Reproductive System

• All cells arise from other cells
• Unicellular organism


  Cell division results in reproduction
• Multicellular organism


  • Cell division usually results in growth or replacement of tissue
  • Division of body cells may also be a method of reproduction if the new cells separate from the parent and form a complete, independent individual
• Cell division


  1. Division of the nucleus (mitosis)

  2. Division of the cytoplasm (cytokinesis)
• Types of reproduction


  • Asexual reproduction
  • Sexual reproduction
• Asexual reproduction


  • Only one parent
  • No special reproductive cells or organs
  • Individual is a separated part of the parent organism
• Some organisms reproduce only asexually, others only sexually, and some can reproduce by either method
• Asexual reproduction


  • Involves only mitotic cell division
  • Offspring have exactly the same hereditary information as parent
  • Offspring show little variation, are nearly identical to each other and to the parent
  • Results in stable characteristics within a species

  
• Forms of asexual reproduction

  • Binary fission
  • Budding
  • Spore formation
  • Regeneration
  • Vegetative reproduction

  
• Binary fission


  Parent divides into approximately two equal parts, each daughter cell becomes a separate individual, no parent is left
• Budding


  Parent divides into two unequal parts, new individual develops as small buds on the outer surface of the parent, the bud may break off and live independently or may remain attached and form a colony, parent and offspring are not of equal size
• Spore formation


  Spores are single, specialized cells that when released germinate and grow to form new individuals, each spore contains the usual components of a cell and is often surrounded by a cell wall

  
• Regeneration


  Ability of an organism to regrow lost body parts, relatively simple animals can regenerate each part into a whole new organism, the power of regeneration decreases as the animal becomes more complex
• Vegetative reproduction


  Roots, stems and leaves can give rise to new plants, undifferentiated cells divide mitotically and then differentiate to give rise to an independent plant, new plant has the same hereditary characteristics as its parent, can occur naturally or be brought about artificially
• Conjugation


  Simplest form of sexual reproduction in protists and other simple organisms, involves formation of a cytoplasmic bridge between two cells of different mating types and exchange of nuclear material
• Sexes in animals


  • Female
  • Male
• Hermaphrodites


  Animals that contain both testes and ovaries, self-fertilization is rare, they exchange sperm with another individual of the same species
• Gametogenesis


  1. Oognenesis: formation of eggs in the ovaries

  2. Spermatogenesis: formation of sperm in the testes
• Oogenesis


  1. Oogonia divide mitotically to form a supply

  2. Oogonia develop into primary oocytes, meiosis stops until sexual maturity

  3. Primary oocyte divides unequally into secondary oocyte and polar body

  4. Secondary oocyte divides unequally into ootid and another polar body

  5. Ootid grows into a mature egg, polar bodies disintegrate

  
• Spermatogenesis


  1. Spermatogonia divide mitotically to produce more spermatogonia

  2. Spermatogonium becomes a primary spermatocyte

  3. Primary spermatocyte undergoes first meiotic division to form secondary spermatocytes

  4. Secondary spermatocytes undergo second meiotic division to form spermatids

  5. Spermatids develop into mature sperm
• External fertilization


  Eggs are fertilized in the environment outside the body of the female, limited to aquatic animals, large numbers of sperm and eggs are released
• Internal fertilization


  Fertilization within the body of the female, characteristic of land animals, requires a specialized sex organ to transfer sperm to the female, fewer eggs are needed but large numbers of sperm are released
• Parthenogenesis


  Development of an unfertilized egg into an adult animal without fusion with sperm, occurs in many insects, rotifers and microscopic animals
• Male reproductive system


  • Testes
  • Epididymis
  • Vas deferens
  • Urethra
  • Seminal vesicles
  • Cowper's glands
  • Prostate gland
• Female reproductive system


  • Ovaries
  • Follicles
  • Oviducts (Fallopian tubes)
  • Uterus
  • Cervix
  • Vagina
• Sperm and egg


  Egg secretes a chemical that attracts the sperm, one sperm breaks through the egg membranes and the sperm nucleus enters the egg cytoplasm, this prevents penetration by any other sperm
• Fertilization and zygote formation

  1. Sperm nucleus fuses with egg nucleus to form diploid zygote

  2. Zygote undergoes cleavage and develops into a blastula as it moves down the oviduct

  3. Blastula implants in the uterus, beginning pregnancy
• Pregnancy and birth


  1. Embryo undergoes gastrulation, forming the three germ layers

  2. Embryo develops into a fetus

  3. Uterine muscles contract, cervix enlarges to permit baby's passage