Embryology of Fertilization

Site of Fertilization

• The Developing Human: Clinically Oriented Embryology, p. 117
• Langman's Medical Embryology, p. 58

The Oocyte at the Time of Fertilization

• Corona radiata - an outer layer of follicular (granulosa) cells
• Zona pellucida - a glycoprotein shell between the corona radiata and the oocyte plasma membrane
• A first polar body within the perivitelline space

Step-by-Step Events of Fertilization

Phase 1: Sperm Capacitation

• During capacitation, a glycoprotein coat and seminal proteins are removed from the sperm acrosome surface
• Membrane components are extensively altered
• Capacitated sperm show no morphologic changes but are more active
• Molecules secreted by the oocyte (resact, speract) orient and stimulate sperm via guanylate cyclase
• Completion of capacitation is required for the acrosome reaction to occur

Phase 2: Penetration of the Corona Radiata

• Capacitated sperm pass relatively freely between the corona radiata cells
• Sperm release hyaluronidase by exocytosis, which dissolves hyaluronic acid between coronal cells, allowing easier passage to the zona pellucida

Phase 3: Penetration of the Zona Pellucida - The Acrosome Reaction

1. The sperm acrosome binds to ZP3 (a zona pellucida glycoprotein) on the zona pellucida surface via the SED1 protein
2. Acrosomal reaction is triggered: multiple perforations form in the acrosomal wall, and the outer acrosomal membrane fuses with the overlying sperm plasma membrane
3. This causes exocytosis of hydrolytic enzymes from the acrosome, including:
   • Acrosin (main acrosomal protease)
   • Hyaluronidase
   • Other hydrolytic enzymes
4. These enzymes locally degrade the zona pellucida, allowing the sperm to tunnel through
5. As sperm penetrates, sperm pre-acrosin binds ZP2 - mediating forward movement

Phase 4: Sperm-Oocyte Membrane Fusion

• The first sperm to completely penetrate the zona pellucida fuses its cell membrane with the oocyte plasma membrane
• Actual fusion occurs between the oocyte membrane and the posterior membrane of the sperm head (the acrosomal cap membrane was shed during the acrosome reaction)
• Key molecular mediators of fusion:
  • IZUMO protein on sperm binds Juno receptor on oocyte
  • ADAMs 1, 2, 3 and CRISP1 on sperm bind integrins, CD9, CD81 on oocyte
  • Disintegrin-integrin interactions mediate initial adhesion
• In humans, both the head and tail of the sperm enter the oocyte cytoplasm; the sperm's plasma membrane is left behind on the oocyte surface

Phase 5: Three Oocyte Responses to Sperm Entry

1. Cortical Reaction (Block to Polyspermy)

• Sperm entry triggers a wave of Ca²⁺ release from vesicles in the oocyte
• This causes exocytosis of proteases from cortical granules lining the oocyte membrane (cortical reaction)
• The wave spreads across the entire oocyte surface
• Released enzymes hydrolyze ZP3 and ZP2, altering the zona pellucida (zona reaction) - converting it into the impenetrable perivitelline barrier
• This permanent block prevents polyspermy (fertilization by more than one sperm)
• The fast block involves immediate membrane depolarization; the slow block is the cortical/zona reaction

2. Completion of Meiosis II

• The Ca²⁺ rise causes the oocyte to complete its second meiotic division (previously arrested at metaphase II)
• This produces:
  • The second polar body (with little cytoplasm, expelled into perivitelline space)
  • The haploid female pronucleus (23 chromosomes + X)

3. Metabolic Activation of the Egg

• The sperm-borne activating factor triggers initial cellular and molecular events of early embryogenesis

Pronuclear Stage and Zygote Formation

1. The sperm head moves toward the female pronucleus, loses its protamines, and its chromatin decondenses - forming the male pronucleus (23 chromosomes)
2. Both pronuclei are surrounded by their own membranes and are visible as distinct vesicular nuclei
3. Maternal and paternal chromosomes do not fuse directly - instead, the pronuclear membranes break down (syngamy)
4. The 46 chromosomes (23 from each pronucleus) align on the first cleavage spindle
5. Fertilization is complete when chromosomes intermingle at metaphase of the first mitotic division - forming the zygote

Results of Fertilization

Key Molecular Players Summary

Clinical Correlates

• Ectopic (tubal) pregnancy: If the embryo cannot be transported to the uterus (e.g., due to chronic salpingitis/tubal adhesions), it implants in the tube, which ruptures causing potentially fatal hemorrhage
• Polyspermy: Penetration by more than one sperm results in triploidy (69 chromosomes) and abnormal, non-viable development
• Male infertility: Evaluated by semen analysis; >20 million sperm/mL needed for fertility; 50% should be motile at 2 hours
• In vitro fertilization (IVF): Capacitation is induced artificially in defined media for several hours; ICSI (intracytoplasmic sperm injection) bypasses zona penetration by directly injecting a single sperm
• Capacitation and src kinase: Both capacitation and the acrosome reaction appear regulated by tyrosine kinase (src kinase)

• The Developing Human: Clinically Oriented Embryology (Moore & Persaud), pp. 113-120
• Langman's Medical Embryology, pp. 58-60
• Junqueira's Basic Histology - Text and Atlas, 17e, pp. 1127-1128
• Guyton and Hall Textbook of Medical Physiology, pp. 4061-4068