52
Xenopus
embryos was best studied using amphibian animals in the
frst half of the 20th century due to practical advantages.
In vitro growth of amphibian embryos in simple salt solutions not only allowed observation of embryogenesis at all
stages but more importantly made embryos accessible to
microsurgery. Once the composition of culture media was
rendered optimal for embryo growth and infection could
be minimized by careful operations, microsurgery became
the key tool in studies of developmental trajectories of tissues. Isolated pieces cultured in vitro (explants), as well
as embryonic parts transplanted into a host embryo, were
examined in detail to gain information about tissue determination and plasticity. Stage- and region-specif c microsurgeries were performed to examine progressive changes
with time and at different embryonic positions in terms of
both inducing and responsive capacity of the manipulated
tissues. Gross morphological and histological analyses
were employed to assess developmental outcomes of the
operations. These experimental embryological approaches
were used to explore tissue interactions involved in AP axis
development. The patterning of the nervous system was
especially investigated in detail due to the ease of identifying morphological and histological features of the brain
regions and the spinal cord, and a range of amphibian species, including salamanders, newts, and frogs (urodeles/
pleurodeles and anurans), were surveyed. These studies
cumulated in several models that aimed to elucidate general
principles underlying AP neural induction and patterning
in amphibians.
Early cell fate mapping studies showed that mesendodermal and ectodermal precursor cells in blastula and early
gastrula embryos had opposite AP polarity on the surface of
the embryos (Slack and Tannahill, 1992; Vogt, 1929). The
prospective anterior neural cells were localized toward the
animal pole whereas the anterior mesendodermal precursors were close to the vegetal side. Internationalization and
morphogenetic movements of mesendoderm during gastrulation led to reversal of mesendodermal AP polarity and
subsequent registration of the AP axis between the germ
layers (Figure 5.1A). The fate map studies raised questions about when the AP tissue identities were developed
and how mesendodermal and neural AP development was
coordinated.
In the neural tissue, morphological and histological
features in the amphibian tadpoles, such as the position of
sensory organs and characteristic patterns of ventricular
thickness and axonal tracks in the brain, made it easy to
distinguish between the forebrain, midbrain, hindbrain, and
spinal cord (Slack and Tannahill, 1992). However, there were
no defnitive features to subdivide the prechordal mesendoderm, and both the notochord and the somites looked morphologically similar along the trunk. The AP distinction of
the mesendoderm was hence limited only to the prechordal
and the trunk regions in induced tissues. Because of this,
the analysis of AP development was done mainly using the
neural tissues. Distinct research approaches, including culture of explants, conjugation of different embryonic parts,
and various types of tissue transplantations, were used by
many groups to address specifc issues concerning AP neural development.
To explore how AP neural features were acquired during embryogenesis, tissue conjugates were made with the
presumptive inducing materials taken from the dorsal lip
of gastrula embryos, tissues from different AP positions
along the archenteron roof, or pieces of the neural plate
at distinct AP levels. These isolates were sandwiched
between one or two pieces of early gastrula-stage animal caps, and the resulting structures were scored for the
appearance of brain or spinal cord traits (Doniach, 1993;
Slack and Tannahill, 1992). The studies showed that early
dorsal lips induced only anterior structures, whereas more
posterior structures also formed when late dorsal lips
were used (Okada and Takaya, 1942). In addition, anterior
pieces of archenteron roof or neural plate induced only
anterior structures in the animal caps, such as brain and
eyes, whereas posterior archenteron roof or neural plate
induced more posterior structures (Doniach, 1993; Slack
and Tannahill, 1992). Though some differences were seen
in inductive activities of archenteron roof and neural plate,
the results from the sandwich studies demonstrated that
AP neural characters could be induced in both temporaland spatial-dependent manners. Forebrain-like structures
were induced by early-stage organizer and anterior tissues,
whereas spinal cord was induced by late-stage organizer
and posterior tissues.
The idea that mesoderm at different AP positions
induced neural tissues of corresponding AP characters
was in fact suggested by an earlier experiment from Otto
Mangold (1933) using a different approach. He inserted
stripes of dorsal mesoderm taken from underneath the
neural plate at different AP positions of early neurula
embryos into the blastocoel cavity of early gastrula-stage
embryos (the Einsteck experiment). Gastrulation movements would push the implanted tissues against the ventral wall of the blastocoel and allow inductive interaction
between the implants and the host. Otto Mangold (1933)
showed that when the implants were taken from the anterior regions, only anterior structures were induced in
the ectopic position. However, when donor tissues were
taken from the posterior regions, ectopic trunk structures
appeared. He thus proposed that different mesodermal
regions contained different neural inducers that could
induce neural tissues with corresponding AP characteristics (Figure 5.1B).
A very different model of AP patterning was suggested
by Pieter Nieuwkoop based on his tissue transplant experiments (Nieuwkoop, 1952a, 1952b, 1952c). Instead of using in
vitro tissue explants or transplantation in gastrula embryos,
he devised a distinctive way of inserting folded f aps of
competent ectoderm into the neural plate of neurula stage
embryos at different axial levels. He then used histological
landmarks to score for formation of different structures in
the folds at later stages. He observed that neural tissues were
induced in the folds along the entire AP axis. Decreased
Xenopus
embryos was best studied using amphibian animals in the
frst half of the 20th century due to practical advantages.
In vitro growth of amphibian embryos in simple salt solutions not only allowed observation of embryogenesis at all
stages but more importantly made embryos accessible to
microsurgery. Once the composition of culture media was
rendered optimal for embryo growth and infection could
be minimized by careful operations, microsurgery became
the key tool in studies of developmental trajectories of tissues. Isolated pieces cultured in vitro (explants), as well
as embryonic parts transplanted into a host embryo, were
examined in detail to gain information about tissue determination and plasticity. Stage- and region-specif c microsurgeries were performed to examine progressive changes
with time and at different embryonic positions in terms of
both inducing and responsive capacity of the manipulated
tissues. Gross morphological and histological analyses
were employed to assess developmental outcomes of the
operations. These experimental embryological approaches
were used to explore tissue interactions involved in AP axis
development. The patterning of the nervous system was
especially investigated in detail due to the ease of identifying morphological and histological features of the brain
regions and the spinal cord, and a range of amphibian species, including salamanders, newts, and frogs (urodeles/
pleurodeles and anurans), were surveyed. These studies
cumulated in several models that aimed to elucidate general
principles underlying AP neural induction and patterning
in amphibians.
Early cell fate mapping studies showed that mesendodermal and ectodermal precursor cells in blastula and early
gastrula embryos had opposite AP polarity on the surface of
the embryos (Slack and Tannahill, 1992; Vogt, 1929). The
prospective anterior neural cells were localized toward the
animal pole whereas the anterior mesendodermal precursors were close to the vegetal side. Internationalization and
morphogenetic movements of mesendoderm during gastrulation led to reversal of mesendodermal AP polarity and
subsequent registration of the AP axis between the germ
layers (Figure 5.1A). The fate map studies raised questions about when the AP tissue identities were developed
and how mesendodermal and neural AP development was
coordinated.
In the neural tissue, morphological and histological
features in the amphibian tadpoles, such as the position of
sensory organs and characteristic patterns of ventricular
thickness and axonal tracks in the brain, made it easy to
distinguish between the forebrain, midbrain, hindbrain, and
spinal cord (Slack and Tannahill, 1992). However, there were
no defnitive features to subdivide the prechordal mesendoderm, and both the notochord and the somites looked morphologically similar along the trunk. The AP distinction of
the mesendoderm was hence limited only to the prechordal
and the trunk regions in induced tissues. Because of this,
the analysis of AP development was done mainly using the
neural tissues. Distinct research approaches, including culture of explants, conjugation of different embryonic parts,
and various types of tissue transplantations, were used by
many groups to address specifc issues concerning AP neural development.
To explore how AP neural features were acquired during embryogenesis, tissue conjugates were made with the
presumptive inducing materials taken from the dorsal lip
of gastrula embryos, tissues from different AP positions
along the archenteron roof, or pieces of the neural plate
at distinct AP levels. These isolates were sandwiched
between one or two pieces of early gastrula-stage animal caps, and the resulting structures were scored for the
appearance of brain or spinal cord traits (Doniach, 1993;
Slack and Tannahill, 1992). The studies showed that early
dorsal lips induced only anterior structures, whereas more
posterior structures also formed when late dorsal lips
were used (Okada and Takaya, 1942). In addition, anterior
pieces of archenteron roof or neural plate induced only
anterior structures in the animal caps, such as brain and
eyes, whereas posterior archenteron roof or neural plate
induced more posterior structures (Doniach, 1993; Slack
and Tannahill, 1992). Though some differences were seen
in inductive activities of archenteron roof and neural plate,
the results from the sandwich studies demonstrated that
AP neural characters could be induced in both temporaland spatial-dependent manners. Forebrain-like structures
were induced by early-stage organizer and anterior tissues,
whereas spinal cord was induced by late-stage organizer
and posterior tissues.
The idea that mesoderm at different AP positions
induced neural tissues of corresponding AP characters
was in fact suggested by an earlier experiment from Otto
Mangold (1933) using a different approach. He inserted
stripes of dorsal mesoderm taken from underneath the
neural plate at different AP positions of early neurula
embryos into the blastocoel cavity of early gastrula-stage
embryos (the Einsteck experiment). Gastrulation movements would push the implanted tissues against the ventral wall of the blastocoel and allow inductive interaction
between the implants and the host. Otto Mangold (1933)
showed that when the implants were taken from the anterior regions, only anterior structures were induced in
the ectopic position. However, when donor tissues were
taken from the posterior regions, ectopic trunk structures
appeared. He thus proposed that different mesodermal
regions contained different neural inducers that could
induce neural tissues with corresponding AP characteristics (Figure 5.1B).
A very different model of AP patterning was suggested
by Pieter Nieuwkoop based on his tissue transplant experiments (Nieuwkoop, 1952a, 1952b, 1952c). Instead of using in
vitro tissue explants or transplantation in gastrula embryos,
he devised a distinctive way of inserting folded f aps of
competent ectoderm into the neural plate of neurula stage
embryos at different axial levels. He then used histological
landmarks to score for formation of different structures in
the folds at later stages. He observed that neural tissues were
induced in the folds along the entire AP axis. Decreased