Morphogens, induction and cytoplasmic determinants
**Morphogenesis is the outcome of correct pattern
•A morphogen is a signal (usually secreted from a
subset of cells) that elicits different cellular responses
at different concentrations.
•A morphogen specifies more than one cell type by
forming a concentration gradient, ie- it diffuses from
itssite of synthesis to become progressively less
concentrated farther from the source of its synthesis.
•Cells respond to different, or threshold,
concentrations, of the morphogen by activating
expression of distinct sets of genes.
•Thresholds can represent the amount of the
morphogen required to bind to receptors for activation
intracellular signaling, or concentrations of
transcription factors required to activate certain genes
•Importantly, different tissues may use the
same gradient system, but will respond to
the gradient in different ways.
•This situation is analogous to reciprocally
transplanting portions of American and
French flags—the segments retain their
identity (American or French), but are
positionally specified (red, white, blue)
according to their new position.
•The final output (color and pattern) is
the product of:
•2) competence of responding cells
*The very first step in patterning the embryo of the fruit
fly, Drosophila melanogaster, is a good example of pattern formation
by a gradient.
*Bicoid is a transcription factor which turns on different genes in different
levels - acting as a morphogen gradient.
The four genes shown in part A (tailless, empty spiracles, hunchback,
and kruppel) are found in different locations within the Drosophila
embryo, as a result of the amount of Bicoid protein at a particular
location in the embryo.
The empty spiracles gene is necessary for proper head formation
Kruppel : Termed as gap gene .. Meant for development of Centre of embryo
Hunchback are the maternal effect genes that are most important for patterning of anterior
parts (head and thorax) of the Drosophila embryo
Tailles : Posterior part
After fertilization, bicoid mRNA from the mother fly begins to be
translated into Bicoid protein in the Drosophila zygote.
image B shows how the Bicoid protein diffuses through the egg
forming a gradient.
High concentrations of Bicoid protein are shown in white on the left (anterior) end of the
zygote, and low concentrations are shown in blue on the right (posterior) end.
Image C shows Bicoid protein in the nuclei of a Drosophila embryo after
a number of rounds of mitosis.The nuclei in the anterior end (left) have
more Bicoid protein than those in the posterior end (right)
Image D shows Kruppel protein in orange and Hunchback protein
in green. The region where the two proteins overlap is yellow. The
colors come from fluorescent dyes attached to antibodies that bind
specifically to these proteins.
•The asymmetric segregation of cytoplasmic determinants
is due to asymmetric localization of molecules (usually
proteins or mRNAs) within a cell before it divides.
• During cell division, one daughter cell receives most or all
of the localized molecules, while the other daughter cell
receives less (or none) of these molecules.
•This result in two different daughter cells, which then
take on different cell fates based on differences in gene
•The localized cytoplasmic determinants are often mRNAs
encoding transcription factors, or the transcription factors
All of the cells in the embryo are
visible on the left side of the image,
while only the P granules are visible
on the right side of the image.The P
granules were fluorescently labelled -
they are the green "dots".
a) A newly fertilized embryo
with dispersed P granules.
b) P granules are localized to the
posterior end of the zygote.
c) After the first division, P granules are
present only in the smaller, posterior
d) Another unequal division gives rise
to a single cell containing P granules.
e) When the larva hatches, P granules
are localized to the primordial germ
-can involve diffusion or direct
-plays an important role in
coordinating the organization of
and organs to establish precise
Induction—the process whereby one
cell or tissue tells another what to do;
thus, the cell or tissue signals to its
neighbor to specify cell fate(s).
1. Instructive interaction
A signal from the inducing cell is necessary for initiating new
gene expression in the responding cell.Without the inducing cell,
the responding cell is not capable of responding in that
Mesoderm induces ectoderm to
form region-specific structures:
thigh mesoderm + wing ectoderm
= thigh feather
Species-specific differences in
mouth parts: mesoderm induces
ectoderm to form
mouth, but ectoderm responds by
making the kind of mouth it
“knows” how to make.
2. Permissive interaction
The responding cell contains all the potentials that are to be
expressed, and needs only an environment that allows expression
of these traits.
1. Developmental Biology BY1101
Morphogenesis I and Positional information:
Chapter 11. Development: Differentiation and
3. Morphogens, their identification and regulation.
4. Morphogen Gradients: From Generation to Interpretation
Annual Review of Cell and Developmental Biology
Vol. 27: 377-407 (Volume publication date November 2011)
Katherine W. Rogers1 and Alexander F. Schier