The number of times a human cell is capable of dividing into two new cells is called:

Question What are stem cells?

Answer

Stem cells are the cells that have an ability to become various types of cells. They are present both during embryonic development (embryonic stem cells) and in the adult body (adult stem cells).

Stem cells have unique properties that other cells do not have.

First, to understand what stem cells are, you need to understand how humans and other mammals develop:

1. Sperm fertilizes an egg and forms a single cell called a zygote.
2. The zygote undergoes a process known as mitosis, in which it replicates its chromosomes (which carry each parent’s DNA) and then divides, resulting in two identical cells. These cells are called totipotent and have the ability to develop into a new organism. The zygote repeats the process of mitosis for about 5 or 6 days creating a small ball of a few hundred cells called a blastocyst.
3. The blastocyst has an outer-layer of cells called the trophoblast, which will eventually form the protective placenta. Inside the trophoblast is a group of cells called the inner cell mass. The organism in a stage between zygote to fetus is called an embryo* and the cells are called embryonic stem cells.
   At this point embryonic stem cells have the ability to become a cell for any part of the body (nerve, muscle, blood, etc.). This ability to become any type of cell in the body is called pluripotent. The difference between totipotent and pluripotent cells is only that totipotent cells can give rise to both the placenta and the embryo.
4. As the embryo grows these pluripotent cells develop into specialized, multipotent stem cells. Multipotent stem cells have the ability to develop specific types of cells (terminally differentiated cells). For example a blood stem cell (multipotent) can develop into a red blood cell, white blood cell or platelets (all specialized cells). There are multipotent stem cells for all of the different types of tissue in the body.

*Embryo: the embryo of mammals is defined as the stage of organism between the first division of zygote and the time it becomes a fetus through further development. For humans, the embryo is defined as the implantation of fertilized egg in the uterus through the eighth week of its development. The embryo will be called a fetus after the eighth week until birth.

Stem Cells are different from other cells because:

1. They can continue to divide for long periods of time: Most cells such as skin cells cannot replicate themselves after a certain period of time. Stem cells are self-sustaining by replicating themselves for a much longer period of time.
2. They are unspecialized: Specialized cells have specific capabilities that allow them to perform certain tasks. For example a red blood cell contains hemoglobin that allows it to carry oxygen. Stem cells have unspecialized capability and do not have tissue- specific structures to perform specialized functions.
3. They can give rise to specialized cells: Stem cells go through a process called differentiation and create special types of cells (muscle, nerve, skin, etc.).

Embryonic stem cells

Embryonic stem cells are the cells within the protective layer of the blastocyst. They are pluripotent, which means they can develop into any of the cells of the adult body. Researchers believe that, because they are pluripotent, and easy to grow, they have the best potential for replacing damaged or lost tissue or body parts.

Adult stem cells

Also known as progenitor cells or somatic stem cells, adult stem cells are located, in small quantities, throughout the body and generate specialized cells for the area they are located. These cells do not renew themselves as well as embryonic stem cells. Still, if these cells are put in a different environment, they may produce a different type of cells from the originating cell.

Stem cell research is an active area of inquiry and scientists are discovering new characteristics of stem cells every day. For example, recent research indicated that multipotent stem cells from one type of tissue (blood) might actually have the ability to generate cells for a different type of tissue (nerve).

Scientists are continuing to search for new sources of adult stem cells. Some of the locations where stem cells have been located include: bone marrow, skin, liver, blood, and the brain. Some adult stem cells, which have already been used to treat illnesses, include hematopoietic stem cells and umbilical cord blood stem cells.

Hematopoietic stem cells are located in the bone marrow and form blood cells. They have been successfully used to treat blood disorders for younger patients. Umbilical cord blood stem cells are located in the blood of the umbilical cord after birth. Umbilical cord stem cells are similar to hematopeitetic stem cells in adults, but they are less mature and have much more potential to differentiate into various types of cells.

Published: 11/19/2019. Author: Science Reference Section, Library of Congress