What is a stem cell?
As per the biological definition, “a stem cell is an undifferentiated cell of a multicellular organism which is capable of giving rise to indefinitely more cells of the same type, and from which certain other kinds of cells arise by differentiation”. These are the foundational cells of every organ and tissue of our body.
What are the types of stem cell?
There are many different types of stem cells from different parts of the body. Broadly in human beings there are two broad types of stem cells- Embryonic stem cells and Adult stem cells. And a newly programmed stem cell has been recently created called Induced pluripotent stem cells (iPSCs). Let’s deep dive into each of these types of stem cells:
Embryonic stem cells – These are stem cells derived from the undifferentiated inner mass cells of a human embryo. They are primarily derived from a four-five-day-old human embryo that is in the blastocyst phase of development. These are the extra embryos that have been created during the process of IVF (in vitro fertilization) where several eggs are fertilized in a test tube, but only one is implanted into the woman. In a normal pregnancy case, the blastocyst stage continues till the implantation of the embryo in the uterus, which occurs by the end of 10th week after all major organs of the body have been created.
Embryonic stem cells are able to differentiate into all derivatives of the three primary germs layers- ectoderm, endoderm and mesoderm. This means they are capable of developing into each of the 200 and more cell types of an adult human body. That is, depending upon the time period they are harvested from the human body, they can give rise to just about any cell of the human body. This distinctive property of embryonic stem cell is known as pluripotency. The other distinctive property of embryonic stem cells is their ability to be able to propagate themselves indefinitely. That is they have unlimited capacity for self-renewal. This is done under certain defined conditions. Because of these two properties, embryonic stem cells are used for tissue replacement therapy after injury or disease. Embryonic stem cells are specifically used for treating the following types of diseases:
- Blindness or spinal cord injuries
- Blood and Immune system related genetic diseases
Embryonic stem cell are very valuable for research purposes like investigating the early human development, the root cause for genetic diseases and for testing drugs.
Adult stem cells or Tissue specific stem cells – Tissue specific stem cells are also known as somatic or adult stem cells. These are more specialized than embryonic stem cells. They are undifferentiated cells, which are found throughout the body. These stem cells can generate different cell types for the specific tissue or organ in which they originate. That means an adult stem call can regenerate the entire organ from a few cells. These are found in children as well as adult humans unlike the embryonic stem cells which are only found at the blastocyst phase of embryo development.
One of the properties that these Adult stem cells possess, along with embryonic stem cells is the ability for self-renewal, indefinitely. In other words, they have the ability to propagate by being undifferentiated, i.e., they can make identical copies of themselves of the specific tissue. The other distinctive property the adult stem cell has is multipotency or in other words, multidifferentiative potential which is the ability to generate progeny of several distinct cell types. That is, they can give rise to mature cell types that have specialized functions.
The adult tissues which are reported to contain stem cells are in bone marrow, peripheral blood, brain, spinal cord, liver, pancreas, blood vessels, skeletal muscles, digestive system, cornea, retina and this list continues to grow through further research.
Adult stem cell have one major drawback. It gets difficult to harvest and culture them up to the required number as compared to embryonic stem cells. However, adult stem cells have been used popularly and widely for the treatment of the following diseases related to bone marrow transplantation:
- Related bone and blood cancer
Induced pluripotent stem cells (iPSCs) – These cells are a type of pluripotent stem cells, generated from adult stem cells but carrying the characteristics of embryonic stem cells. These cells can propagate indefinitely and also give rise to every other cell type in the body (like neurons, heart, skin). This has been a groundbreaking development since it overcomes the drawback of the characteristics of the adult stem cells which are restricted to treatment of the same tissue or organ where they originate. Currently experiments are going on to find alternative ways to create the iPSC.