stem cells

3 general properties of stem cells

1. stem cells are capable of dividing by mitosis and self-renew for long periods

   • they can go through numerous cycles of cell divisions to form more stem cells

   • each new cell has the potential to remain as a stem cell

2. stem cells are unspecialised

   • they do not have a specialised physiological function → unlike red blood cell or muscle cell

3. stem cells can differentiate to give rise to specialised cells

   • differentiation is a process which is controlled by differential switching on and off of genes → involving transcription factors [internal cues]

   • differentiation is also controlled by external signals [external cues]

   • stem cells destined for differentiation exit the cell cycle at G₀ phase

classification of stem cells

there are two ways that stem cells can be classified, based on:

1. potential to give rise to certain types of cells → potency

2. location in the body

potency

stem cells can be classified based on their potential to give rise to different cells

• all cells in the body arise from the single-cell zygote formed after fertilisation

• potency: totipotent, pluripotent and multipotent

totipotent stem cells

having the potential to differentiate into any cell type form whole organisms

• totipotent stem cells are also pluripotent and multipotent

• zygotic stem cells are totipotent

  • include the zygote (single-cell stage) and early embryo cells up to the 8-cell stage (morula) of development in humans

• function: cells differentiate to form all cell types in the body of the organism and the placenta

pluripotent stem cells

having the potential to differentiate into almost any cell type except the placenta

• pluripotent stem cells are also multipotent

• embryonic stem cells (ESC) are pluripotent

  • found in the inner cell mass of a blastocyst

• function: cells differentiate to form almost any cell type in the body of the organism except the placenta

multipotent stem cells

having the potential to differentiate into a limited range of cell type

• lymphoid stem cells and myeloid stem cells are multipotent → classified adult stem cells

  • they can differentiate into a limited range of blood cells

• function: multipotent stem cells give rise to cells to replace dead and worn-out cells in the tissue where are found

location

• stem cells can also be classified based on their location in the body

  • e.g. some adult stem cells, umbilical cord stem cells

adult stem cells

adult stem cells are undifferentiated cells present in differentiated tissue or organ

• functions: they replace dead and worn-out cells in the tissue that they are found → due to wear and tear or old age

• found in both children and adult

blood stem cells → lymphoid and myeloid stem cells

• found in the blood and bone marrow

• are multipotent

• functions

  1. blood cells have limited life spans, ranging from less than a day to a few months, and are continually replaced by the division and differentiation of blood stem cells

  2. lymphoid and myeloid stem cells ensure constant renewal of blood cells

  3. lymphoid stem cells differentiate to form white blood cells like T lymphocyte and B lymphocytes, natural killer cells (for immune response)

  4. myeloid stem cells differentiate to form red blood cells and other types of white blood cells like neutrophils

other examples of adult stem cells

gut epithelial stem cells

• found in the lining of the digestive tract

• are multipotent

skin stem cells

• found in the basal layer of the epidermis and at the base of hair follicles

• are multipotent

bone marrow stromal (mesenchymal) cells

• found in bone marrow

• also known as mesenchymal stromal cells (MSC)

• are multipotent

neural stem cells

• found in the brain

• are multipotent

umbilical cord blood stem cells

• cord blood contains blood stem cells

• are multipotent

• function: cord blood stem cells ensure constant renewal of foetal blood

comparison between totipotent, pluripotent and multipotent

	totipotent	pluripotent	multipotent
examples	zygotic stem cells	embryonic stem cells → cells derived from the ICM of the blastocyst	• lymphoid stem cells • myeloid stem cells
self-renewal capability	limited self-renewal capability, because they no longer exist after a certain stage in the development of the embryo	limited self-renewal capability, because they no longer exist after a certain stage in the development of the embryo	undergo continuous self-renewal throughout the lifetime of the organism
cell specialisation ability and function	can differentiate into any cell type, including the placenta, to form whole organisms	can differentiate into almost any cell type in the body of the organism except the placenta	can differentiate into a limited range of cell type form new cells to replace dead and worn-out cells of the same cell type in the tissue that they are found