Stem Cells

Stem cells have been in the news quite a bit lately. However, there is a lot of confusion or misunderstanding about what stem cells actually are. I have noticed that in many news stories, both in print and on tv, a basic explanation of stem cells is missing. So, I am going to go over in very simple language what a stem cell is, what it does and why they are so important. Most of this information comes from various classes I have taken, but there will be some additional resources at the bottom.

First, there are actually different types of stem cells. When you are discussing stem cells, it may not matter what actual types you are talking about, but the fact that there is not just one type of stem cell is significant. All animals basically start off the same way. Cells begin to divide once conception takes place. This mass of cells are stem cells! They are referred to as totipotent stem cells because they can become anything, anything at all. Any organ or tissue can be made from these cells.

At a certain point, enough cells are gathered together and a process known as gastrulation occurs. What gastrulation does, is take the inital lump of cells and divides them into three separate and different layers of cells. These three layers become everything in the body. They are known as ectoderm (the outer layer), mesoderm (middle layer) and endoderm (inner layer). Think of an oreo cookie. The top cookie is ectoderm, the yummy middle is mesoderm and the bottom cookie is endoderm. These three layers are called germ layers and are also stem cells. These are known as pluripotent stem cells because they can differentiate (or become) into body parts depending on which layer they originated. Ectoderm makes things like skin and the nervous system. Mesoderm makes things like muscles and bones and a couple of organs. Endoderm results in most of the organs of the body and the deep skin known as the dermis. So, for example, mesoderm cells have the potential to become heart cells or kidney cells. Cool, huh?

Once these germ layers are in place, they begin to become more specialized and form the precursors of the actual tissues in the body. For example, some of the mesoderm becomes mesenchyme which then becomes connective tissues like tendons or bones (yes, bones are connective tissues, so is blood for that matter). Mesenchyme is also considered a stem cell, but instead of being able to form anything that mesoderm is capable of, it can only become one of the various types of connective tissue. These type of cells are known as multipotent stem cells because they can become multiple tissues within a specific tissue category like connective. With me so far?

So, to recap and bottom line it:

1. Conception= totipotent stem cells (can become anything!)

2. Gastrulation-ectoderm, mesoderm, endoderm= pluripotent stem cells (can form only the body parts from each germ layer. Example: mesoderm can form muscles OR kidney cells).

3. More differentition= Multipotent stem cells (can form only what the differentiated cell can form like mesenchyme can form connective tissue but not muscle).

So that is it for a basic understanding of stem cells. In my next blog post, I will discuss how stem cells are made with embryonic and adult cells.

More information on stem cells here:
Wikipedia Stem Cells
Stem Cells from the National Institute of Health
Pictures of gastrulation from Med.unc.edu