In the early stages in the life of an organism, fairly unspecialised cells divide to produce more cells.

The nuclei in these cells undergo the process of mitosis, which ensures that the genetic material (DNA) they contain is faithfully copied and passed on to daughter cells. As a result, all the cells in each organism contain identical genetic material.

As these cells develop, they gain certain features by acquiring or modifying parts of the cell ('sub-cellular structures') which make them specialised in order to carry out particular functions. Different genes within their genetic material are activated to control this process, for instance by producing different proteins.

In animals, this differentiation occurs at an early stage, and as growth in volume proceeds, cell division is required to increase cell numbers.

An early embryo is just a ball of identical cells, derived from the division of a zygote (egg fertilised by a sperm) but in a matter of just a few days these cells divide and move, forming into three layers which take up different tasks within the organism: The outer layer forms a covering layer - the skin - and part of it rolls up and sinks down a little, forming nervous tissue. In a middle layer beneath this, cells develop into different tissues such as muscle and bone, and in the central layer the digestive system develops.

Of course all these developments of general cells into specialised cells and tissues to perform particular functions are effectively one-way transitions.

When an animal becomes an adult, ths cell division slows down or stops. As cells wear out, they need to be replaced, and wounds also need to be repaired. These processes rely on cell division of undifferentiated cells such as stem cells.

Within bone marrow cells are dividing continuously to provide red and white blood cells. Red blood cells only last for about 120 days, and before they enter into the blood circulation they produce the protein haemoglobin, then eject their nuclei and take on a disk-like shape with a dimple in the middle, which enables them to pass along the smallest blood vessels.

Skin wears away at the surface and is replaced from below. Hair grows continuously and is replaced by division from a layer in the skin Similarly, cells lining the gut are subject to wear and tear so they need to be replaced by cell division.

Cell specialisations in the human body

Cell type	Special features
Skin cells	- waterproof (keratin) - hairs (keratin) - pigmentation (melanin)
Blood cells	- red blood cells have haemoglobin, no nuclei, round shape - lymphocytes produce antibodies - phagocytes can change shape to engulf invading microorganisms
Neurones (Nerve cells)	- tube-like extensions to carry impulses
Bone cells	- produce a flexible outer matrix of collagen - this is reinforced ('mineralised') by calcium phosphate
Muscle cells	- protein fibres (actin and myosin) to contract and change cell length

Plant cells also increase in number and differentiate as they grow, but there is sometimes less limitation to this growth.

It is said that many types of plant cells retain the ability to differentiate throughout life. In specialised regions known as meristems, cells divide repeatedly, causing growth.

For instance, meristems at root tips and shoot tips cause these structures to grow in length, and meristems in cambium tissue within vascular bundles can produce more xylem and phloem tissue, resulting in 'secondary thickening' within plant stems.


Cell specialisations in green plants

Cell type	Special features
upper and lower leaf epidermis	- no chloroplasts - allow light to pass - produce waxy layer (protection)
mesophyll cells in middle of leaf	- many chloroplasts - to carry out photosynthesis
paired guard cells	- uneven cell wall thickness - curvature opens stomata - chloroplasts (photosynthesis activates this)
phloem cells	- perforated end plates for cytoplasmic strands - no nuclei
companion cells	- connection with phloem cells alongside
xylem cells	- thickened side cell walls - reinforced with lignin - but end walls lost