Presented by :-

Bhupen Chandra Koch

Deptt. Of Zoology

Gauhati University

Concept of MorphogenA brief introduction:-cell fate can be specified by specific amounts of soluble molecules secreted at a distance from the target cells. Such a soluble molecule is called a morphogen, and a morphogen may specify more than one cell type by forming a concentration gradient. The concept of morphogen gradients had been used to model another phenomenon of regulative development: regeneration. It had been known since the 1700s that when hydras and planarian flatworms were cut in half, the head half would regenerate a tail from the wound site, while the tail half would regenerate a head

Allman (1864) had called attention to the fact that this phenomenon indicated a polarity in the organization of the hydra. It was not until 1905, however, that Thomas Hunt Morgan 1905,1906 realized that such polarity indicated an important principle in development. He pointed out that if the head and tail were both cut off a flatworm, leaving only the medial segment, this segment would regenerate a head from the former anterior end and a tail from the former posterior end never the reverse. Moreover, if the medial segment were sufficiently small, the regenerating portions would be abnormal. Morgan postulated a gradient of anterior-producing materials concentrated in the head region. The middle segment would be told what to regenerate at both ends by the concentration gradient of these materials. If the piece were too small, however, the gradient would not be sensed within the segment. (It is possible that there are actually two gradients in the flatworm, one to instruct the formation of a head and one to instruct the production of a tail.

In the 1930s through the 1950s, gradient models were used to explain conditional cell specification“The French flag analogy."

Imagine a row of "flag cells," each of which is capable of differentiating into a red, white, or blue cell. Then imagine a morphogen whose source is on the left-hand edge of the blue stripe, and whose sink is at the other end of the flag, on the right-hand edge of the red stripe. A concentration gradient is thus formed, being highest at one end of the "flag tissue" and lowest at the other. The specification of what type of cell any of the multipotential cells in this tissue will become is accomplished by the concentration of the morphogen. Cells sensing a high concentration of the morphogen become blue. Then there is a threshold of morphogen concentration below which cells become white. As the declining concentration of morphogen falls below another threshold, the cells become red.

Different tissues may use the same gradient system, but respond to the gradient in a different way. If cells that would normally become the middle segment of a Drosophila leg are removed from the leg- forming area of the larva and placed into the region that will become the tip of the fly's antenna, they differentiate into claws. These cells retain their committed status as leg cells, but respond to the positional information of their environment. Thereby, they became leg tip cells claws. This phenomenon, said Wolpert, is analogous to reciprocally transplanting portions of American and French flags into each other. The segments will retain their identity (French or American), but will be positionally specified (develop colors) appropriate to their new positions.

Identifying morphogen

For a diffusible molecule to be considered to be a morphogen, it must be demonstrated that:-

Cell respond directly to that molecule.The differentiation of those cells depends upon the concentration of that molecule.

Two commonly studied morphogn are:-1)Zebra fish nodal protein.2)Rationic acid as morphogen.

Nodal m-RNA made by egg before fertilization accumulated in the blastomere that will form the dorsal margin of the zebra fish embryo.

These cells will activate Gooscoid genes. Whose products commit these blastomeres to Became the cell that instruct the anterior portion of the head to form (Gorae et al. 2005)cells slightly further away from the dorsal margin activated the floating head gene, which commits the cell to became notochord and slightly further from the dorsal margin cell , activated the no tail gene that is important for forming the trunk and tail muscles. Experiments performed by Shank & Schier (2001) provide the evidence that these dorsal cell fold are co-ordinately regulated by different amounts of nodal proteins.Nodal protein is a morphogen and the gradient amount of nodal results the commitment of cells to different fates.

Rationic acid as morphogenRationic acid, a derivative of vitamin A is a small

hydrophobic molecule with an important role in local signalling in vertebrate development like the steroid and thyroid hormones, it diffuses across the plasma membrane unaided and binds to intra cellular receptors, the complex receptor and rationic acid, then functions as a transcription factor. A variety of experiments has shown that rationic acid can alter cells positional values development and it can have effect on the antero-poatero axis. The developmental abnormalities induced by rationic acid are apparently the result of its interference with the normal establishment of the Hox gene expression pattern.

For example:-Treatment of early mouse embryos with rationic

acid results in homeotic transformation of vertebrate, both anterior and posterior transformations being induced depending on the time of treatment. It is likely that this effect of rationic acid is mediated in part by the action of Hox gene expression and studies of cells in culture show that the Hox gene can be induced by rationic acid in a concentration dependent manner. As their is evidence for a gradient in rationic acid along the antero-posterior axis in the mouse embryo,it could be important &activating Hox gene in normal antero postero pattern.

Rationic acid also has striking effects on regenerating amphibian limbs:-

Exposing a degenerating limb to rationic acid results in the blastema, becoming proximatised, i.e. the limb degenerates as it had originally been amputed at the more proximal site, for example, if the limb is amputed through the radius and ulna treatment of rationic acid will result not only in the region of the elements distant to the cut but also to the production of an extra complete radius and ulna. The effect of rationic acid is dose dependent & with a high dose it is possible to regenerate a whole, extra limb including the part of the shoulder girdle on a limb from which only the hand has been amputed. Rationic acid can, therefore, alter the proximo-distal positional values of the blastema making it more proximal.

Cadherins and cell AdhesionRecent evidence shows that boundaries between tissues can indeed be created both by (1) different cell types having different types of cell adhesion molecules and (2) different cell types having different amounts of cell adhesion molecules. There are several classes of molecules that can mediate cell adhesion. The major cell adhesion molecules appear to be the cadherins. As their name suggests, they are calcium-dependent adhesion molecules. Cadherins are critical for establishing and maintaining intercellular connections, and they appear to be crucial to the spatial segregation of cell types and to the organization of animal form (Takeichi 1987). Cadherins interact with other cadherins on adjacent cells, and they are anchored into the cell by a complex of proteins called catenins . The cadherin-catenin complex forms the classic adherens junctions that connect epithelial cells together. Moreover, since the catenins bind to the actin cytoskeleton of the cell, they integrate the epithelial cells together into a mechanical unit.

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