L5 Mol & Morphlgcl Mechsms of Gastrulation

• Gastrulation

  • rearrangement of cells to establish three germ cell layers
  • first step in morphogenesis
• morphogenesis

  • process whereby individual cells undergo complex movement that generate the organ rudiments
• Three Germ Layers

  1. Ectoderm - outer layer
  2. Endoderm - lines digestive tract
  3. Mesoderm - partly fills the space between the endoderm and the ectoderm
• Blastopore

  • indentation or groove through which cells move inside to form the endoderm and mesoderm
  • fate of blastopore used as basis for classifying animals into:
  • Protostome - develop into mouth (most animals)
  • Deuterostome - develop to anus, secondary mouth formation (vertebrates & echinoderms)
• Animal Kingdom

  1. porifera
  2. cnidaria
  3. nematodes
  4. platyhelminthes
  5. annelida
  6. mollusca
  7. arthropoda
  8. echinodermata
  9. chordata
• Morphogenetic Movements

  1. invagination
  2. involution
  3. epiboly
  4. delamination
  5. ingression
• Invagination is the infolding of a cell sheet of an embryo (sea urchin mesoderm)

  
• Involution - inturning or inward movement of a cell sheet over the basal surface of an outer later (amphibian mesoderm)

  
• Epiboly - spreading movement of an epithelium to envelope a yolk mass of cells (ectoderm formation of amphibian)

  
• Delamination - splitting of one layer of cells into two parallel layers (mammalian and bird hypoblast formation)

  
• Ingression - movement of individual cells or small group from an epithelium to cavity (sea urchin mesoderm)

  
• Derivatives of Ectoderm

  1. epidermis of skin and its derivatives
  2. epithelial lining of mouth and rectum
  3. sense receptors in epidermis
  4. cornea and lens of eye
  5. nervous system
  6. adrenal medulla
  7. tooth enamel
• Derivatives of Endoderm

  1. epithelial lining of digestive tract except mouth and rectum
  2. epithelial lining of respiratory system
  3. liver
  4. pancreas
  5. thyroid
  6. parathyroids
  7. thymus
  8. lining of urethra
  9. urinary bladder
  10. reproductive system
• Derivatives of Mesoderm

  1. notochord (in animals retaining it as adults)
  2. skeletal system
  3. muscular system
  4. circulatory system
  5. excretory system
  6. reproductive system (except germ cells which differentiate during cleavage)
  7. dermis of skin
  8. lining of body cavity
  9. adrenal cortex
• Gastrulation in Sea Urchins

  • begins at vegetal pole of the blastula
  • mesenchyme cells migrate into the blastocoel
  • vegetal pole forms from the remaining cells of the vegetal pole and buckles inward through invagination
  • newly formed cavity - archenteron or gastrocoel
  • open through blastopore which will become the anus
• Gastrulation in Frogs (mesolecithal)

  1. group of cells on the dorsal side of the blastula begins to invaginate (animal pole). This forms a crease along the region where gray crescent formed
  2. cells continue move from the embryo surface into the embryo by involution
  3. these cells become the endoderm and mesoderm. cells on the embryo surface will form ectoderm.
• Before gastrulation in chicks, the embryo consists of an upper layer (epiblast) and a lower layer (hypoblast)
• The embryo in chicks forms from a blastoderm and sits on top of a large yolk mass
• During gastrulation in chicks, epiblast cells move towards the midline of the blastoderm and then into the embryo towards the yolk
• The midline thickens during gastrulation, forming the primitive streak
• Hypoblast cells contribute to the sac that surrounds the yolk and establish a connection between the yolk and the embryo
• Movement of different epiblast cells during gastrulation forms the ectoderm, endoderm, and mesoderm
• Gastrulation (amphioxus or sea urchin)

  A) animal

  B) vegetal

  C) blastocoel

  D) blastopore

  E) archenteron

  F) mesenchyme

  G) ectoderm

  H) endoderm

  I) archenteron

  J) mesenchyme

  
• Gastrulation (frog embryo)

  A) animal

  B) vegetal

  C) dorsal lip

  D) blastocoel

  E) archenteron

  F) mesoderm

  G) archenteron

  H) endoderm

  I) ectoderm

  J) notochord

  K) blastopore

  L) yolk plug

  M) ventral lip

  
• Early development in a chick, whose embryo is spread out on the surface of a massive yolk

  A) blastodisc

  B) yolk

  C) embryo

  D) yolk

  E) primitive streak

  F) hensen's node

  G) hensen's node

  H) primitive groove

  I) primitive groove

  J) hensen's node

  K) epiblast

  L) blastocoel

  M) yolk

  N) hypoblast

  O) endoderm