Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 136

D. rerio lead to the failure in the development of the primary superficial epithelium
and gastrulation defects ( Sabel et al., 2009 ).
During gastrulation, formation of two or three embryonic germ layers occurs,
respectively in diploblastic (lower invertebrates) and triploblastic (higher inverte-
brates and all vertebrates) organisms. Formation of these layers results from directed
cell movements and proliferation of cells. A better-known mechanism of directed
cell movement is that in which migrating cells have membrane receptors that, on
their way to target sites, detect and bind specific ligands (primarily growth factors
and hormones) released by other cells, which at this stage should be maternal by
origin. Not much is known about the forces driving the movement and the shape of
migrating cells. The most likely candidates are contractile forces generated by the
actin-myosin cytoskeleton ( Martin, 2010 ).
As a result of invagination, the dorsal side of the embryo now consists of three
germ layers. Later, the endoderm and mesoderm will produce the majority of the
organ systems, including musculature and skeleton as well as ectoderm, which will
give rise to the nervous system and epidermal derivatives, such as various forms of
integument, hairs, nails, etc.
Cell division is now driven by the nucleocytoplasmic ratio instead of maternal
cyclins, and cell differentiation is determined by extracellular signals instead of
asymmetric distribution of maternal factors in cells. Production of changes in cells
under the influence of factors released by other cells, which is observed during
gastrulation and during the whole development of an individual, was termed induc-
tion in a famous article published in 1924 by Hans Spemann and Hilde Mangold
( Spemann and Mangold, 1924 ). As a result of the interaction between the preor-
ganizer and the Nieuwkoop center, Spemann's organizer, which is responsible for
the patterning of embryonic germ layers during gastrulation, develops. Embryonic
structures homologous to the amphibian Spemann's organizer in birds and mam-
mals were given the name nodes or Knoten (German for nodes ), in 1876 by the
physiologist Viktor Hensen (1835-1924). Hensen used the term to describe a mass
of cells in the 7-day rabbit embryos in which the germ layers were indistinguish-
able ( Blum et al., 2007 ).
Embryonic Induction
Hans Spemann (1869-1941), one of the founders of experimental developmental
biology, is credited with the discovery of the embryonic organizer. He entered biol-
ogy unconventionally; beginning as an apprentice in his father's topic business, he
studied preclinical medicine, finally settling in the field of zoology. His research
culminated with the discovery of the organizer, made public in 1924 in an article
coauthored with Hilde (Proescholdt) Mangold (1898-1924), a young biologist work-
ing on her thesis in his laboratory at Freiburg University, Germany. They implanted
cells of the dorsal lip of a salamander species embryo to the opposite side of another
salamander species embryo and observed that the latter developed a new notochord,
CNS, somites, etc., giving rise to a Siamese twin organism ( Figure 3.14 ). Thus, they