How does gene regulation result in differentiation of different organs?

[PainterGuy]

I don't know much about biology but the following two questions have always puzzled me.

1: If each human body cell contains the same genes (from 20,000 to 25,000) then how different cells in different parts of body do different things. A liver cell, for example, does not have the same biochemical duties as a nerve cell. Yet every cell of an organism has the same set of genetic instructions, so how can different types of cells have such different structures and biochemical functions?

2: When a human baby is born, each body organ is small and as the baby grows so do the different body parts like eyes, liver, lungs, size of hands, etc. What stops these organs from overgrowing once they have grown to a certain size. I think it has to do something with genes but still I'd appreciate it if you could give me some explanation.Helpful links:
1: http://www.dnaftb.org/36/
2: https://medlineplus.gov/genetics/understanding/howgeneswork/geneonoff/
3: https://www.nature.com/scitable/topicpage/gene-expression-regulates-cell-differentiation-931/
4: /watch?v=mcEV3m9SG9M (insert www.youtube.com in front)

 

[docnet]

There are a special subset of genes called master regulator genes, which help determine which genes are turned on in each cell. This process starts during the early stages of development. When there are only 8 cells of a person, every cell is exposed to the same environmental signals . After the 8-cell stage, cell fates diverge because the environments in which cells are in differ from one location of the embryo to other locations. The detailed and rigorous study of the development of living organisms is called developmental biology.

What causes tissues to stop growing after a certain size? There is no one answer for all, but I believe most answers will relate the growth rates of cells to their environments. For example, the availability of growth factors secreted by neighboring cells and the availability of oxygen via blood a affects the cell's internal self-regulating mechanisms to switch on or off in certain scenarios. This is a simple explanation of what prevents cells from growing out of control.

Here is a simple explanation of a common motif seen in developmental biology, certain tissues produce molecular signals that diffuse to other cells to affect their DNA transcription. Other cells can use these molecular signals, which may be transcription factors or peptide ligands, to turn on different subsets of differentiating genes to become a specified cell type. After the molecular signals are used up, the other cells are no longer creating these signals and the genes for tissue differentiation are again turned off.

You can get a solid idea of human biology by taking 2-3 years worth of biology in undergraduate university. Some important courses are the following.

Introductory biology
biochemistry
molecular biology of canceer
physiology
developmental biology
molecular/cellular immunology
Genetics

 

[BillTre]

These are important and not fully understood biological control mechanisms.

Yet every cell of an organism has the same set of genetic instructions, so how can different types of cells have such different structures and biochemical functions?

As you seem to have figured out, different genes are turned on in different cells.
In addition to the more obvious genes, there are a lot regulatory genes whose protein products job is to bind DNA or other DNA associated proteins to modulate (turn on, turn up, turn down, or turn off) the function of other genes.
The many different regulatory gene products cooperate to determine how and when a particular gene poduces its mRNA. The translation of the mRNA into protein can also controlled by other factors.
This is all very complicated and has been heavily researched for many years.
Wikipedia article on gene regulation.

How different cells end up expressing the proper genes appropriate to their position within an organisms has also been heavily researched and is the object of many studies in developmental biology.
It has recently become possible to determine what individual genes are being expressed in different individual cells during development. This has lead to a lot of recent studies on many organisms mapping out these patterns of gene expression. This is pretty cutting edge stuff.
Here is a Science magazine article on such a study. It is difficult for me to tell if it is open access since I have a subscription, but here are some figures from it to give you some idea of the kind of things they are looking at:

What stops these organs from overgrowing once they have grown to a certain size.

In addition to things like growth factors which @docnet mentioned, there are additional size regulators.
Overall growth of an animal is is often regulated by nutrition and phase of life. for example, humans grow relatively slowly until puberty, then rapidly, than pretty much stop growing. This is considered hormone controlled and under the influence of gene expression.

The proportions of different organs within an organism of a particular size are probably controlled by mechanisms similar those mentioned by @docnet.

 

[atyy]

This review may be helpful:

Organ-Size Regulation in Mammals
Alfredo I. Penzo-Méndez and Ben Z. Stanger
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4563708/