Answering your first question

1. It is the amount of heterochromatin in different regions of the genome which decides what genes to be transcribed and what not.

2. First answer partly answers your second question as well. Stem cells by keeping their cell cycle short have less amount or levels of heterochromatin and in this case it is facultative heterochromatin makes it feasible to express genes when and where required.

3. Differentiated cells once differentiated retains the cellular memory (chromatin landscape is the same) henceforth expresses same genes. This is what makes liver like liver, kidney like kidney and neurons like neurons.

4. Third answer is the answer for your fourth question as well.

Thank you for ur valuable answer sir, yes, i agree its all bcoz of gene expression and memory of the particular cell. Hope, there must be something initiating point for that gene expression too....All the cells are rose from single cell, and depending upon organ its gene expression varies with the help of memory of first differentiated cell.

facultative heterochromatin makes it feasible to express genes when and where required and Differentiated cells once differentiated retains the cellular memory - I agree, stem cells gonna differentiated to organ specific type of cells... again it falls on gene expression... Now What is the deciding factor for differentiation of stem cell , Who is deciding stem cell to become endothelial cell, epithelial cell, fibroblast cell, etc...., what is triggering the gene expression to make the first differentiated cell, so that further it retains the cellular memory to proceed with its function to be liver as liver and neuron as neuron etc..

my 2nd que is, how does the cell knows that the organ has reached its specific structure? Eg. liver in a triangle shape, intestine to be tubular shape, shape of heart, bean shaped kidney, etc...

stem cells inherently maintains low levels of heterochromatin by keeping the cell cycle short which means it is ready for differentiation when it has necessary factors.

Your second question answer is cells maintain its shape by having a unique cellular memory (means its unique chromatin landscape) , as I said in my previous example with neurons . Neurons once differentiated cannot go back.

In simple words its the heterochromatin patterns which reflects cellular identity.

Thank you sir, My second question is different sir, An organ is made up of cell, Eg. stem cell differentiated into hepatocytes, multiple copies are produced through cell proliferation and cell division, finally complete liver has been formed,.. now what decides the shape of organ, what is the check point for cells to stop proliferation to notify the cells that the organ has reached its structure???? Hope, now u have got the point in my question sir.