Why do cells vary in shape and function when they have the same genome and the same organelles?

Question

Why do cells vary in shape and function when they have the same genome and the same organelles. For example: why do all cells have nuclei but red blood cell's don't; why can't the cells of a eye perform the function of the tongue; and the list goes on and on. If you say it is because of gene regulation, so then how does the RNA know to transcribe the specific genes that are necessary to that particular cell?

Answer 1

This question actually has a very simple answer on the surface, as you already suspect, but becomes incredibly complex the more you look into it. But before we tackle that, let's address the red blood cell. The RBC is not born without a nucleus, the nucleus, after all the right genes are transcribed and the right proteins made, is ejected during cellular development. This results in the concave donut-like shape of the RBC which provides a higher surface area to volume ratio for oxygen diffusion into and out of the cell. So all cells at some point have or had a nucleus, because without one, the cell receives no instruction on what to be, which brings us to your question. You are right, it is about gene regulation, but RNA does not tell the cell which genes to transcribe, RNA is the result of transcription, and transcription is stimulated by the action of proteins called transcription factors in the nucleus. These proteins, though, don't know what to do unless other protein signals enter through the cell membrane or originate within the undifferentiated cell. Research is ongoing into determining exactly what type of proteins regulate this process and how, but it is certain that its a complex pathway that begins with signals entering embryonic stem cells (ESTs) and stimulating them, through closely regulation transcription, to differentiate into the cells of one of the three germ layers of the developing embryo. Without reviewing the literature myself and reporting it here, I cannot give you an incredibly detailed explanation, but methylation of DNA to suppress expression, transmembrane proteins to receive signals and transduce them, and various other cascade proteins and signaling molecules, including kinases which activate or inhibit other proteins, are involved. Though scientists understand that the signal transduction of stem cell differentiation is stimulated by either paracrine signals (from nearby cells), autocrine signals (from within the cell), or endocrine signals (from far away through the action of hormones), they are still working out the various signaling pathways leading to differentiation. One example though is the Notch pathway, which involves a paracrine interaction with the Notch cell surface receptor of the EST, followed by a complex cascade of signals resulting in the entry of the Notch intracellular domain into the nucleus, stimulating transcription of particular genes. Complex, right? And that's just one example.

So here is the simple version: Cells, for whatever reason, whether through chemical signals from each other or different cell types, or because of physical contact with another cell, become a particular type because of the genes in them that are stimulated to be expressed. This information then is passed on to daughter cells if the cell type is one that divides. All cells in your body carry the same set of genes, but not all of them are expressed in all cells. Only the skin cell genes are expressed in skin cells, or the nerve cell genes in the nerve cell, etc. Stem cell differentiation is a complex process that scientists don't entirely understand, but your question is timely. In the science literature, you can find information about adult stem cells (discovered in the bone marrow in the 1950s), and induced pluripotent stem cells (iPSCs), the latter of which are adult somatic cells (differentiated body cells) induced to become undifferentiated stem cells in the lab. Both types of cells can be used in stem cell research, potentially bypassing the ethical considerations that limit this research currently. While research into uses of adult stem cells continues, it appears that iPSCs are being used in wildlife conservation efforts by the laboratory growth of animal products such as leather and meat. But who knows, perhaps one day, with a lot more research, an eyeball cell can be turned into a tongue cell with the use of iPSC technology.

Some information about stem cells and a little about differentiation can be found at the NIH website below.

https://stemcells.nih.gov/info/basics/2.htm

Answer 2

Even though all cells within a larger organism have the same DNA sequence and thus the same genes, the expression of genes is variable within different cells.

So in the most basic way, your eye is an eye because the eye genes are on and not a heart because the hear genes are off.