Stem Cells Flashcards
What’s cell differentiation
The process of a cell becoming specialised. Despite being differentiated in structure and function, all body cells within an organism have the same DNA. Differentiation involves the expression of some genes but not others in the cells genome.
Stem cells
All cells in plants and animals begin as undifferentiated cells and originate from mitosis or meiosis. They are not adapted to any particular function (unspecialised) and they have the potential to differentiate to become any one of the range of specialised cell types in the organism. These undifferentiated cells are called stem cells.
Stem cells are able to undergo cell division again and again, and are the source of new cells necessary for growth, development and tissue repair. Once stem cells have become specialised they lose the ability to divide, entering the g0 phase of the cell cycle.
The activity of stem cells has to be strictly controlled. If they do not divide fast enough then tissues are not efficiently replaced, leading to ageing. However, if there is uncontrolled division then they form masses of cells called tumours, which can lead to cancer.
Stem cell potency
A stem cells ability to differentiate into different cell types is called potency. They greater the number of cell types it can differentiate into the greater its potency. Stem cells differ depending on the type of cell they can turn into:
-Totipotent- these stem cells can differentiate into any type of cell. A fertilised egg, or zygote and the 8 or 16 cells from its first few mitosis divisions are totipotent cells, which are destined eventually to produce a whole organism. They can also differentiate into extra-embryonic tissues like the amnion and unbilicus.
-Pluripotent-these stem cells can form all tissue types but not whole organisms. They are present in early embryos and are the origin of the different types of tissue within an organism.
-multipotent- these stem cells can only form a range of cells within a certain type of tissue. Haematopoetic stem cells in bone marrow are multipotent because this gives rise to the various types of blood cell.
Differentiation
Multicellular organisms like animals and plants have evolved from unicellular (single-celled) organisms because groups of cells with different functions working together as one unit can make use of resources more efficiently than single cells operating on their own.
In multicellular organisms cells have to specialise to take on different roles in tissues and organs. They may be required to form barriers such as skin or be motile such as sperm cells. Cells have adapted to different roles in an organism and so have many shapes (and sizes) and often contain different organelles.
Erythrocytes (RBC) and neutrophils (WBC) are both present in blood. They look very different because they have different functions. When cells differentiate they become adapted to their specific role. What form this adaptation takes is dependent on the function of the tissue, organ and organ system to which the cell belongs.
All blood cells are derived from stem cells in the bone marrow.
Replacement of red and white blood cells
Mammalian erythrocytes are essential for transport of oxygen around body. Have few organelles- more room for haemoglobin.
Due to the lack of nucleus and organelles they only have a short lifespan of around 120 days. They therefore need to be replaced constantly. The stem cell colonies in the bone marrow produce approximately three billion erythrocytes per kilogram of body mass per day to keep up with the demand.
Neutrophils have an essential role in the immune system. They live for only about 6 hours and the colonies of stem cells in bone marrow produce in the region of 1.6 billion per kg per hour. This figure will increase during infection.
Sources of animal stem cells
-embryonic stem cells- these cells are present at a very early stage of embryo development and are totipotent. After about seven days a mass of cells, called a blastocyst, has formed and the cells are now in a pluripotent state. They remain in this state in the fetus until birth.
-Tissue (adult) stem cells- these cells are present throughout life from birth. They are found in specific areas such as bone marrow. They are multipotent, although there is growing evidence that they can be artificially triggered to become pluripotent. Stem cells can also be harvested from the umbilical cords of newborn babies. The advantages of this source are the plentiful supply of umbilical cords and that invasive surgery is not needed. These stem cells can be stored in case they are ever needed by the individual in the future, and tissues cultured from such stem cells would not be rejected in a transplant to the umbilicus owner.
Sources of plant stem cells
Stem cells are present in meristematic tissue (meristems) in plants. This tissue is found wherever growth is occurring in plants, eg at the tips of roots and shoots.
Meristematic tissue is also located sandwiched between the phloem and the xylem tissues and this is called the vascular cambium. Cells originating from this region differentiate into the different cells present in xylem and phloem tissues. In this way the vascular tissue grows as the plant grows. The pluripotent nature of stem cells in the meristems continues throughout the life of the plant.
Stem cells transplanted into specific areas have the potential to treat certain diseases such as:
-heart disease-muscle tissue in the heart is damaged as a result of heart attack, normally irreparably-this has been tried experimentally with some success already.
-type 1 diabetes- with insulin-dependent diabetes the body’s own immune system destroys the insulin-producing cells in the pancreas; patients have to inject insulin for life- this has been tried experimentally with some success.
-Parkinson’s disease- the symptoms (shaking and rigidity) are caused by the death of dopamine-producing cells in the brain; drugs currently only delay the progress of disease.
-Alzheimer’s disease- brain cells are destroyed as a result of the build up of abnormal proteins; drugs currently only alleviate the symptoms.
-macular degeneration-this condition is responsible for causing blindness in the elderly and diabetics; scientists are currently researching the use of stem cells in its treatment and early results are very encouraging.
-birth defects-scientists have already successfully reversed previous untreatable birth defects in model organisms such as mice.
-spinal injuries-scientists have restored some movement to the hind limbs of rats with damaged spinal cords using stem cell implants.
Stem cells are already used in such diverse areas as
-treatment of burns-stem cells grown on biodegradable meshes can produce new skin for burn patients, this is quicker than the normal process of taking a graft from another part of the body.
-drug trials-potentially new drugs can be tested on cultures of stem cells before being tested on animals and humans.
-developmental biology-with their ability to divide indefinitely and differentiate into almost any cell within an organism, stem cells have become an important area of study in developmental biology. This is the study of the changes that occur as multicellular organisms grow and develop from a single cell, such as a fertilised egg- and why things sometimes go wrong.
Ethics
The removal of stem cells from embryos normally results in the destruction of the embryos, although techniques are being developed that will allow stem cells to be removed without damage to embryos.
There are not only religious objections but moral objections too- many people life begins at conception and the destruction of embryos is murder. There is a lack of consensus as to when the embryo itself has rights, and also who owns the genetic material that is being used for research.
This controversy is holding back progress of research. The use of umbilical cord stem cells overcomes these issues to a large extent, but these cells are merely multipotent, not pluripotent like embryonic stem cells, thus restricting their usefulness. Adult tissue stem cells can also be used but they do not divide as well as umbilical stem cells and are more likely to have acquired mutations. Developments are being made towards artificially transforming tissue stem cells into pluripotent cells. Induced pluripotent stem cells (iPSCs) are adult stem cells that have been genetically modified to act like embryonic stem cells and so are pluripotent.
The use of plant stem cells doesn’t raise the same ethical issues as animal cells.
