Cell Specialisation and Stem Cells

Differentiation is the process by which an unspecialised cell becomes a specialised cell suited to a particular function.

One genome — differential gene expression. Every body cell carries the same set of genes (the same genome). Cells differ because each cell type expresses a different subset of those genes — switching some genes ON and others OFF. The proteins produced from the active genes determine the cell's structure and function.

• It is the pattern of gene expression, not the genes themselves, that makes a neuron different from a muscle cell.
• This is why a skin cell and a liver cell, with identical DNA, look and behave completely differently.

Specialised cells: structure matched to function.

• Red blood cells (erythrocytes) — biconcave disc with no nucleus and few organelles → maximises space for haemoglobin and surface area for gas exchange; flexible to squeeze through capillaries.
• Sperm cells — a flagellum for swimming, many mitochondria in the midpiece for ATP, and an acrosome of enzymes to penetrate the egg.
• Root hair cells — a long, thin projection that greatly increases surface area for absorbing water and mineral ions from soil.
• Palisade mesophyll cells — column-shaped, packed with chloroplasts near the upper leaf surface to maximise light absorption for photosynthesis.
• Neurons — long axons to transmit electrical impulses over distance.

The recurring theme: form fits function — each adaptation can be explained by the job the cell performs.

Stem cells are unspecialised cells defined by two properties:

1. Self-renewal — they divide repeatedly to make more stem cells.
2. Potency — they can differentiate into one or more specialised cell types.

Types of potency (decreasing range):

• Totipotent — can form every cell type, including the placenta/extra-embryonic tissues. Example: the zygote and the cells of the very early embryo.
• Pluripotent — can form any cell type of the body but not the placenta. Example: embryonic stem cells of the blastocyst.
• Multipotent — can form a limited range of related cell types. Example: haematopoietic (bone-marrow) stem cells forming the different blood cells.
• Unipotent — can form only one cell type but can still self-renew. Example: cells that renew skin or liver.

Sources of stem cells:

• Embryonic — taken from the early embryo (blastocyst); pluripotent and highly versatile, but obtaining them destroys the embryo.
• Umbilical cord blood — collected painlessly at birth; multipotent; easily stored.
• Adult / somatic (tissue) stem cells — found in bone marrow, skin, etc.; usually multipotent or unipotent; an exact tissue match avoids rejection.
• Induced pluripotent stem cells (iPSCs) — adult cells reprogrammed in the lab back to a pluripotent state by switching on key genes; avoid embryo destruction and can be patient-matched.

Therapeutic uses (learn these two named examples):

• Stargardt's disease — an inherited form of macular degeneration that destroys photoreceptors. Retinal cells grown from stem cells can be transplanted to restore vision.
• Leukaemia — a cancer of white blood cells. A bone-marrow (haematopoietic stem-cell) transplant replaces the patient's diseased blood-forming stem cells (after their own are destroyed) with healthy donor stem cells.

Ethical considerations.

• Use of embryonic stem cells destroys an embryo, which raises questions about the moral status of the embryo.
• iPSCs and cord-blood stem cells avoid this objection because no embryo is destroyed.
• Other concerns: risk of tumour formation, source consent, and equitable access to costly therapies.

As a cell grows, its volume increases faster than its surface area. For a cube of side $L$, surface area scales with $L^2$ while volume scales with $L^3$, so the surface-area-to-volume (SA:V) ratio falls as $L$ rises.

• The cell surface membrane controls the exchange of materials (oxygen, nutrients, wastes) and heat.
• Volume sets the demand for those materials and the rate of waste production.
• If a cell gets too large, its surface becomes too small relative to its volume to supply the inside fast enough → exchange becomes limiting.

Consequences:

• Cells stay small to keep a high SA:V ratio.
• Cells specialise their shape to raise SA:V where exchange matters (e.g. the long projection of a root hair cell, the folded membranes of an absorptive cell).
• Larger organisms must be multicellular, dividing labour among many small specialised cells served by transport systems, rather than relying on one giant cell.