B2.3: Cell Specialisation

What is cell differentiation?

The process by which unspecialised cells become specialised by switching certain genes on or off, leading to different proteins and functions.

Do all cells in one organism have the same DNA?

Yes – they have identical DNA, but different genes are expressed in different cells.

What is a zygote?

The diploid single cell formed at fertilisation.

What is a morula?

A solid ball of 16–32 cells formed by mitotic divisions of the zygote.

What is a blastocyst?

A hollow ball of cells formed ~5–6 days after fertilisation, containing:

• Trophoblast (forms placenta)

• Inner cell mass (ICM) (forms embryo)

What are morphogens?

“Form-giving” signalling molecules secreted by certain cells in the embryo that diffuse and form concentration gradients.

How do morphogen gradients affect cell fate?

Cells at different distances from the morphogen source experience different concentrations → activate different genes → become different cell types.

Why are morphogen gradients efficient for development?

They allow complex body patterns to form using relatively few genes, based on positional information.

What are stem cells?

Undifferentiated cells capable of self-renewal and differentiation into one or more specialised cell types.

Name the two key properties of all stem cells.

1. Self-renewal (can divide to make more stem cells)

2. Potency (ability to differentiate into other cell types)

What are totipotent stem cells? Give an example.

• Can form any cell type, including placental tissues

• Can give rise to a complete organism

• Example: first 8 cells of the morula.

What are pluripotent stem cells? Give an example.

• Can form all body cell types, but not extra-embryonic tissues

• Cannot form a whole organism

• Example: inner cell mass cells of the blastocyst, embryonic stem cells, induced pluripotent stem cells (iPSCs).

What are multipotent stem cells? Give examples.

• Can differentiate into a few closely related cell types

• Examples: bone marrow stem cells (→ all blood cells), umbilical cord stem cells.

What are unipotent stem cells? Give an example.

• Can only produce one specific cell type

• Example: liver stem cells → only liver cells.

What are induced pluripotent stem cells (iPSCs)?

Adult differentiated cells (e.g. skin cells) that have been reprogrammed back to a pluripotent state by changing gene expression.

Give one medical application of pluripotent stem cells.

Making retinal cells to treat diseases like age-related macular degeneration or Stargardt’s disease.

How does cell size affect SA:V?

As a cell grows, volume increases faster than surface area, so SA:V decreases.

Why is a high SA:V ratio important for cells?

It allows efficient exchange of gases, nutrients, and wastes across the membrane.

What happens if a cell becomes too large?

• SA:V becomes too low

• Exchange of materials becomes too slow

• Cell may not get enough nutrients or remove waste fast enough → cannot survive.