topic 11.4: Sexual Reproduction

• Spermatogenesis and oogenesis


  • Both involve mitosis, cell growth, two divisions of meiosis and differentiation
  • Result in different numbers of gametes with different amounts of cytoplasm

  
• Spermatogenesis


  1. Spermatogonia divide to produce spermatocytes

  2. Meiosis to produce spermatids

  3. Differentiation into mature sperm

  
• Oogenesis


  1. Oogonia divide by mitosis during fetal development

  2. Oogonia enlarge and undergo meiosis I, stopping at prophase I

  3. At puberty, some follicles develop each month in response to FSH

  4. Oocyte completes meiosis I, dividing unequally to form a polar body

  5. Oocyte continues into meiosis II, halting at metaphase II

  6. Polar bodies eventually degenerate
• Spermatogenesis and oogenesis both involve mitosis, cell growth, two divisions of meiosis and differentiation

  
• Processes in spermatogenesis and oogenesis result in different numbers of gametes with different amounts of cytoplasm

  
• When the follicle is mature


  It ruptures to release the secondary oocyte into the fallopian tube
• If the cell is fertilized


  The oocyte completes meiosis II, forming the ovum
• The remaining follicle cells form the corpus luteum, which secretes progesterone
• Spermatogenesis and oogenesis


  1. Mitosis

  2. Cell growth

  3. Two divisions of meiosis

  4. Differentiation

  
• Spermatogenesis and oogenesis


  Processes that result in different numbers of gametes with different amounts of cytoplasm

  
• Oogenesis resources


  • http://highered.mheducation.com/sites/00724958
  • 55/student_view0/chapter28/animation__maturat
  • ion_of_the_follicle_and_oocyte.html
  • http://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::6
  • 40::480::/sites/dl/free/0072495855/63089/28_02_1.swf::Structure%2
  • 0of%20the%20Ovary%20and%20the%20Developmental%20Sequence
  • %20of%20the%20Ovarian%20Follicles
  • http://www.wiley.com/college/jenkins/0470227583/ani
  • mations/index_25_03_01.html
• Fertilization in animals can be internal or external
• External fertilisation


  • Fish
  • Amphibians
• External fertilisation


  Eggs are released followed shortly by sperm, susceptible to environmental variation, animals produce large quantities of eggs and sperm to compensate for losses
• Internal fertilisation


  • Reptiles
  • Birds
  • Mammals
• Internal fertilisation


  Sperm is deposited into the female, in easy reach of the ova, during intercourse, to prevent dehydration of gametes or the developing embryo
• Fertilization involves mechanisms that prevent polyspermy
• Fertilisation


  1. Sperm pushes through follicular cells and binds to receptors in zona pellucida

  2. Enzymes released from acrosome digest zona pellucida

  3. Membranes of sperm and ovum fuse, stimulating cortical reaction to harden zona pellucida and Ca2+ influx to complete meiosis II

  4. Sperm nucleus deposited into ovum's cytoplasm and fuses with ovum nucleus to form diploid zygote

  
• Implantation of the blastocyst in the endometrium is essential for the continuation of pregnancy
• Blastocyst formation and implantation

  1. Morula forms after mitotic divisions

  2. Unequal divisions form fluid-filled cavity, now a blastocyst with inner cell mass and outer layer

  3. Blastocyst embeds in endometrium, gains nutrients and oxygen from endometrium tissue fluid and capillary network
• HCG stimulates the ovary to secrete progesterone during early pregnancy


  • Menstruation ceases
  • No further mature eggs are released
• Estrogen and progesterone are secreted by the placenta once it has formed
• The placenta facilitates the exchange of materials between the mother and fetus
• Placenta


  • Chorionic villi increase surface area for exchange of substances
  • Placental cells secrete hormones like HCG, estrogen and progesterone
• Birth


  1. Rise in estrogen levels stimulates birth process

  2. Estrogen increases responsiveness of uterine wall to oxytocin

  3. Contractions stimulate stretch receptors, signalling brain to release more oxytocin

  4. Oxytocin stimulates stronger uterine contractions

  5. Contractions continue briefly after birth to eject placenta, then oxytocin levels fall and contractions cease
• Animal size


  Positively correlated with gestation period and development of newborn young
• Altricial mammals give birth to relatively helpless, incompletely developed offspring. Precocial mammals give birth to offspring that are mobile and able to defend themselves.
• There is a positive correlation between body mass and gestation period, but mammals with the same gestation period can have widely varying body masses
• Animals with longer gestation periods give birth to more developed offspring