Embryonic Induction

1
Embryonic Induction

• Induction is the process by which one group of
  cells produces a signal that determines the fate
  of a second group of cells. This implies both the
  capacity to produce a signal by the inducing
  cells and the competence of the responding cells
  to receive and interpret the signal via a signal
  transduction pathway. Amphibians are the most
  extensively studied vertebrates for
  investigations into embryonic induction. There
  are two major inductive events during early toad
  development
• mesoderm induction.
• neural induction.

2

• Mesoderm Induction
• Mesoderm induction occurs over an extended period
  of time in the equatorial region of the embryo
  from about the 32-cell stage to the beginning of
  gastrulation. The requirement of induction for
  production of mesoderm is evident by comparing
  the embryonic fate map (which shows the fates of
  regions of the embryo during normal development)
  to the specification map (which shows what tissue
  explants can do in isolation) during cleavage
  stages.
• The fate map shows that about half of the
  mesoderm arises from cells above the equatorial
  pigment boundary, whereas no cells from above the
  pigment boundary will form muscle or notochord in
  isolation. The vegetal cells are not equal in
  their inductive capacities. The dorsal vegetal
  cells induce axial mesoderm of the dorsal
  midline (notochord and segmented muscle), whereas
  the remaining vegetal cells induce ventrolateral
  mesoderm (mesothelium, mesenchyme, blood cells).
• These observations have led to the proposal that
  there are two mesoderm-inducing signals
• A general vegetal signal that operates around the
  circumference and induces ventral mesoderm.
• A dorsal vegetal signal that induces the axial
  mesoderm (i.e., Spemann's organizer).
• The general vegetal signal
  apparently remains operational in ventralized
  embryos produced by UV irradiation of fertilized
  eggs these radially symmetrical embryos have the
  normal amount of mesoderm, but all of it is
  ventral in character. The dorsal vegetal signal
  is dependent upon dorsalization factors.

3

• Neural Induction
• Spemann's Organizer substance
• The developmental significance of the
  dorsal lip of the blastopore, which is derived
  from the gray crescent, was initially
  demonstrated by Spemann and Mangold. The dorsal
  lip material transplanted to the ventral side of
  a host gastrula would induce a secondary embryo.
  The transplanted dorsal lips induced the
  overlying host ectoderm to develop as neural
  tissue through vertical signaling.
• Spemann-Mangold experiment
• Regional determination by the chordamesoderm
• Planar (lateral) vs. vertical signaling
• The involuting dorso-anterior mesoderm
  induces the adjacent mesoderm to form anterior
  neural tissue. As the mesoderm migrates toward
  the former animal pole, it induce to form
  anterior neural tissue. With the advancement of
  involution , ectoderm is induces the formation of
  posterior neural elements. According to this
  model, the anteroposterior character of the
  neural ectoderm is dependent. Interestingly, when
  BMP-4 signaling is blocked in animal caps, neural
  tissue is formed in the absence of neural
  inducers.

4

• BMP-4 is expressed throughout the gastrula of
  Xenopus, except for the dorsal lip and animal cap
  regions. Both the mesoderm and ectoderm are
  patterned by antagonizing signals, with BMP-4
  acting to ventralize them. If the effects of
  BMP-4 are counteracted, the mesoderm develops
  dorsal properties, and the ectoderm becomes
  neural tissue.
• At this point, three secreted factors have
  been identified as likely neural inducers
• Noggin Secreted factors that can dorsalize the
  mesoderm.
• Chordin Secreted factors that can dorsalize the
  mesoderm.
• Follistatin Is an activin antogonist that can
  also dorsalize mesoderm when injected as mRNA.

5
(No Transcript)
6
(No Transcript)
7
(No Transcript)
8
(No Transcript)
9
(No Transcript)
10
(No Transcript)
11
(No Transcript)
12
(No Transcript)
13
(No Transcript)