3297.5.1 Amphibians
7.5.1.1 The Organizer
As a part of her doctoral thesis under Hans Spemann (1869–1941), Hilde Proescholdt
(later Mangold) (1898–1924) worked with embryos from two differently pigmented
species of salamander. She transplanted the dorsal lip of the blastopore from one
embryo onto the ventral side of a host embryo. The dorsal lip is the site where gas-
trulation initiates in amphibians, and is visible as a small region of darkly pigmented
cells at the margin. These experiments were particularly challenging because she
could only obtain embryos from natural mating of a species that had only one mat-
ing season a year, and in the pre-antibiotic era, the operated embryos did not survive
very well in the pond water she used to incubate her embryos (Hamburger 1984 ).
She repeated the experiment several hundred times over the course of 2 years in
order to obtain the six survivors that formed the basis of her Nobel prize winning
paper (Spemann and Mangold 1924 ). The grafted tissue induced an entire second-
ary body axis. The transplant differentiated into notochord, muscle and some gut,
and the overlying host cells adopted neural fates (Spemann and Mangold 1924 ). The
mesoderm of the dorsal lip is sufficient to induce the overlying ectoderm to form
neural tissue, and to organize an entire secondary body axis. Spemann called this
tissue the Organizer, and laid out three characteristics that functionally define orga-
nizer tissue. The Organizer tissue should induce neural tissue from naïve ectoderm,
self-differentiate into notochord and prechordal plate, and reprogram surrounding
tissue within the mesoderm.
7.5.1.2 Mesoderm Induction
Johannes Holtfreter (1901–1992) used a different approach to determine how the
germ layers interact to produce the embryo. He developed culturing conditions that
decreased the incidence of bacterial infection of operated embryos (Holtfreter
1929 ). He found that when raised in particular salt conditions, amphibian embryos
gastrulate abnormally, such that the mesoderm and endoderm egress from the
embryo instead of internalizing and migrating underneath the prospective ectoderm
(Holtfreter 1933 ). In these exogastrulae, the mesoderm never underlies the ecto-
derm, and the neural plate never forms. He also performed explant studies, in which
he removed cells of the animal cap from the blastula, and cultured them in his solu-
tion. The caps quickly healed into round spheres, but only differentiated into “atypi-
cal ectoderm” (Fig. 7.6a) (Holtfreter 1938a, 1938b). This was surprising because,
according to Vogt’s fate map, some of the cells he explanted would become neural
tissue if left in its original location. The Organizer experiment, Holtfreter’s exogas-
trulae and the animal cap explants all showed that mesodermal tissue interacts with
the ectoderm to generate the neural plate. This interaction was termed “induction”
because it involved communication between the germ layers and was presumably
7 Establishment of the Vertebrate Germ Layers