Cell differentiation: Most types of animal cell differentiate at an early stage.

The idea

In animals, differentiation mostly happens while the organism is still developing as an embryo. Early embryo cells are unspecialised and can divide by mitosis to produce more cells; as development proceeds, these cells become specialised for particular jobs. Once most animal cells have differentiated, they usually lose the ability to divide and cannot change into a different cell type. This is why mature nerve and muscle cells, for example, are permanently specialised. A few animal cells, such as some stem cells in bone marrow, remain unspecialised and can still divide and differentiate to replace worn-out cells. By contrast, many plant cells keep the ability to differentiate throughout the plant's life, which is a useful comparison in exam answers.

What earns the marks

• Most animal cells differentiate early in development, while the organism is still an embryo.
• Early embryo cells are unspecialised and divide by mitosis before they become specialised.
• After differentiation, most animal cells cannot divide again or change into another cell type.
• Some animal cells remain unspecialised, for example stem cells in bone marrow, and can divide and differentiate to replace cells.
• Early specialisation allows an animal embryo to develop the range of tissues and organs it needs.

Common mistakes and how to put them right

• Saying all animal cells differentiate at an early stage.

  Put it right: Most do, but some stem cells remain unspecialised and can differentiate later.

• Believing differentiated animal cells can change into any other cell type.

  Put it right: Most lose this ability once specialised.

• Confusing 'early stage' with 'before fertilisation'.

  Put it right: Differentiation happens after the fertilised egg begins to divide and develop.

• Stating that plant cells also lose the ability to differentiate early.

  Put it right: Many plant cells retain it throughout life.

Exam tips

• Use the qualifier 'most' when describing animal cell differentiation, and give an exception such as bone marrow stem cells.
• Contrast animal and plant differentiation in one sentence to show the difference clearly.
• Link early differentiation to the idea that an embryo needs many specialised tissues as it grows.
• Avoid absolute words such as 'all' or 'never' unless they are scientifically correct.