Importance of morphogens , induction and cytoplasmic

determinants in early developmentPresented by

Sakshi SaxenaASU2013010200124

IBT Ivth sem

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Introduction

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Introduction

**Morphogenesis is the outcome of correct pattern formation

•A morphogen is a signal (usually secreted from asubset of cells) that elicits different cellular responsesat different concentrations.

•A morphogen specifies more than one cell type byforming a concentration gradient, ie- it diffuses from itssite of synthesis to become progressively lessconcentrated farther from the source of its synthesis.

•Cells respond to different, or threshold,concentrations, of the morphogen by activatingexpression of distinct sets of genes.

•Thresholds can represent the amount of themorphogen required to bind to receptors for activation intracellular signaling, or concentrations oftranscription factors required to activate certain genes

Morphogens

•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) isthe product of:•1) morphogen•2) competence of responding cells

Morphogens

Example

*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 formationKruppel : 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 embryoTailles : 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.

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•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 expression.

•The localized cytoplasmic determinants are often mRNAs encoding transcription factors, or the transcription factors themselves.

Cytoplasmic determinants

Example : The early  Caenorhabditis elegans embryo.

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".

Example : The early  Caenorhabditis elegans embryo.

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 cell. 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 cells. 

Induction

-can involve diffusion or direct cell-cell contact-plays an important role in coordinating the organization of cells, tissues,and organs to establish precise arrangements.

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).

Modes of inductive interactions

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 particular way.

Example:

Mesoderm induces ectoderm to form region-specific structures:thigh mesoderm + wing ectoderm = thigh feather

Example:

Species-specific differences in mouth parts: mesoderm induces ectoderm to formmouth, but ectoderm responds by making the kind of mouth it “knows” how to make.

Modes of inductive interactions

2. Permissive interactionThe responding cell contains all the potentials that are to be expressed, and needs only an environment that allows expression of these traits.

Modes of inductive interactions

Recap!!

REFERENCES1. Developmental Biology BY1101P. MurphyLecture 8Morphogenesis I and Positional information:2. KAPBiology DeptKenyon College

Chapter 11.  Development: Differentiation and Determination

3. Morphogens, their identification and regulation.Tabata T, Takei Y.4. Morphogen Gradients: From Generation to InterpretationAnnual Review of Cell and Developmental BiologyVol. 27: 377-407 (Volume publication date November 2011)Katherine W. Rogers1 and Alexander F. Schier