Dev. Bio

• Gastrulation


  The process where the blastula is rearranged into a three-layered embryo called a gastrula
• Future germ layers

  • Future ectoderm
  • Future mesoderm
  • Future endoderm
• SURFACE VIEW

  • Blastocoel
  • Shrinking
• CROSS SECTION


  • Archenteron
  • Gastrulation and the three-layer formation
• The terms "gastrula" and "gastrulation" were coined by Ernst Haeckel, in his 1872 work "Biology of Calcareous Sponges"
• Characters of the gastrulation stage

  • It is the first stages of cellular differentiation
  • Embryonic cells begin to follow in specific layers through morphogenetic movements
  • The three embryonic layers (external Ectoderm, medium Mesoderm and Endoderm internal layer)
  • Cellular divisions lose synchronization between them
  • The metabolism changes within the cells and the oxidation process becomes the dominant
  • Nucleus at this stage has more impact than before in the process of control of development and shows chromosomes or paternal genes and its effect
• Gastrulation


  The rearrangement of the cells of a blastula into a three-layered embryo, called a gastrula, which has a primitive gut or called archenteron
• The three embryonic germ layers produced by gastrulation

  • Ectoderm - forms the outer layer
  • Endoderm - lines the digestive tract and lungs
  • Mesoderm - partly fills the space between the endoderm and ectoderm
• The blastula consists of numerous cells, the positions of which were established during cleavage
• During gastrulation, these cells are given new positions and new neighbors, and the multilayered body plan of the organism is established
• The cells that will form the endodermal and mesodermal organs are brought to the inside of the embryo, while the cells that will form the skin and nervous system are spread over its outside surface
• The three germ layers-outer ectoderm, inner endoderm, and interstitial mesoderm-are first produced during gastrulation
• In addition, the stage is set for the interactions of these newly positioned tissues
• This fate map diagram of a Xenopus blastula shows: cells whose fate is to become ectoderm in blue and green, cells whose fate is to become mesoderm in red, cells whose fate is to become endoderm in yellow
• Notice that the cells that will become endoderm are NOT internal, yet. It will become internal during gastrulation stage
• Derivatives of germ layers

  • Ectoderm - skin and nervous system
  • Mesoderm - muscles, blood, organs
  • Endoderm - gut lining and derivatives
• Types of cell movement during gastrulation

  • Invagination - Infolding of cell sheet into embryo
  • Involution - Inturning of cell sheet over the basal surface of an outer layer
  • Ingression - Migration of individual cells into the embryo
  • Delamination - Splitting or migration of one sheet into two sheets
  • Epiboly - The expansion of one cell sheet over other cells
• Examples of cell movement types

  • Invagination - Sea urchin endoderm
  • Involution - Amphibian mesoderm
  • Ingression - Sea urchin mesoderm, Drosophila neuroblasts
  • Delamination - Mammalian and bird hypoblast formation
  • Epiboly - Ectoderm formation in amphibians, sea urchins, and tunicates
• Sea urchin gastrulation

  1. The vegetal pole of the blastula flattens

  2. Some cells change shape and move inward to form the archenteron

  3. Other cells break free, becoming mesenchyme

  4. Thin extensions of mesenchyme cells attach to the overlying ectoderm

  5. The archenteron elongates, assisted by the contraction of mesenchyme cells

  6. The mouth will form where the archenteron meets ectoderm
• Gastrulation in the frog

  1. The frog blastula is many cell layers with blastocoel at the end of cleavage stage

  2. The blastula start to form the dorsal lip were the cell in the animal pole start to move from the outside to the inside via the dorsal lip of Blastopore

  3. Cells of the dorsal lip originate in the gray crescent area and invaginate to the inside of the embryo

  4. Cells continue to move from the embryo surface to the inside of the embryo by involution

  5. These invagenated cells become the endoderm and mesoderm

  6. The blastocoel is replaced by a new cavity inside the gastrula stage called archenteron

  7. The surface of the embryo is now ectoderm, the innermost layer is endoderm, and the middle layer is mesoderm

  8. The blastopore encircles a yolk plug when gastrulation is completed
• Cell Movements in Amphibian Gastrulation

  • Gastrulation begins on dorsal side Below the equator, in region of grey crescent
  • Cells invaginate to form a slender blastopore
  • Dorsal lip of blastopore will become important organizing region of embryo (Spemann organizer)
  • Cells become elongated as they contact the inner surface (Bottle cells)
• How the blastopore lip forms

  1. A small group of cells change shape, narrowing at the exterior edge of the blastula

  2. This change in cell shape, called apical constriction, creates a local invagination, which pushes more interior cells upwards and begins to roll a sheet of cells towards the interior

  3. The constricted cells are called bottle cells, due to their shape
• Bottle Cell Function in Xenopus


  • Bottle cells form the leading edge of the archenteron (primitive gut)
• Next steps in amphibian gastrulation

  1. Involution of the cells at the marginal zone (outer sheet spreads over inner sheet)

  2. Cells from Animal pole undergo epiboly

  3. Converge at the blastopore, When reach blastopore, travel inward

  4. Bottle cells continue to migrate, form leading edge of archenteron (primitive gut)
• Gastrulation in Amphibian


  1. Mass of yolk left by surrounding blastopore = yolk plug

  2. Cells from the dorsal lip (the first cells that migrated inward) form the leading edge of the archenteron
• After the process of lining, the fetus transforms from a hollow ball with a single layer, into a three-layer outer body known as ectoderm, internal known as endoderm, and cells that migrate from the outside to the inside are the middle class, which is known as the Mesoderm
• From these three embryonic layers, all organs of the fetus arise later
• Surface view


  View of the top of the organism
• Cross section

  View of the organism cut through the middle
• Blastocoel


  Cavity inside the blastula
• Archenteron


  Primitive gut that forms during gastrulation
• Gastrulation in Amphibian


  1. Blastopore forms

  2. Pericardial plug decreases

  3. Primitive line forms

  4. Ectoderm, endoderm and mesoderm form
• Gastrulation in Amphibian


  • Mass of yolk left by surrounding blastopore = yolk plug
  • Cells from dorsal lip become prechordal plate
  • Cells that involute form chordamesoderm
• Ectoderm


  Outer embryonic layer
• Mesoderm


  Middle embryonic layer
• Endoderm


  Inner embryonic layer
• Gastrulation is caused by at least two hypotheses: 1) Mesoderm induction in the blastula stages, 2) Mesodermal patterning in the gastrula stages
• Mesoderm induction hypothesis


  • POF and BMP signals induce mesoderm
  • Nieuwkoop center and Wnt/beta-catenin signals dorsalize mesoderm
• Mesodermal patterning hypothesis


  • Local accumulations of gene products determine cell behaviors
  • Spemann organizer regulates fate of other areas
• Epiblast


  Upper layer of cells in the blastoderm