Ch. 42

• Details in Human Fertilization:

  • Sperm bores through remaining follicular cells called the corona radiata
  • Acrosome: cap on sperm that releases enzymes digesting zona pellucida (jelly coat)
  • Sperm surface molecules bind to egg receptors to ensure same species fertilizes the egg
  • Sperm nucleus enters egg and fuses with egg nucleus forming a zygote
  • Prevention of polyspermy:
  • "Fast block": sperm membrane touching egg membrane causes depolarization
  • "Slow block": cortical granules release enzymes turning zona pellucida into an impenetrable fertilization membrane, Juno molecules disappear
• Embryonic Development:

  Cellular Stages of Development:

  • Morula: solid ball of cells from repeated cell division without growth
  • Blastula: hollow ball of cells with a blastocoel cavity
  • Inner Cell Mass (ICM) develops into the embryo

  Tissue Stage of Development:

  • Gastrula: layered ball of cells from invagination

  Layers:

  • Ectoderm: outer layer forming nervous system, epidermis, lens of eye
  • Mesoderm: middle layer forming musculoskeletal, cardiovascular, excretory, and reproductive systems
  • Endoderm: inner layer forming epithelial lining of digestive and respiratory systems and associated glands
  • Archenteron: new internal cavity becoming gut tube
  • Blastopore: opening into archenteron becoming anus or mouth

  Organ Stage of Development Neurula:

  • Nervous system forms from ectoderm:
  • Neural plate then neural tube
  • Anterior end becomes brain, rest becomes spinal cord
• • Neural crest: band of cells developing where neural tube pinches off from ectoderm, will become skin, muscles, etc.
  • Induced by notochord, which becomes vertebral column in vertebrates
  • Somites:
  • Midline mesodermal tissue not becoming notochord
  • Two longitudinal masses of tissues block off into somites
  • Somites become muscles associated with axial skeleton, causing segmentation
• Developmental Processes:

  Introduction: Development involves cellular differentiation, morphogenesis, and apoptosis

  Cellular Differentiation:

  • Every cell initially totipotent
  • Mechanisms: Cytoplasmic Segregation and Induction
  • Developmental Potential: Totipotent, Pluripotent (ICM), Unipotent

  Morphogenesis:

  • Movement of cells changing shape and form of body parts
  • Pattern formation enables morphogenesis
  • Common sets of genes direct pattern formation
  • Example: Drosophila

  Apoptosis:

  • Programmed cell death needed for normal development of eye, nervous system, and phalanges