
  • Corpus luteum has to be transformed to corpus luteum graviditatis
    
    • ​Produces sufficient amount of P4

Question 22

Maternal recognition of pregnancy

Interspecies differences

Answer 22

Primates

• Blastocysts secrete human chorionic gonadotropin (hCG), a protein hormone
  
  • ​hCG binds to LH-receptors in corpus luteum and stimulates continued secretion of P4

Horse

• EPF (Early Pregnancy Factor) and E2 have a role in the inhibition of the PGF_2α​ synthesis

Ruminants

• The early ruminant embryo secretes a protein called interferon tau
  
  • OTP1, BTP1: Bovine trophoblast protein 1
  • Exposure of the endometrium to this hormone inhibit the synthesis of PGF_2α, blocking the signal for luteolysis
  • Corpus luteum survives and P4 levels are maintained

Dogs

• Following ovulation, the pattern of P4 secretion is the same pregnant or not
  
  • Ø maternal recognition of pregnancy

Swine

• Embryos are migrating before implantation and secret estrogen, which changes the direction of PGF_2α secretion to exocrine secretion
• If number of embryos less than 4
  
  • Less E2 → luteolysis → abortion

Figure: progesterone profile of pregnant and non-pregnant bitch