TOPIC: EMBRYONIC INDUCTION AND ORGANIZER
LECTURE NO:05
BSC(HONS.) PART 1-PAPER II-GROUP B
DATE: 26TH MARCH 2020
AUTHOR:NIRMAL KUMARI
EMBRYONIC INDUCTION
Embryonic induction defines as the process of
communication between cells required for their
differentiation, morphogenesis and maintenance.
In amphibian embryos, the dorsal ectodermal cells in a
mid-longitudinal region differentiate to form a neural
plate, only when the chorda-mesoderm is below it.
Chorda-mesoderm is the layer formed by cells
invaginated from the region of the dorsal blastoporal lip,
which forms the roof of archenteron.
Mangold (1927) selected a small part of dorsal
blastoporal lip from an early gastrula of Triturus cristatus
and grafted it at a place near the lateral lip of the
blastopore of the host gastrula of T. taeniatus. The graft
cells grew in number and spread inside the host gastrula
to form an additional chorda-mesoderm at this place. This
chorda-mesoderm subsequently induced the ectoderm of
the host gastrula to form an additional neural tube.
The graft cells themselves formed an additional
notochord. As the host gastrula developed further, it grew
into a double embryo joined together. One of the embryos
was the regular one, while the second was the induced
one. The latter did not develop a complete head. This
experiment clearly showed that the dorsal blastoporal lip
of the blastula had the ability to induce the formation of
the neural plate in the ectoderm of the host. This
phenomenon is called neural induction. Other parts of an
embryo can similarly induce the formation of other
structures. This influence of one structure in the
formation of another structure is called embryonic
induction.
In fact, the entire development of an organism is due to a
series of inductions. The structure, which induces the
formation of another structure, is called the inductor or
organizer. The chemical substance that is emitted by an
inductor is called an evacuator. The tissue on which an
evacuator or inductor acts is called the responsive tissue.
(a) Historical Background of Embryonic Induction:
For the discovery of neural induction, the German
embryologist, Hans Spemann and his student, Hilde
Mangold (1924) worked a lot and for his work Spemann
received Nobel Prize in 1935.
These two scientists performed certain heteroblastic
transplantations between two species of newt, i.e.,
Triturus cristatus and Triturus taeniatus and reported
that the dorsal lip of their early gastrula has the capacity
of induction and organization of presumptive neural
ectoderm to form a neural tube and also the capacity of
evocation and organization of ectoderm, mesoderm and
endoderm to form a complete secondary embryo.
They called the dorsal lip of the blastopore the primary
organizer since it was first in the sequence of inductions
and as it had the capacity to organize the development of
a second embryo. Later on, the primary organizer was
reported to exist in many animals, e.g. in frogs (Daloq and
Pasteels, 1937); in cyclostomes (Yamada, 1938); in bony
fishes (Oppenheimer, 1936); in birds (Waddington, 1933)
and in rabbit (Waddington, 1934).
Primary organizer and neural induction have been
reported in certain pre-vertebrate chordates, such as
ascidians and Amphioxus (Tung, Wu and Tung, 1932). In
1960 and 1963 Curtis investigated and reported that the
organizer of gastrula of Xenopus laevis can be
distinguished in the cortex of gray crescent of a fertilized
egg.
Holtfreter (1945) gave an account of how an enormous
variety of entirely unspecific substances-organic acids,
steroids, kaolin, methylene blue, sulphhydryl compounds,
which had nothing in common except the property of
being toxic to sub-ectodermal cells-produced neurulation
in explants. Barth and Barth (1968, 69) provided further
information about the chemical nature of embryonic
induction.
Types of embryonic induction:
Lovtrup (1974) classified different types of embryonic
induction into two basic categories-endogenous and
exogenous inductions.
Endogenous induction:
Certain embryonic cells gradually assume new
diversification pattern through the inductors that are
produced by them endogenously. Due to these inductors,
these cells undergo either self-transformation or self-
differentiation. Examples of such induction were reported
in Mesenchymal cells of ventral pole of Echinoid and in
small sized, yolk-laden cells of dorsal lip of amphibian
blastopore.
Exogenous induction:
When some external agent or a cell or a tissue is
introduced into an embryo, they exert their influence by a
process of diversification pattern upon neighbouring cells
through contact induction. This phenomenon is called
exogenous induction. It may be homotypic or heterotypic
depending on the fact that whether the inductor provokes
the formation of same or different kind of tissues
respectively (Grobstein, 1964). In homotypic induction, a
differentiated cell produces an inductor. The inductor not
only serves to maintain the state of the cell proper, but
also induces adjacent cells to differentiate according to it,
after crossing the cell boundaries. Best example of the
heterotypic exogenous induction is the formation of a
secondary embryonic axis by an implanted presumptive
notochord in amphibians.
Experimental evidences to induction:
Spemann and Mangold (1924) transplanted
heteroplastically a piece of the dorsal lip of the blastopore
of an early gastrula of pigmented newt, Triturus cristatus
and grafted it near the ventral or lateral lip of the
blastopore of the early gastrula of pigmented newt T.
taeniatus. Most of the graft invaginated into the interior
and developed into notochord and somite’s and induced
the host ectoderm to form a neural tube, leaving a narrow
strip of tissue on the surface.
With the development of host embryo, an additional
whole system of organs was induced at the graft –
placement area. Except for the anterior part of the head,
almost a complete secondary embryo comprising of the
additional organs was formed. Posterior part of the head
was present as indicated by a pair of ear rudiments. Since
in this experiment the type of transplantation involved
was heteroplastic, it was found that notochord of
secondary embryo consisted exclusively of graft cells; the
somites consisted partly of graft and partly of host cells
(Fig. 3.5). Few cells, which did not invaginate during
gastrulation, were left in the neural tube. The bulk of the
neural tube, part of the somites, kidney tubules and the
ear rudiments of the secondary embryo consisted of host
cells. The graft becomes self-differentiated and at the
same time induces the adjoining host tissue to form spinal
cord and other structures including somites and kidney
tubules. Spemann (1938) described dorsal lip of the early
gastrula as a “primary organizer” of the gastrulative
process.However, organization of the secondary embryo
results from a series of both inductive interactions and
self-differentiate changes in the host and donor tissues.
Hence, now a days the term “embryonic induction” or
“inductive interactions” is preferred. The part, which is
the source of induction, is called “inductor”.
Fig. Induction of secondry embryo in triturus by
transplanting a piece of dorsal lip to the future belly
region of another gastrula(A-B) and C-E are the stage of
resulting primary embryo with a secondry embryo
attached to it,where F is the T.S of same embryo.
Characteristics of the organizer:
Organizer has the ability for self-differentiation and
organization. It also has the power to induce changes
within the cell and to organize surrounding cells,
including the induction and early organization of neural
tube. Primary organizer determines the main features of
axiation and organization of the vertebrate embryo.
Induction is a tool-like process, utilized by this
center of activity through which it affects changes in
surrounding cells and as such influences organization and
differentiation. These surrounding cells, changed by the
process of induction, may in turn act as secondary
inductor centers with abilities to organize specific sub-
areas.
Thus, the transformation of the late blastula into an
organized condition of the late gastrula appears to be
dependent upon a number of separate inductions, all
integrated into one coordinated whole by the “formative
stimulus” of the primary organizer located in the pre-
chordal plate area of the endodermal -mesodermal cells
and adjacent chorda-mesodermal material of the early
gastrula.
Regional specificity of the organizer:
Vital-staining experiments of Vogt with newt eggs have
shown that the material successively forming the dorsal
blastoporal lip moves forward as the archenteron roof.
Transplants taken from this region are also able to induce
a secondary embryo or the belly of a new host i.e. the
archenteron roof acts as a primary inductor in essentially
the same way as does the dorsal lip tissue proper. The
inductions of neural inductor are found to be regionally
specific and the regional specificity is imposed on the
induced organ by the inductor.
Therefore, the inductive capacity of the blastoporal lip
varies both regionally and temporally. Most of the dorsal
and dorso-lateral blastoporal material is necessary for a
graft to induce a more or less complete secondary
embryo. Spemann (1931) demonstrated that during
gastrulation anterior part of the archenteric roof
invaginates over the dorsal lip of the blastopore earlier.
Dorsal blastopore lip of the early gastrula contains the
archenteric and deuterocephalic organizer and the dorsal
blastopore lip of the late gastrula contains the
spinocaudal organizer. Inductions produced by the dorsal
lip of the
blastopore taken from the early and the late gastrula
differ in accordance with exception; the first tends to
produce head organs and the second tends to produce
trunk and tail organs (Fig.).
Fig: The separation of neural inductor into head & trunk
organizer
As invagination continues and the dorsal lip no longer
consists of prospective head endo-mesoderm but
progressively becomes prospective trunk mesoderm; it
acts as a trunk-tail inductor. The most caudal region of
the archenteron roof, in fact, specifically induces tail
somites and probably other mesodermal tissues. The
archenteron roof induces entirely different class of
tissues; various neural and meso-ectodermal tissues by
its anterior region and various mesodermal tissues by its
most posterior region.
Therefore, differences in specific induction capacities
exist between head and trunk level of archenteron roof
and are related to the regional differentiation of the
neural tissue into archencephalic (including fore-brain,
eye, nasal pit), deuterencephalic (including hind-brain,
ear vesicle) and spinocaudal components. Thus,
archenteron roof consists of an anterior head inductor
including an archencephalic inductor and a
deuterencephalic inductor and a trunk or spinocaudal
inductor.
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