Stem Cells, Part 1 - What are they?

One of the most promising areas of medical research is the use of human stem cells. Stem cells are specific undeveloped cells in the human body that can be induced to develop into one or more of the specialized cells of mature human tissue. Stem cells can develop into blood cells, brain cells, kidney cells, or any of the other types of cells that make up our bodies.

This post starts a series of articles that will delve into the science of stem cells; what they are, how they are formed and develop, their present and potential future medical uses, and the controversy surrounding them. Since this is a medical subject, it would be easy for confusing jargon and jaw-breaking terminology to creep in. At the risk of oversimplification in some cases, this article will avoid the Latinate words as much as possible and will use commonly understood terms wherever possible.

To understand stem cells and why they can be so important, it will first be necessary to describe the normal development of a human being from conception to birth.

How a Baby is Formed

Conception occurs as a result of sexual intercourse. The man contributes sperm cells and the woman contributes an ovum, a human egg cell. Both types of cells, sperm and ovum, are incomplete, each having only 23 of the 46 chromosomes required, but they combine into a single complete cell known as a zygote (fertilized ovum). This occurrence is referred to as conception.

At this point, the zygote is a complete entity, capable of developing into a baby, as long as the conditions are right, and as long as a few more key milestones occur as nature designed. The zygote starts to divide inside its outer surface, once every 12 to 20 hours as it passes down the fallopian tube towards the uterus. At any time during the first 14 days, the zygote may separate into two separate zygotes; this is how twins are formed.

By day three, when the zygote consists of a cluster of 16 cells, it has entered a new phase of development that goes beyond simple division. This cluster, which we shall call a pre-embryo, starts to resemble a hollow sphere. By about day five, the pre-embryo has entered the uterus.

It is at this point that the identical cells of the pre-embryo start to differentiate into specialized cells. The outward-facing cells attach to the lining of the uterus and form the placenta, and the inward-facing cells start to form the embryo. By day ten the pre-embryo is firmly attached, and blood circulation begins in the placenta. Medically, this is considered the start of pregnancy.

The process of differentiation leads to the cells in the pre-embryo (textbook name: blastocyst) to organize into three distinct layers, referred to as the germ layers. The term “germ layer” is not related to infection; it comes from the same root as “wheat germ” or “germinate”. Each germ layer is a region of the hollow sphere. Each layer starts to develop specialized cells of specific types. The outer layer will develop into skin cells and brain cells. The middle layer develops into blood cells, muscle cells, and kidney cells. The inner layer becomes lung cells and the cells of various organs. And just to make it confusing, a fourth type of primitive cell is the germ cells, which become sperm in a male or ova in a female.

These four types of cells are referred to as stem cells. The analogy of development is with a plant, where the stem leads to branches, the branches to twigs, and the twigs to leaves, flowers, and all the other glorious plant parts. Stem cells branch into the four types described above, and these in turn develop into more specialized types of cells, eventually leading to more than two hundred different types of cells in the human body.

At about two weeks after conception, the pre-embryo starts to develop a linear pattern of cells known as the primitive streak. It is at this point that it is no longer possible to separate into twins, and it is at this point that the organism is referred to as an embryo. The primitive streak will eventually develop into the central nervous system.

For the next eight weeks the embryo enters a period of rapid development, with the undifferentiated cells forming into organs, limbs, and other bodily structures. At ten weeks, all the recognizable features are present; arms, legs, head and eyes. At this point, it becomes a fetus.

The fetus, which at ten weeks is less than two inches long and weighs less than an ounce, spends the next six months growing in size and complexity inside the mother. At birth, the average child is about 20 inches long and weighs about seven and a half pounds.

The human body, besides being equipped with over two hundred different types of specialized cells, also continues to grow stem cells throughout the body. These stem cells proliferate throughout a lifetime, and eventually develop into one of the two hundred types of cells. This allows the body to grow and to repair damage.

Embryonic and Adult Stem Cells

A stem cell is a cell that can continue to divide and proliferate for an entire lifetime, and develop into specialized cells. Stem cells are categorized in two ways; by their origin, and by their potential to develop a certain way.

All the cells in the pre-embryo are called “pluripotent”; that is, they have the power to develop into any of the germ layer cells, and to further differentiate into more specialized types of cells.

As they start to specialize, they organize into groups by capability. For example, outer layer stem cells can become a number of different types of cells, but they will not become others; that job falls to the middle layer or inner layer types of cells. This restriction leads to the category “multipotent”. As development continues, further specialization occurs, to the point where a stem cell can be considered “unipotent”, that is, only capable of developing into a single type of cell.

A fully developed human, baby to senior citizen, will have throughout his body a collection of different kinds of stem cells, which are usually multi- or unipotent. These cells are referred to as “adult stem cells”. We all have them.

The stem cells in the pre-embryo and embryo are the newest and most potent, capable of differentiating into all types of specialized cells. They can, however, only be obtained by removing them from embryos. Consequently, these are referred to as “embryonic stem cells”.

Next: Stem Cells, Part 2 - How are they obtained?