Stem cells and potency

Cell Structure, Reproduction and Development · Stem cells, gene expression and phenotype · note 1 of 6

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In short

A stem cell is an undifferentiated cell that can divide by mitosis and differentiate into specialised cells. Totipotent cells can differentiate into any cell type, including the cells of the placenta, and form a whole organism. Pluripotent cells can form any cell type in the body but not extra-embryonic tissues. The morula is a solid ball of cells and the blastocyst is a hollow ball.

Stem cell: An undifferentiated cell that can divide by mitosis to produce more stem cells and can differentiate into specialised cell types.
Totipotent: Able to differentiate into any type of cell, including the cells of the extra-embryonic tissues such as the placenta, so can form a complete organism.
Pluripotent: Able to differentiate into almost any type of cell in the body, but not extra-embryonic tissues, so cannot form a complete organism.
Morula: A solid ball of about 16 cells formed by early cell divisions (cleavage) of the zygote.
Blastocyst: A hollow ball of cells with a fluid-filled cavity, an outer layer of cells that forms the placenta, and an inner cell mass of pluripotent cells.

After fertilisation, the zygote divides repeatedly by mitosis, without growing, to form a morula. Its cells are totipotent at first. Further divisions produce a blastocyst: the outer layer (the trophoblast) forms the placenta and the inner cell mass becomes the embryo. The cells of the inner cell mass are pluripotent. They are known as embryonic stem cells and can be collected and grown in a culture to make specialised cells.

Sequence from a zygote to 2-cell and 4-cell stages, a morula of about 16 totipotent cells, and a blastocyst with an outer layer (trophoblast) that forms the placenta, a fluid-filled cavity and a pluripotent inner cell mass; arrows show inner cell mass cells differentiating into a neurone, a muscle cell and a red blood cell. — Morula cells are totipotent; cells of the inner cell mass are pluripotent.

Exam tip:

The difference to remember: totipotent cells can form any cell including the placenta (the whole organism); pluripotent cells can form any body cell but not the placenta.

Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026

Frequently asked questions

What is the difference between pluripotent and totipotent stem cells?

Totipotent cells, such as those in the early morula, can become any type of cell including the placenta, so they can form a whole organism. Pluripotent cells, such as the inner cell mass of the blastocyst, can become any body cell but not the extra-embryonic tissues.

How does a cell become specialised?

A cell becomes specialised through differential gene expression. Only some of its genes are transcribed to active mRNA, which is translated into specific proteins. These proteins either control cell processes as enzymes or form part of the cell's structure, so the cell takes on a particular shape and function.

How can one gene code for more than one protein?

After a eukaryotic gene is transcribed to pre-mRNA, introns are removed and exons are joined together. By joining the exons in different combinations (alternative splicing), different mRNAs are made from the same gene and are translated into different proteins.

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