Reproduction 5

• Fertilization


  The process by which a sperm cell and an egg cell (oocyte) fuse to form a new organism

  
• Oocyte transport

  1. Ovulated oocytes picked up by contracting fimbriae

  2. Transported to ampullary-isthmic junction

  3. Remains viable for 6 – 24 hrs (viability species dependent)

  
• Sperm deposition and movement

  1. Millions of sperm deposited – only small numbers reach the site of fertilization

  2. After deposition, retrograde loss of semen (loss is species dependent)

  3. Boar's semen forms a plug to minimize loss

  4. Semen of other species activates coagulation proteins to minimize loss

  5. Short rapid phase of sperm transport

  6. Establishment of spermatozoal reservoirs

  7. Prolonged phase of slow release from the reservoir

  8. Sperm orient themselves into the thin mucus during estrus

  9. Moved through oviduct by cilia of oviduct and uterine contractions

  10. Collection of large numbers of sperm at various sites (reservoirs)

  11. Slow release of sperm from reservoirs ensures continued entry of sperm into ampulla

  12. Sperm remain viable for 24+ hours

  
• Capacitation


  Freshly ejaculated sperm are unable to fertilize an egg until they undergo a series of changes including removal of adherent seminal plasma proteins, re-organization of plasma membrane lipids and proteins, and modifications and/or removal of sperm surface components by secretions from female reproductive tract

  
• Fertilization

  1. Sperm-ovum encounters mediated by chemotaxis

  2. Sperm in ampulla become hyperactive

  3. Hyaluronidase and other lytic enzymes facilitate sperm penetration of corona radiata

  4. Sperm attach to the zona pellucida by binding to zona protein 3 (ZP3) – initiates acrosomal reaction

  5. Penetration: Acrosome releases enzymes to digest glycoprotein layer (zona pellucida) surrounding egg

  6. Activation: Sperm physically contacts egg plasma membrane, final meiotic division

  7. Nuclei fusion: Fusion of sperm nucleus with egg nucleus to form a zygote

  8. Completion of meiosis – formation of the female pronucleus

  9. Sperm nuclear envelope disintegrates – DNA decondensation – formation of male pronucleus

  10. Formation of ootid – both male & female pronuclei present

  11. Male & female pronuclei migrate to center of ovum, nuclear envelopes disintegrate, mixing of maternal & paternal chromosomes – syngamy, re-establishment of diploid chromosome number - zygote

  12. Prevention of polyspermy: Exocytosis of cortical granules in oocyte's cytoplasm – cortical reaction or zona reaction, releases mucopolysaccharides, proteases, plasminogen activator, acid phosphatases and peroxidases, inactivates the ZP3 receptors and remodels the zona pellucida – zona block, remodelling of the vitelline membrane – vitelline block

  
• Pregnancy


  The period of foetal development in the womb, from implantation to birth
• Definitions

  • Nulliparous: Never conceived
  • Primiparous: 1 pregnancy
  • Multiparous: 2 or more pregnancies
  • Amniotic fluid: The liquid surrounding and protecting the foetus within the amniotic sac throughout pregnancy
  • Amniotic sac: The membrane within the uterus that contains the fetus and the amniotic fluid
  • Conceptus: An embryo and its associated membranes
  • Dystocia: Difficult labour; prolonged or slowly progressing labour
  • Embryo: While the newly formed animal is developing its various types of tissues; from fertilization to the primordial stages of organ development; about 30 days in dogs, cats, sheep, pigs and almost 60 days in horses, cattle, humans
  • Foetus: After the tissues are acquired until birth or hatching
  • Gestation: The period of foetal development in the womb, from implantation to birth
  • Lochia: The vaginal discharge of blood, mucus and tissue from the uterus after birth
  • Meconium: The first faeces of a mammalian infant; composed of materials ingested in the uterus - intestinal epithelial cells, amniotic fluid, cellular debris
  • Placenta: The vascular organ that supplies the foetus with maternal blood and nutrients through the umbilical cord

  
• Blastogenesis


  1. Begins with fertilization and ends with hatching of blastocyst

  2. Cell cleavage

  3. Transport of conceptus to uterus

  4. Morula formation

  5. Blastocyst formation

  6. Zonal hatching

  7. Zygote has two hemispheres: Animal pole (faster dividing, smaller cells, develops into ectoderm) and Vegetal pole (slower dividing, larger cells, develops into endoderm)

  8. Cleavage begins within an hour of fertilization, series of mitotic divisions into a blastomere, zygote -> morula -> blastocyst (blastula) stage, about 12 divisions resulting in a solid ball of blastomeres

  9. Pattern of cleavage divisions depends on amount of yolk in zygote: Birds and reptiles have meroblastic cleavage, mammals have holoblastic cleavage

  10. Zonal hatching: Lysis of the zona pellucida; facilitates implantation

  
• Embryogenesis

  1. Begins with the hatching of the blastocyst and ends with the formation of the placenta and primordial organ systems

  2. Involves antiluteolysis (maternal recognition of pregnancy), differentiation of primary germ cell layers and subsequent organogenesis, development of extra-embryonic membranes, formation of the placentas

  3. Maternal recognition of pregnancy: Successful pregnancy depends on the prolongation of the life-span of the CL, embryo sends a signal (estrogens or proteins) that either prevent the secretion of PGF2 or have direct luteotropic effect

  4. In sows, the effect of estrone is local and not systemic, absence of embryos in one horn leads to PGF2 release and luteolysis of CL in both ovaries

  5. In ruminants, trophoblast cells produce interferon-tau (IFN-tau) that inhibits the expression of receptors for oxytocin and estrogen in luminal and superficial glandular epithelial cells

  
• Nogenesis

  • Development of extra-embryonic membranes
  • Formation of the placentas
• Foetal growth period

  • Longest stage of gestation
  • Begins when placental formation and organogenesis is complete
  • Conceptus now known as a fetus - offspring recognizable as a member of a given species
  • Rapid growth of the fetus - size increases geometrically
  • Most weight gain occurs during last third of gestation
  • Histotrophic nutrition replaced by hemotropic nutrition
• Maternal nutrition

  Adequate maternal nutrition crucial: Demands of growing fetus, Dam's body stores to deal with stress of parturition, lactation and to shorten subsequent breeding intervals
• Placenta

  • Temporary organ that develops in the uterus during pregnancy
  • Provides the nutrients and oxygen to the developing embryo and foetus for development and growth; transient endocrine organ (eCG, hCG, progesterone, relaxin)
  • Maternal (uterine lining) and foetal side (foetal membranes)

  
• Puerperium

  • Post-partum period when reproductive tract returns to its normal non-pregnant state
  • Uterine involution; myometrial repair and resumption of ovarian function
  • Myometrial contractions and lochia expulsion (dairy cows, lochia discharge occurs at day 2-9 post-partum)
  • Elimination of bacterial contamination from the uterine lumen
  • Resumption of estrous cycle

  
• Profile of hormones involved in parturition