Bio Review Notes #35
EMBRYOLOGY: CONTROL OF DEVELOPMENT
Performance Objectives:
Many developmental changes are controlled by "organizers," which induce changes in cell growth. Experiments on amphibians show that the dorsal lip is an organizer for gastrulation movements and that cells on the roof of the archenteron are an organizer for the formation of the nervous system. Many organizers contain RNA. Hormones may control developmental processes like metamorphosis.

 Organizer theory:   organizers are substances secreted by cells that induce changes in the growth of other cells. They were first studied by transplant experiments on amphibians.
  • Many organizers contain RNA. Chemical extracts of organizers are effective inducers, but ribonuclease (RNAse) abolishes this ability.
  • Various mutational agents (mutagens) and cancer-causing agents (carcinogens) are also effective inducers.

Dorsal lip is an organizer for gastrulation. Dorsal lip transplanted to another embryo induces gastrulation in a second place and results in a "double gastrula."

Notochord (roof of the archenteron) is an organizer for the formation of the nervous system.
  • If normal gastrulation is prevented, no nervous system forms. (Stripping the jelly-like capsule from amphibian eggs prevents the dorsal lip from tucking in; neural tube never forms.)
  • "Double gastrula" (see above) develops two neural tubes.
  • Notochord transplanted to another embryo induces a second nervous system to form; transplants between species show that induced nervous systems contain no cells from the transplant.
  • Cell-free extract from notochord can induce a second neural tube.

Role of hormones:   The insect hormone ecdysone causes chromosome puffing, which exposes DNA sequences needed for metamorphosis and wing development. In larval amphibians (tadpoles), the hormone thyroxin causes metamorphosis into an adult. Destroying thyroxin prevents metamorphosis; giving thyroxin early induces premature metamorphosis.

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