sexual intercourse and fertilisation

• For fertilisation, sperm needs to be brought into contact with an ovum 

  • Usually through sexual intercourse
• Penis must become enlarged and firm (erection) 

  • Blood rushing into spaces of erectile tissue 
  • Sexual excitation initiates blood flow 
• When sexual stimulation of penis within vagina becomes sufficiently intense, rhythmic contractions of epididymis, vas deferentia, seminal vesicles, prostate gland occur 

  • Propel contents of ducts and glands into urethra and out body (ejaculation)
  • Rapid heartbeat, increase blood pressure, increase breathing rate
• When female is sexually stimulated, erectile tissue fills with blood 

  • Reduces size of vaginal opening - increase stimulation of penis 
  • Copious secretions of mucus by glands around cervix and region of vaginal opening 
  • Lubricate epithelial lining of vagina 
• Insemination : deposition of sperm within vagina 

  • Sperm is release in vagina at entrance to uterus 

  Sperm travels through cervix → body of uterus → uterine tube 

  • Muscular contraction of uterus and uterine tube help transport sperm
  • Muscular contraction of uterine tube and beating action of cilia transport ova toward uterus 
• Hundreds of millions of sperm are deposited, only a few thousand reach uterine tube 

  • Sperm mortality is high - why a large number of sperm are required
• Secondary oocyte is surrounded by : 

  • Corona radiata - follicle cells held together by cementing materials that contain acid
  • Zona pellucida - glycoprotein matrix surrounding plasma membrane of oocyte
• Outer surface of acrosome on head of sperm contains enzyme

  • Capable of breaking down acid holding together corona radiata 
  • Amount in a single sperm is small and ineffective. When several thousand surround oocyte, there is enough to loosen cells - why a large number of sperm are required
  • Allows one sperm to penetrate corona radiata 
• Once sperm encounters zona pellucida - acrosomal reaction is initiated 

  • Digestive enzymes from acrosome are released
  • Enzymes breaks down glycoprotein matrix , giving sperm access to plasma membrane of oocyte 
  • Plasma membranes fuse and nucleus of sperm enters ovum 
• Entrance of one sperm stimulates formation of fertilisation membrane around oocyte 

  • Prevents entry of any more sperm 
• Once sperm has entered : 

  • Tail is absorbed 
  • Head begins of move through cytoplasm in the form of a male pronucleus 
  • Secondary oocyte completes second meiotic division
  • Nucleus of oocyte develops into female pronucleus 
  • Fuse to form zygote