B2.3 Cell specialisation - stem cells student reading
These panels contain information about stem cells. This information could be used for the pairs activity as it covers all the details from the DP Biology guide.
B2.3.1 - Following fertilization, stem cells are produced by mitosis and then develop into specialized cells by differentiation. Concentration gradients (of gene transcription factors can switch on genes) leading to specialisation of cells in early embryo development.
B2.3.2 - Stem cells can divide endlessly and differentiate along different pathways.
Stem cells are cells that divide by mitosis to form either two stem cells, thus increasing the size of the stem cell "pool", or one daughter cell that goes on to differentiate, and one daughter that retains its stem-cell properties.
It is still unknown exactly which cell becomes which. However, several genes have been found whose activity prevents a daughter cell from differentiating.
The cells of the inner cell mass can differentiate along many different pathways and much research has been carried out using Drosophila fly embryos. Gradients in the concentrations of gene transcription factors have been shown to affect the genes of the cells. Some genes are promoted by high concentrations, others are inhibited by high concentrations, and so the genes which are switched on (expressed) in one cells will be different from those which are active in another cell. This way the cells in different places can follow different pathways of differentiation and become different specialized cells.
B2.3.3 - Location and function of stem cell niches in adult humans includes:
- Bone marrow
- Hair follicles are suitable examples.
The stem cell niche can maintain the cells or promote their proliferation and differentiation.
Although all living cells have many things in common — such as a plasma membrane and cytoplasm — different types of cells have their own unique structures and functions. Cells with different functions generally have different shapes that suit them for their particular job.
All types of cells have come from division of the first zygote cell, and they were produced by the division and then the differentiation of stem cells.
Even as adults, humans maintain stem cells in stem cell niches. These cells are multipotent stem cells as they can only produce a limited range of cell types. Research is looking at trying to reprogramme these multipotent stem cells in the hope of providing new treatments to repair damaged tissues.
Two well known examples of stem cell niches in the human body are the bone marrow which makes a range of blood cells and hair follicles which make the cells of the growing hair. (see below)
Bone marrow
The bone marrow has an extensive blood supply and many cavities where stem cells proliferate and differentiate.
Haematopoietic stem cells in the bone marrow have the unique ability to give rise to all of the different mature blood cell types and tissues.
This diagram gives just a small selection of cell types produced to illustrate the importance of this stem cell niche.
B2.3.4 - Differences between totipotent, pluripotent and multipotent stem cells
- Cells in early-stage animal embryos are totipotent but soon become pluripotent.
- Stem cells in adult tissue such as bone marrow are multipotent.
Types of Stem Cells
Several adjectives are used to describe the stem cells. These words relate to the number of different kinds of differentiated cell that they can become.
- Totipotent cells. In mammals, totipotent cells have the potential to become any type in the adult body and any cell of the extraembryonic membranes (e.g., placenta). The only totipotent cells are the fertilized egg and the first 4 or so cells produced by its cleavage (as shown by the ability of mammals to produce identical twins, triplets, etc.).
Stem cells can usually replace themselves by dividing, but in mammals, the expression "totipotent stem cells" isn't quite right because totipotent cells cannot make more of themselves. Therefore the term "Totipotent cells" is better.
- Pluripotent stem cells. These are true stem cells, with the potential to make any differentiated cell in the body (but probably not those of the placenta which is derived from the trophoblast). Pluripotent stem cells occur naturally: Embryonic Stem (ES) Cells. These can be isolated from the inner cell mass (ICM) of the blastocyst — the stage of embryonic development when implantation occurs. For humans, excess embryos produced during in vitro fertilization (IVF) procedures are used. Harvesting ES cells from human blastocysts is controversial because it destroys the embryo, which could have been implanted to produce another baby (but often was simply going to be discarded).
- Multipotent stem cells. In adult tissue such as bone marrow there are stem cells with can become several different types of related cells, for example lymphocytes and phagocytes. There are also cells in the hair follicles of mammals which are also pluripotent. When the hair is growing they migrate to the bulb region, where they proliferate (divide) and differentiate to form cells of the inner and outer sheath, hair matrix, and hair shaft.
Test yourself with these questions
Which is the best description of stem cells?
Strictly speaking stem cells should be able to divide continuously, and so in essence they much produce new stem cells when they divide.
What is the function of gradients in the concentrations of gene transcription factors in early embryo development?
Different concentrations of transcription factors in different parts of the embryo body cause different sets of genes to be transcribed.
Which of the following statements are true and which are false?
| A totipotent cell can differentiate to become any type of cell. | ||
| A pluripotent cell forms in the early embryo from division of totipotent cells. | ||
| Multipotent cells can differentiate to become any cell type. |
Multipotent cells can only become one of a limited type of cells, these are found in stem cell niches in adults.
Which of the following are both stem cell niches in mammals?
The two examples given in the Biology guide are hair follicles and bone marrow. There are other stem cell niches.
Which of the following happens rapidly after the formation of a zygote in mammals?
The first divisions of the zygote form totipotent cells which soon become pluripotent stem cells. These cells divide by mitosis and begin to differentiate. They do migrate, but growth happens later, once the embryo is implanted in the uterus.
Credit:
Some of the content of this page has been adapted from the page titled 16.12: Regulating Gene Expression in Cell Development - Gene Expression in Stem Cells is shared under a CC BY-SA 4.0 licence and was authored, remixed, and/or curated by Boundless.