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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.
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:
1. A general vegetal signal that operates around the circumference and
induces ventral mesoderm.
2. 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.
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
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.