Animal Development II Notes

Animal Development II

Objectives

• Describe the significance of endoderm, mesoderm, and ectoderm in triploblastic animals.
• Compare gastrulation between urchins, frogs, and birds.
• List tissues that each germ layer forms.
• Explain how a coelom develops.
• Explain the basics of embryonic induction.

Urchin Gastrulation

1. Vegetal pole invaginates
   • Becomes archenteron – endoderm
   • Outer layer of cells – ectoderm
2. Archenteron elongates
   • Contraction of mesenchyme cells
3. Mouth forms where archenteron meets ectoderm

• The vegetal pole of the blastula flattens.
• Some cells change shape and move inward to form the archenteron.
• Other cells break free, becoming mesenchyme.
• Thin extensions of mesenchyme cells attach to the overlying ectoderm.
• The archenteron elongates, assisted by the contraction of mesenchyme cells.
• The mouth will form where the archenteron meets ectoderm.
• The blastopore will form the anus of the mature animal.

Gastrulation in Frogs

• Moderate Amount of Yolk
  • Smaller and smaller cells in animal hemisphere
  • Larger cells in vegetal hemisphere
  • Invagination of cells within grey crescent
  • Blastopore
  • Animal pole blastomere cells move over dorsal lip
  • Push into blastocoel
  • Animal hemisphere cells divide faster
  • Move around vegetal hemisphere cells
  • Cells reaching blastopore move inward
  • Develop into endoderm and mesoderm
• Gastrulation begins when cells in the region of the gray crescent move inward, forming the dorsal lip of the future blastopore.
• Cells of the animal pole spread out, pushing surface cells below them toward and across the dorsal lip. These cells move into the interior of the embryo, where they form the endoderm and mesoderm.
• The archenteron expands, eliminating the blastocoel. The blastopore lip forms a circle, with cells moving to the interior all around the blastopore; the yolk plug is visible through the blastopore.

Gastrulation in Birds

• Very Yolky Eggs
  • Blastodisc develops on top of yolk
  • Blastopore forms as primitive streak
    • Groove, with Hensen’s node at anterior end
  • Ectoderm cells migrate inwards through groove & node
    • Become endoderm & mesoderm
    • Displaces hypoblast
  • Similar in placental mammals
    • Hypoblast becomes placenta
• Posterior epiblast cells change shape and thicken, forming the primitive streak.
• Cells migrate, converging at the primitive streak and causing it to elongate.
• The primitive streak narrows and lengthens, forming the primitive groove-the chick blastopore. Cells migrate inward through the primitive groove and Hensen's node.
• Cells generated in Hensen's node and passing into the gastrula migrate anteriorly and form head structures and notochord.
• Surface cells move toward the groove and into the gastrula. The hypoblast is displaced by spreading endoderm. Cells moving over the sides of the primitive groove form mesoderm and endoderm.

Coelom Formation

• Deuterostomes
  • Mesoderm forms – out-pouching of archenteron
• Protostomes
  • Specific blastomeres give rise to each tissue
  • Mesoderm forms from many cell divisions
• Germ layer interactions influence morphogenesis
  • Animal achieves final form/shape

Germ Layer Fates

• Ectoderm
  • Epidermis, hair, claws, sweat glands
  • Brain & Nervous System
  • Pigment cells
• Endoderm
  • Lining of digestive tract & associated organs
  • Lungs, liver, gall bladder, pancreas
• Mesoderm
  • Notochord, heart, blood, blood vessels
  • Urogenital system, muscles, bones, dermis

Notochord Induces Formation of Neural Tube

• Cells moving through blastopore
  • Separate from roof of archenteron
  • Develop into notochord
• Notochord induces overlying ectoderm
  • Form dorsal nerve cord (Neurulation)
  • Signals pass from notochord into ectoderm
    • Neural plate formed
    • Plate edges roll up à groove
    • Walls of groove expand and fuse to form neural tube
      • Vertebrate tube
      • Anterior end develops three swellings
        • Forebrain, midbrain, hindbrain

Neural Crest Cells

• Cells dissociate from closing neural tube
  • Between neural tube and overlying ectoderm
  • “Wandering” multipotent cells
    • Migrate and form various structures
      • Sensory neurons and part of ANS
      • Many skull bones
      • Pigment cells

Mesoderm Forms Tissues of Middle Layer

• On either side of notochord…
  • Somites
    • Repeating segments
    • Modified during development
      • Vertebrae, ribs, muscles, cartilage, dermis
  • Intermediate mesoderm
    • Urinary & Reproductive systems
  • Lateral plate mesoderm
    • Lines the body cavity
    • Muscles of digestive tract
    • Circulatory system – including heart

Positional Information Guides Vertebrate Limb Formation

• What a cell will be depends on where is it located.
• Morphogens
  • Inducers that diffuses to surrounding cells
    • Different concentrations have diff. effects
      • E.g. vertebrate limbs
        • Paddle-shaped limb bud
        • Group of cells at limbs base (Zone of Polarizing Activity - ZPA)
          • Secretes Sonic Hedgehog (Shh) à forms gradient
            • Cells w/high dose à little finger
            • Cells w/low dose à thumb

Extraembryonic Membranes – birds, reptiles, egg-laying mammals

• Protect & nourish embryo
• Amniotic egg (reptiles, birds, mammals)
  • Contained aqueous environment
  • Germ layers help create the membranes that allow this

1. Yolk sac

   • Extension of endoderm & inner lateral plate mesoderm
   • Grow around yolk – enclose it
   • Blood vessels and cells

2. Allantois

   • Extension of endoderm & inner lateral plate mesoderm
   • Storage of metabolic wastes

3. Amnion

   • Ectoderm & outer lateral plate mesoderm
   • Surrounds embryo, fluid filled sac
   • Protection – shock absorbing

4. Chorion

   • Beneath egg shell
   • Limits water loss
   • Creates membrane to facilitate gas exchange with outside environment