Blastulation

• Blastulation

  • During cleavage, the embryo divides repeatedly, converting a single large cell into many smaller cellscalled blastomeres• During this period: no growth, only subdivision of mass until normal somatic cell size is attained• At the end of cleavage, the zygote has been divided into hundreds-thousands of cells [blastula stage]
• • Cleavage subdivides the mass of the zygote until a cluster of cells called a blastula is formed
  • In most mammals, cluster of cells is called blastocyst
  • In most animals, layer of cells formed around the blastocoel ~ central fluid-filled cavity
  • • In the blastula stage, the embryo consists of a few hundred to several thousand cells poised for further development
  • Development continues beyond the blastula to form one or two more germ layers in a gastrula stage
• Gastrulation

  • Inward folding of outer blastomere
  • Creation of germ layers
  • Invagination - forms a second germ layer by bending one side of the blastula
• Archenteron/gastrocoel

  • Internal pouch/gut cavity

  Blastopore

  • Opening of the gastrocoel

  Ectoderm

  • outer layer

  Endoderm

  • inner layer
• Germ Layers

  • Groups of cells that behave as a unit during the early embryogenesisDiploblastic
  • Mesoderm – germ layer lying between the outer ectoderm and inner endoderm
  • Ectoderm – germ layer surrounding the blastocoel
  • Endoderm – germ layer lining the gut
• Dipoblastic

  • 2 germ layers: ectoderm and endoderm
  • 2 cavities: gut and bastocoel
  • Sea anemones and jellyfishes
• Tripoblastic

  • 3 germ layers
  • Bilateria
• Mesoderm formation

  Acoelomate plan

  • Mesoderm completely fills the blastocoel
  • 1 body cavity: gut

  Pseudocoelomate plan

  • Mesoderm lines the outer edge of blastocoel
  • 2 body cavities: gut and blastocoel -> pseuocolem

  Coelomate plan

  • True coelom develops via schizocoelous and enterocoelous
• Schizocoelous

  • Mesoderm forms a band of tissue lining the gut
  • Coelem is a result of apoptosis
• Enterocoelus

  • Cells from central portion of the gut begin to grow outward as pouches
  • Pouches expand to blastocoel
  • Forms the mesodermal rings that becomes the coelom
• Body Cavity/Coelom

  • Body cavity - internal space
  • Digestive tract/gut cavity
  • Space between the gut and the outer body wall