Stem cells

Types of stem cells

• zygotic stem cell (totipotent)

• embryonic stem cells (pluripotent)

• blood stem cell (pluripotent)

• adult stem cell e.g myeloid and lymphoid stem cell (multipotent, derived from blood stem cell)

• neural stem cell (multipotent)

characteristics of stem cell

1. self-renewal by mitosis

   • asymmetric (one cell with same differentiation potential, one pregenitor cell with limited differentiation potential)

   • symmetric (both are genetically identical with same differentiation potential)

2. undifferentiated

   • i.e can be differentiated through differential gene expression

   • e.g RBC progenitor can undergo differentiation to become a RBC

3. unspecialised

   • do not have cell specific features to carry out specific function

zygotic stem cell

• derived from a ball of cells called the morula

• they are totipotent

  • can differentiate into any cell type to form whole organisms, including the extra-embryonic tissues (e.g placenta)

embryonic stem cells

• derived from the inner cell mass of the blastocyst

• they are pluripotent

  • can differentiate into most types of cell except extra-embryonic tissues

• functions is to divide and differentiate into multiple specialized cells to give rise to specific organs

  • specifically, they give rise to the embryonic germ layers — endoderm, mesoderm, ectoderm

  • different tissues and organs are derived from these layers

differences between zygotic and embryonic stem cells

Feature	Zygotic stem cells	Embryonic stem cells
Source of cells	Derived from the cells of the morula	Derives from the inner cell mass of the blastocyst
Potency	Totipotent – they can divide and then their progenitor cells can differentiate into any cell type including extra-embryonic tissues such as placenta, amnion, chorion	Pluripotent – they can divide and then their progenitor cells can differentiate into any cell type except extra-embryonic tissues such as placenta, amnion, chorion
Normal function	Divide via mitosis and differentiate to form a blastocyst that eventually develops into the whole organism	Divide via mitosis and differentiate into specialised cell types to form specific organs of the organism
Use in therapy	Not used in therapy; Difficult to harvest due to the small number of cells in the morula (~16) and their rapid division to form a blastocyst (~2-3 days)	Use in therapy; Easier to harvest from the blastocyst and have approximately the same differentiation potential as zygotic stem cells.

adult stem cells

• derived from specialized tissue e.g myeloid and lymphoid stem cells are derived from bone marrow

• they are multipotent

  • can differentiate into specific lineage of cells

• undergo asymmetric division

• functions is to continuously generate differentiated cells for growth, repair and maintenance of tissues

myeloid and lymphoid stem cells

• derived from blood stem cells in bone marrow which is pluripotent

• they are multipotent as they can differentiate into various mature blood cells

  • myeloid can differentiate into RBC

  • lymphoid can differentiate into natural killer cells

• functions is daily replacement of worn out blood cells from normal wear and tear, fighting infections and disease and injury

describe the features of myeloid and lymphoid stem cells and discuss their use in treatment of leukemia

• they divide continuously by mitosis to produce new stem cells

• they are undifferentiated by have the ability to differentiate where there is an increase in the structural complexity of the cell, accompanied by a more specific function

• they are unspecialized and do not have cells specific structures to carry out specific functions

• they are derived from specialized tissues like from bone marrow

• they are multipotent as they are able to differentiate into a limited range of cell type by differential gene expression and produce only cells of a specific lineage

treatment:

• blood stem cells can be obtained from patient or a donor and preserved

• most cancerous cells in patient are killed during chemotherapy

• the stem cells are then re-injected after treatment is completed, where they are able to asymmetrically divide to form stem cells and progenitor cells which differentiate into RBCs, WBCs and platelets to keep the body healthy and help fight infections

allogenic stem cell transplant

• from donor

• pro:

  • stem cells are free of cancer cells

• cons:

  • risk of tissue rejection by the recipient

  • may have difficulty finding a match

autologous stem cell transplant

• from patient

• pros:

  • no risk of tissue rejection as paitent’s pwn cells are being used

  • no need to find a matching donor

• cons:

  • stem cells harvested may not be free of cancer cells and may reintroduce cancer cells into the body

Somatic Cell Nuclear Transfer (SCNT)

• involves removing the nucleus of an unfertilised egg, replacing it with the nucleus of a somatic cell of a donor

• it is then stimulated by artificial stimulation like electrical impulses

• when put in surrogate mother → reproductive cloning

• when allowed to grow into embryos then used to develop into kidney, lungs, heart etc → therapeutic cloning

Induced pluripotent stem cells (iPSCs)

• specialised adult cells are transformed using genes for transcription factors to enable expression of genes important for having embryonic stem cell-like properties I.e self-renewal, undifferentiated, unspecialised

• they have the same properties as ES cells but are not embryonic stem cells

advantages of using iPSCs in patients

As iPSCs are derived from patient,

• there will be no tissue rejection

• there is no need for intake of immunosuppressant drugs to prevent tissue rejection which makes patient vulnerable to opportunistic infections

• it overcomes the problem of finding suitable donors who is a genetic match for the patient

• there is no risk of infection from donor tissue

• there are less ethical issues associated with their use than embryonic stem cells as they do not involve the destruction of embryos

suggest how insertion of genes coding for transcription factors can cause a differentiated cell to become pluripotent

• the trancription of inserted genes and translation of mRNA results in the synthesis of transcription factors

• which will then activate or inhibit expression of specific genes whose proteins enable cell to divide continuously, showing pluripotent characteristcs.

explain how telomerase may contribute to the re-programming of mouse skin cells into stem cells

• telomerase plays a part in maintaining the length of telomeres

• which counteracts the end-replication problem

• allowing the cell to divide continuously

ethical implications of use of stem cells

1. destruction of embryos

   • extraction of pluripotent stem cells from inner cell mass involved destruction of a blastocyst

   • some people may view the early embryo as a potential human being hence this is seem as anti-life

2. creating embryos for stem cell research and therapy through SCNT

   • deliberately creating embryos and destroying them in the process of research is seen by pro-life groups as violating respect for human life

3. medical risks of oocyte retrival for SCNT

   • oocytes have to be retrieved from women’s ovaries for in-vitro development of ES cells via SCNT

   • as it is an invasive procedure, it carries a high risk of developing complications

4. safety and efficiency of treatment

   • when stem cells are used in disease treatment, there is a question of safety and efficiency in treatment and there may also be unknown long-term consequences