8.3 Cell Fates
177
Fig. 8.7 Stages of Drosophila oocyte growth. Colored letters denote the localization of various
messenger RNA molecules, which encode addressing for the delivery of particular proteins along
the network of microtubules; the black dot is the nucleus (Bor et al, 2014)
The oocyte becomes polarized early on, as determinants of the anterior and posterior ends (indicated in Fig. 8.7) appear at the respective locations specifying the
major body axis of the future fly. This used to be the only axis of ancient radially symmetric creatures. The appellations of particular messenger RNA molecules
bringing instructions to set this axis are of no importance to us: they are specific to
Drosophila, the darling of all geneticists, and are not present even in the genomes
of other flies. The anterior end is called the animal pole and its opposite, the vegetal
pole. Although establishing this axis is fundamental to embryo patterning, it is not
conserved among different species, which may use different strategies to set it up.
In amphibians, represented by their own model animal, the Xenopus frog, the egg
possesses a distinct polarity even before it is fertilized.
Next comes the dorso-ventral axis. It first appeared in bilateral animals, which
emerged in the Cambrian Explosion. Lewis Wolpert (2002) writes: “We are much
more like flies in our development than you might think”. But not quite: there
is an evolutionary surprise here, first suggested by Geoffroy Saint-Hilaire (1822).
What is dorsal in Drosophila and its arthropod relatives, is ventral in our relatives,
chordates. The way this axis is set up in a radially symmetric egg appears to be of no
consequence. Thus, in Xenopus it is determined by the sperm’s entry point (Wolpert,
2002). There are, of course, less trivial distinctions between the development of
arthropods and chordates.
8.3 Cell Fates
Cell fates have to be fixed early in the development, before the embryo has grown
too large to be reached by morphogenetic signals. The outcomes are decisive: transplanted cells maintain the fate bestowed on them by their original position. In this
way, experimentalists have caused limbs and appendages to grow in the wrong places.
Francis Crick (1970), supporting Wolpert’s idea of positional information, estimated
that a morphogen profile could be established by diffusional transport in a reasonable
time, within a few hours, in a millimeter-sized embryo, but would take a full day in
a centimeter-sized animal. Although diffusion is not the only transport mechanism
177
Fig. 8.7 Stages of Drosophila oocyte growth. Colored letters denote the localization of various
messenger RNA molecules, which encode addressing for the delivery of particular proteins along
the network of microtubules; the black dot is the nucleus (Bor et al, 2014)
The oocyte becomes polarized early on, as determinants of the anterior and posterior ends (indicated in Fig. 8.7) appear at the respective locations specifying the
major body axis of the future fly. This used to be the only axis of ancient radially symmetric creatures. The appellations of particular messenger RNA molecules
bringing instructions to set this axis are of no importance to us: they are specific to
Drosophila, the darling of all geneticists, and are not present even in the genomes
of other flies. The anterior end is called the animal pole and its opposite, the vegetal
pole. Although establishing this axis is fundamental to embryo patterning, it is not
conserved among different species, which may use different strategies to set it up.
In amphibians, represented by their own model animal, the Xenopus frog, the egg
possesses a distinct polarity even before it is fertilized.
Next comes the dorso-ventral axis. It first appeared in bilateral animals, which
emerged in the Cambrian Explosion. Lewis Wolpert (2002) writes: “We are much
more like flies in our development than you might think”. But not quite: there
is an evolutionary surprise here, first suggested by Geoffroy Saint-Hilaire (1822).
What is dorsal in Drosophila and its arthropod relatives, is ventral in our relatives,
chordates. The way this axis is set up in a radially symmetric egg appears to be of no
consequence. Thus, in Xenopus it is determined by the sperm’s entry point (Wolpert,
2002). There are, of course, less trivial distinctions between the development of
arthropods and chordates.
8.3 Cell Fates
Cell fates have to be fixed early in the development, before the embryo has grown
too large to be reached by morphogenetic signals. The outcomes are decisive: transplanted cells maintain the fate bestowed on them by their original position. In this
way, experimentalists have caused limbs and appendages to grow in the wrong places.
Francis Crick (1970), supporting Wolpert’s idea of positional information, estimated
that a morphogen profile could be established by diffusional transport in a reasonable
time, within a few hours, in a millimeter-sized embryo, but would take a full day in
a centimeter-sized animal. Although diffusion is not the only transport mechanism