(4) Explaining the role of animal stem cells and human reproduction

Development in the embryo, foetus and growing baby

Stem cells are unspecialised (undifferentiated) cells that divide by mitosis to produce daughter cells of many different types of specialised cells - a process of cell differentiation.

Most types of animal cells only differentiate at an early stage whereas many plant cells retain the ability to differentiate throughout life.

In mature animals, adult cell division is mainly restricted to replacement of damaged or dead cells and adult stem cells cannot differentiate into all the specialised cells needed in an animal.

Most differentiation occurs as an organism develops - embryonic stem cells are found in early human embryos.

Unlike embryonic animal cells, adult animal stem cells can only partially differentiate into a few types of cell e.g. specific tissues to repair or replace damaged cells.

Initially, from the fertilised egg,  the cells in an embryo are all the same and referred to as embryonic stem cells and divide by mitosis (reminder diagram below of the context of discussing stem cells in detail here).

Stem cells are undifferentiated and have not changed into a specialised cells in the developing embryo.

Stem cells are found in the early human embryo as it develops in the womb.

Since they are unspecialised, they are able to divide and ultimately produce any type of specialised cell (like those described so far on this page), but stem cells lose this ability as the animal matures.

It seems remarkable that all the different types of cell found in the human body all come from a few cells in the early embryo!

AND, this emphasises how important stem cells are for growth and development.

In human embryos the cells are unspecialised as far as the eight cell stage after three cell divisions by mitosis.

The process of stem cells becoming specialised is called cell differentiation.

Every type of cell in your body is derived from these stem cells.

Cell differentiation enables the embryo to grow and develop tissues - groups of specialised cells working together to perform a particular function e.g. skin, muscle, organs etc.

Adults also have stem cells in their bone marrow (spongy tissue in bones) but these can only be converted into a few specific type of cells - so only quite limited specialisation is possible.

The stem cells in the bone marrow are important in replacing dead or damaged cells e.g. producing new skin or red blood cells, but they are not as versatile as embryonic stem cells - they cannot produce any type of cell.

All body cells contain the same genes, this differs from specialised cells in which most genes are not active.

This means specialised cells only produce the specific proteins they need.

Stem cells can switch any gene 'on' or 'off' during their development.

Genes which are switched on ('active') facilitate the production of proteins that will determine the type of specialised cell a stem cell becomes

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Summary of learning objectives and key words or phrases

Be able to explain the role of animal stem cells in human reproduction and development of the foetus in the embryo and development into the birth of a baby.

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