Video Transcript
In this video, we’ll find out what
stem cells are, where they’re found, and how they can be used to treat a range of
human disorders. We’ll also discover why human stem
cells are such a controversial topic in biology by evaluating the arguments for and
against their use in modern medicine. Let’s get started.
The brain, the heart, and the eyes
are all examples of organs in the human body. And the reason we consider them to
be organs is because they’re made up of tissues containing differentiated cells. A cell is differentiated if it’s
committed to being one particular cell type that’s specialized for its function. For example, the brain contains
neurons, which possess dendrites and long axons to carry electrical signals to and
away from the cell body. The heart contains cardiac
pacemaker cells, which are able to initiate and regulate the heartbeat. And the eyes contain retinal
photoreceptor cells, which are adapted to respond to light.
However, these cell types weren’t
always specialized in this way. In fact, they all originated from
an undifferentiated cell known as a stem cell. Stem cells show no specialization
and therefore have the potential to differentiate into any one of over 200 different
cell types in the human body. This happens a lot during our early
development when we grow from a tiny ball of cells into a fully fledged human
being. But it can also happen during our
adult lives, with stem cells functioning as a repair system for damaged tissues.
So where do we find human stem
cells? A human possesses three different
types of stem cells from three different sources over the course of their
lifetime. The first is found only a few days
after fertilization of the egg when a small ball of cells called an embryo has
formed. These cells are known as embryonic
stem cells, and they will go on to form all the tissues and organs which make up a
human. The second source of stem cells is
the umbilical cord. This forms during the seventh week
of pregnancy linking the embryo to the mother and is removed after birth. You can see where your umbilical
cord was once attached by looking down at your belly button. The blood vessels of the umbilical
cord in particular are very rich in stem cells.
The final type are adult stem
cells. Adult is a slightly confusing way
to describe these cells as they are not only present in adults but also in children
and babies. Adult stem cells are found in small
numbers in most of the tissues and organs of the body. An important feature of stem cells
is that they can’t all differentiate into the same number of cell types. We describe embryonic stem cells as
being pluripotent. This means they can differentiate
into any cell type of the human body apart from those which exist outside the
embryo. Embryonic stem cells are the most
useful type as they can be used to replace damaged or diseased cells in any part of
the body.
Umbilical cord and adult stem cells
are described as multipotent. This means they can differentiate
into a limited number of different but closely related cell types. A good example of multipotent stem
cells are the ones located in the bone marrow. As you can see in the diagram,
these stem cells can give rise to all the different cell types found in the blood:
the red blood cells, the white blood cells, and the platelets. But would they be capable of
differentiating into a cell of, say, the liver? No, they wouldn’t. Although this makes them less
useful than pluripotent stem cells, we can still use them for targeted therapies to
treat diseases such as leukemia, which is a blood cancer.
It’s not just animals that have
stem cells. They’re also found in plants. Plant stem cells are mainly found
in the growing tissues of the plant, which are called the meristems. These are located at the tips of
the shoots and roots and are therefore known as the shoot apical meristem and the
root apical meristem. Some plants also have lateral
marrow stem tissues, which allow growth around the stem, allowing it to become
larger in diameter and to produce the thick woody outer layer we see in many large
plants and trees. Meristem cells are totipotent,
which means they’re capable of differentiating into any cell type of any tissue or
organ in the plant. In fact, totipotent stem cells are
capable of eventually differentiating into a whole new organism, which pluripotent
stem cells will be unable to do.
Stem cells from both animals and
plants have become an increasingly important field of biology presenting exciting
applications in many areas of our lives. For example, if you take a plant
cutting containing a shoot apical meristem and replant it under the appropriate
conditions, it will develop into a complete plant which is a clone of the
original. This is a quick and cheap technique
which is widely used in horticulture to mass-produce plants which have desirable
characteristics. Some researchers also suggested
that meristem cells can be used in antiaging cosmetics as they have special
properties, which mean they can rejuvenate the skin.
So what can we use human stem cells
for? Human stem cells present exciting
possibilities in medicine and research to treat many life-threatening diseases. For example, adult stem cells can
be transplanted from the bone marrow of a healthy donor into a patient who is
suffering from a blood disorder, such as leukemia, which affects the white blood
cells. The stem cells will then
differentiate into healthy white blood cells to replace the faulty ones. Embryonic stem cells also show a
lot of potential for replacing damaged cells to treat serious conditions. For example, researchers have shown
that embryonic stem cells can be differentiated into dopamine-producing neurons,
which could be used in the treatment of Parkinson’s disease.
Parkinson’s disease is a condition
where dopamine-producing neurons in the brain gradually die, leading to problems
with movement and speech. Although, on one hand, the ability
to cure people of life-changing diseases like Parkinson’s is really positive, on the
other hand, the use of stem cells, particularly those taken from embryos, is pretty
controversial because it presents a number of issues which we’ll discuss later in
the video. In order to get around some of
these issues, scientists have developed a new stem cell line called induced
pluripotent stem cells.
These are produced by taking
differentiated body cells and reprogramming them back into pluripotent stem
cells. These cells can be used in the same
way as embryonic stem cells, with the great advantage being that patients will
receive their own reprogrammed cells. So the problem of rejection can be
avoided. Induced pluripotent stem cells have
already been utilized effectively to rebuild the cornea in the eye and restore
vision in patients who are losing their sight. Another technique called cell
fractionation can be used to collect stem cells from a patient or donor.
First, a sample is taken, for
example, from the bone marrow or in a separate procedure from the blood found in the
umbilical cord. Next, a process called
centrifugation is used to separate the different populations of cells in the sample
based on their different densities. The desired stem cell population
can then be cultured and used for a particular therapy. Cell fractionation not only makes
the sorting of stem cells much easier, but it can also be useful for breaking up and
separating the components of any cell type to determine the composition of their
organelles and other subcellular structures.
We’ve already said that stem cells
are quite a controversial area of science. So let’s consider some of the
arguments for and against their use. Before we evaluate the arguments,
it’s useful to divide them into ethical, scientific, and social issues. An ethical issue is anything that’s
informed by somebody’s idea about what’s right and wrong. A scientific issue is anything that
presents an opportunity or a challenge for scientists. And a social issue is anything that
affects people in our society. Here, we’ll present the ethical
issues in orange, the scientific issues in blue, and the social issues in green.
First, let’s address the arguments
in favor of using stem cells in medicine and medical research. If a couple is unable to conceive
naturally, they can sometimes be assisted by a process called in vitro
fertilization, or IVF for short. During IVF, more embryos will be
created than end up being used. Embryonic stem cells are therefore
usually taken from the embryos that are left over and would otherwise be
discarded. Adult, induced pluripotent, and
umbilical cord stem cells are always taken with permission from voluntary
donors.
Stem cells have the potential to
treat and even cure many life-threatening conditions, such as cancer and Parkinson’s
disease. Stem cells also show potential for
growing entire organs, which could then be used in organ transplantation. Because the new organ would be made
of the patient’s own cells, it would not be rejected by the body’s immune
system. By testing new therapies on tissues
grown from stem cells, we can make significant improvements to existing medical
treatments.
Now, let’s have a look at the
arguments that oppose stem cell use. Some people believe that each
embryo is a potential human person who has not consented to having embryonic stem
cells taken from them. Although most of us can choose to
donate adult stem cells from our bone marrow to help treat patients with leukemia,
it involves an invasive procedure, which puts many people off signing up to the stem
cell register. It’s possible that the process used
to produce induced pluripotent stem cells might introduce potentially harmful
mutations. Stem cell treatments are expensive,
whether you pay individually or if they’re publicly funded by the taxpayer. Finally, a lot of money is made
from the harvesting and storage of human stem cells, and this can lead to patients
being exploited.
Now we’ve learned all about stem
cells, let’s have a go at a practice question.
Macular degeneration is when the
light-sensitive cells in the center of your eye stop working. Which of the following best
explains how stem cells could be used to treat someone with macular
degeneration? (A) Stem cells can be stimulated to
differentiate into light-sensitive cells to replace the damaged ones. (B) Stem cells can be used to
create an entire eyeball and replace the eye of someone with this condition. (C) Stem cells can be inserted into
the eye to fix the damaged cells. (D) Stem cells can stimulate the
immune response to repair the damaged cells. Or (E) stem cells cannot be used to
treat this condition.
Let’s remove the multiple-choice
options for now and explore how stem cells can be used to treat conditions like
macular degeneration. You may recall that organs such as
the eye contain tissues made of specialized cells that are adapted for their
function. For example, the retina tissue of
the eye is made of cells that are specialized by being sensitive to light. We say that specialized cells such
as these are differentiated because they’re committed to being specialized for their
function. Not all cells are differentiated
however. Some cells — which can be found in
developing human embryos, in the umbilical cord, in some tissues of the developed
human body or that can be artificially made from reprogrammed skin cells — are
undifferentiated, and these are known as stem cells.
Because they’re not specialized,
stem cells are capable of differentiating into a range of different cell types. This is how all our tissues and
organs are formed during development and how they repair themselves if they become
damaged during adult life. Stem cells are also very useful to
us because they can be used in medicine and research. This question is about macular
degeneration, which is one of the conditions that could be treated using stem
cells. The macula is the region of the
retina shown in orange on this diagram. A macular degeneration is when the
light-sensitive cells that make up this region gradually stop working over time,
leading to sight loss, particularly in the center of the sufferer’s field of
vision.
Fortunately, stem cells show great
potential for treating macular degeneration. Scientists could modify stem cells
with specific proteins called transcription factors, which would stimulate them to
differentiate into light-sensitive cells. These healthy cells could then be
inserted into the macula to replace the damaged ones.
Now we can return to the
multiple-choice options and answer this question. The statement that best explains
how stem cells could be used to treat someone with macular degeneration is (A). Stem cells can be stimulated to
differentiate into light-sensitive cells to replace the damaged ones.
Let’s summarize what we’ve learnt
in this video by reviewing the key points. Stem cells are unspecialized animal
or plant cells that are capable of differentiating into a range of cell types. They can be totipotent,
pluripotent, or multipotent. There are four types of human stem
cell, which are all derived from different sources. Stem cells have many different
uses, particularly in the fields of medicine and research. However, there are many issues
surrounding these uses, which makes stem cells quite a controversial area of
biology.
