3.5 Stem cells

3.5

Stem cells

KEY IDEAS

stem cell

Multicellular organisms consist of a large number of different types of cells that communicate and work together to ensure the survival of the whole organism. Each cell has a specific function, controlled by its genetic material and the chemical messengers that surround it. Damaged cells need to be replaced. For example, when the skin is damaged, each of the different cells that make up the layers of the skin (epidermis, dermis and hypodermis) along with the nerves, lymph vessels and sweat glands need to be replaced quickly before bacteria can cause damage. Each layer requires a different type of cell with their own specialised functions. The most efficient way for all of these cells to be produced at the same time is through the use of stem cells.

Types of stem cells

Stem cells are undifferentiated cells, effectively blank canvases that can be anything the a cell that is capable body needs them to be. Stem cells divide to produce daughter cells that can either continue as of forming different cell types stem cells or differentiate into specialised cells with specialised functions. These unique cells have three main characteristics. Stem cells: • can continue to reproduce themselves for long periods • are undifferentiated • can produce differentiated cells with specialised functions. There are two main types of stem cells: embryonic and adult stem cells. Embryonic stem cells

In this topic, you will learn that: ✚ stem cells are specialised cells that can differentiate into more than one type of cell ✚ the two main types of stem cells are embryonic and adult stem cells ✚ stem cells can be totipotent, pluripotent or multipotent ✚ the use of embryonic stem cells involves bioethical considerations. FIGURE 1 Damaged skin cells can only regenerate in humans if the damage is no deeper than the epidermis layer; otherwise, scar tissue forms. DRAFT ONLY - NOT FOR SALE Embryonic stem cells are found in an embryo, from a zygote to the inner cell mass of a blastocyst. These cells can use almost any part of their genetic material. This means they can become any type of specialised cell in the body and can endlessly reproduce themselves.

Embryonic stem cells are more flexible than adult stem cells in how they reproduce and the types of differentiated cells they can become. Embryonic stem cells are used as treatments for diseases such as Parkinson’s disease and even in spinal cord injuries. Scientists have discovered that the placenta has higher concentrations of embryonic stem cells than the umbilical cord. Adult stem cells Adult stem cells can reproduce themselves. They can also produce daughter cells that can become more than one cell type. However, adult stem cells are more limited than embryonic stem cells. Each adult stem cell can only produce a small range of differentiated cells. For example, in the lining of the intestines, there are stem cells at the base of deep folds. As older cells at the top of the fold die and are shed from the lining, the stem cells at the base produce new cells that differentiate into replacement cells. Adult stem cells are in bone marrow (producing red blood cells, white blood cells and platelets), the spinal cord, dental pulp, blood vessels, skeletal muscle, skin epithelia, the liver and many more locations.

Hierarchy of cell potency

Cell potency is a way of describing the number of different cell types a stem cell can produce. Cells with higher levels of potency can produce more cell types than cells with a lower potency. Totipotent stem cells Totipotent stem cells can produce cells that can differentiate into any cell type in the organism. These cells are only found in early stage embryos, from a zygote to a morula. Once the embryo has reached the blastula stage, the cells of the inner cell mass cannot become placental cells, and therefore are no longer called totipotent. Pluripotent stem cells The inner cell mass of a blastula is pluripotent. Pluripotent stem cells can differentiate into most of the cell types in a multicellular organism, except placental cell types. The production of these cells occurs in a series of stages (starting with the germ layers), gradually limiting the possible cell types the stem cell can become. Multipotent stem cells Multipotent stem cells are usually only found in adults. Their ability to produce different cell types is often limited. For example, the stem cells in the bone marrow can produce any of the cells in the blood (red blood cells, white blood cells and platelets), but they cannot produce any other cell type.

totipotent stem cell an undifferentiated cell that can later differentiate into any type of cell multipotent stem cell a stem cell that can only differentiate pluripotent stem cell a stem cell that can differentiate into any cell type within a broad group FIGURE 2 A 3D rendering of embryonic stem cells. DRAFT ONLY - NOT FOR SALE into a limited number of closely related cell types

unipotent stem cell

a stem cell that can only form one cell type on division

Unipotent stem cells

The most common cells in the human body are fully differentiated cells or unipotent stem cells . When they reproduce, their daughter cells have the same function as the parent cell. They can only be used for the regeneration of their own kind of cells.

Study tip

Toti potent stem cells can form the tot al range of cells. Pluri potent stem cells can form plur al or many cells. Multi potent stem cells can form multi ple types of cells. Uni potent stem cells can only form one type of cell.

Totipotent Pluripotent Oocyte (including sperm) Morula Blastocyst Inner cell mass Multipotent Inner cell mass Ectoderm Mesoderm Endoderm Unipotent DRAFT ONLY - NOT FOR SALE Skin Nerves Bones Muscles Liver Thyroid

FIGURE 3 The process of forming the different cells that become more specialised with each step

Bioethical considerations

There are bioethical concepts to consider when using embryonic stem cells. Stem cell research offers great hope for treatment of diseases such as diabetes, spinal cord injury and Parkinson’s disease. However, many people are opposed to the used of human embryos. Some ethical considerations are outlined in Case studies 3.5A and 3.5B and Challenge 3.5.

CASE STUDY 3.5A

Induced pluripotent cells There are ethical considerations that may stop people using embryonic stem cells. Because of this, scientists have looked at ways of using adult stem cells and have developed a method of forcing (inducing) a multipotent adult cell to become a pluripotent cell. This is done by using a virus to introduce new genetic material into donor adult cells. The new genetic material causes inactive DNA to become active, giving the cells the chance to differentiate into a new type of cell. In this way, skin cells have become induced pluripotent cells that differentiated into new heart muscle cells and even started synchronised beating. This process is not easy. It is only 0.01–0.1% efficient, which means that only one cell in 1000–10 000 is transformed into a pluripotent cell. It has been suggested that many of these cells could start undergoing uncontrolled growth and form tumours. There have been some human trials in using these cells to repair aged-related muscular degeneration, with limited success. Adult broblasts

Reprogramming factors Induced pluripotent cells Cardiac muscle cells Nerve cells Blood cells Bone cells FIGURE 4 Adding new genetic material to differentiated adult cells can induce pluripotent abilities. DRAFT ONLY - NOT FOR SALE

CASE STUDY 3.5B

Axolotl conservation versus research needs

Axolotls can harness their own pluripotent stem cells at any point in their development, which means they can replace a lost limb once, twice or even 100 times. This incredible ability means that axolotls are one of the most scientifically studied salamanders. Unfortunately, axolotls are critically endangered in the wild because of urban sprawl and pollution of freshwater environments. It is estimated that only a few hundred now reside in the canals of Mexico City, their only habitat. However, these animals are abundant in captivity; they are largely bred for home aquariums and research purposes. Scientists want to understand and harness their unique capabilities. Axolotls have become a model for tissue regeneration and cancer treatments. Inbreeding in captivity has resulted in a loss of genetic diversity, increasing the chance of the populations contracting diseases. A few scientists are now breeding and releasing axolotls into the Mexican canals in an attempt to save the species and increase genetic diversity of wild populations. Axolotls’ resilience to diseases and injuries make them one of the most interesting critically endangered population regeneration projects to date. critically endangered population regeneration projects to date. DRAFT ONLY - NOT FOR SALE

FIGURE 5 Axolotls can harness their own pluripotent stem cells to replace lost limbs.

CHALLENGE 3.5

Waste not, want not?

Describe and explain

1 Define ‘stem cell’. 2 Use an example to explain: a totipotent stem cells b pluripotent stem cells c multipotent stem cells d unipotent stem cells. 3 Read Case study 3.5B. Name the type of stem cells that axolotls use to regenerate lost limbs.

Justify your answer.

Apply, analyse and compare

4 Bone marrow transplantation was one of the first stem cell treatments used. Explain why this is an effective treatment for leukaemia (cancer of the blood) patients. 5 Compare the advantages and disadvantages of using embryonic stem cells or adult stem cells for the repair of damaged spinal tissue in a patient with quadriplegia.

When some couples have difficulty becoming pregnant, they may use in vitro fertilisation. This process involves generating embryos by artificially fertilising an egg with a sperm in a Petri dish before implanting it in the mother. To increase the chances 6 Explain the advantage of using induced pluripotent cells for repairing damaged tissue. Design and discuss 7 Read Case study 3.5A. Discuss how an induced pluripotent cell could express some ‘cancer-like’ properties. 8 Consider the ethical reasons for using embryonic stem cells to treat adult diseases and disabilities. 9 If you had a choice, would you use scientific funding to research the regenerative properties of axolotls and help discover more effective ways of combating cancer or would you use the funding to help with conservation efforts to save the wild populations of axolotl in Mexico City? 10 Scientists have recently added a gene to skin cells that allows the cells to be induced into embryonic stem cells. The scientists then grew the cells for 10 days into a blastocyst (the start of an embryo). Discuss the ethical considerations that this process raises (i.e. just because we can, should we?). CHECK YOUR LEARNING 3.5 of success, many eggs are usually fertilised and the zygote that is considered the most viable is implanted. This requires an embryologist to grade the embryos by examining their morphological features under a microscope and assigning a quality score to each one. The parents may decide what to do with the remaining embryos. The unused embryos may be kept in storage, disposed of, used for research, or donated to another person or couple. Some people argue that these embryos should be used as stem cells to replace damaged tissue in other patients, or to produce vaccines. Other people suggest that they are potential humans and need to be given the chance to survive. 1 Describe the ethical advantages and disadvantages for each of the four choices about what to do with remaining embryos: store, dispose of, use for research or donate. 2 Which decision would you make? Explain the reasoning behind your decision. DRAFT ONLY - NOT FOR SALE