Stem cells and an introduction to cell differentiation and specialisation

Specialised cell examples are described, their different functions explained

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

 This page will help you answer questions such as ...

 Why do cells become specialised? What is the function of an egg cell?

 What is the function of a sperm cell? What are stem cells?

Why are cells described as haploid or diploid?

How can stem cell research help certain medical conditions?

Know and understand that cells may be specialised to carry out a particular function - differentiation.

Pre-reading: Introduction to plant and animal cell structure and function gcse biology revision notes

These notes about cell specialisation and specialised cell function are developed in detail on other pages.



The specialisation of cells

Undifferentiated cells are called stem cells and develop into all the different types of cells an organism needs.

On the right is a diagram of the basic structure of an animal stem cell which has the features characteristic to most cells e.g. membrane, nucleus of DNA/RNA for instructions, mitochondria for respiration, cytoplasm, vacuoles, ribosomes for making proteins etc.

A stem cell nucleus contains ALL the instructions to switch genes 'on and off' so it has the ability to change into any specialised cell needed by an organism.

Multicellular organisms (eukaryotic) contain a variety of cells, with different structures, which are adapted - specialised, to perform a variety of functions.

Differentiation is the process by which a cell develops into its specialised role.

Cells which have a particular structure adapted for a particular function are called specialised cells.

In cell differentiation, cells become specialised by switching genes off and on to form tissues with particular functions.

In the process of differentiation the stem cells develop different sub-cellular structures to turn into the different types of cells - specialisation.

The specialised cells can now carry out their specific function.

Most differentiation occurs as an organism develops - stem cells are found in early human embryos.

In most animal cells, this ability to differentiate is lost at early stage of development once the cells have become specialised. But, most plant cells retain the ability to differentiate throughout the life of the plant.

The cells that differentiate in mature animals are mainly used for replacing damaged/dead cells e.g. skin or blood cells.

Skin cells live for 14 to 21 days and blood cells have a lifetime of 80 to 120 days.

Other cells, called stem cells, are undifferentiated. See later section on stem cells and their potential uses


Examples of specialised animal cells - adapted to their functions

Note that stem cells are undifferentiated and have not changed into a specialised cells in the developing embryo.

Gamete cells - the function of egg cells and sperm cells in sexual reproduction

Egg cells and sperm cells are the specialised cells of sexual reproduction.

In sexual reproduction the nucleus of an egg cell fuses with the nucleus of a sperm cell to produce a fertilised egg.

The fertilised egg develops into an embryo.

Both the sperm cell and an egg cell are referred to as being haploid, because their nuclei only contain half the number of chromosomes that you find in a normal body cell.

This ensures that when the egg and sperm nuclei combine at fertilisation the created cell will have the right number of chromosomes (now referred to as a diploid cell).

 

The egg cell - its structure and its adapted functions

  Simple diagram of egg cell

The principal function of the egg cell is to convey the female DNA and to provide nutrients for the developing embryo in the early stages of the organism's development.

The egg cell has a haploid nucleus (half set of chromosomes) and the cytoplasm contains the nutrients to feed the growing embryo.

Immediately after fertilisation the egg cell's membrane changes structure to stop another sperm getting into the egg cell and this ensures the right amount of DNA is present in the fertilised cell.

See also CELL DIVISION - cell cycle - mitosis and meiosis in sexual reproduction

 

The sperm cell - its structure and its adapted functions

  Simplified diagram of sperm cell

The sperm cell, like the egg cell has a haploid nucleus (half of the full set of required chromosomes).

The principal function of a sperm cell is to convey the male's DNA to the female's egg.

The sperm cell has a long tail and streamlined head to enable it to swim to the egg.

A sperm cell contains lots of mitochondria (sites of respiration - releasing energy) in the middle section to provide the energy for it to swim to the egg cell.

At the front of the head of a sperm cell is an acrosome where enzymes are stored. These enzymes are needed so that the sperm cell can digest its way through the membrane of the egg cell to fertilise it.

See also CELL DIVISION - cell cycle - mitosis and meiosis in sexual reproduction

 

Ciliated Epithelial Cells

  Simple diagram of epithelial cells

Organ surfaces are lined with epithelial cells. Some types of epithelial cells are adapted with hairs called cilia on the top of the cell's surface. The function of these ciliated epithelium cells is to move substances in one particular direction along the surface of the tissue. The hair-like structure of the cilia beat to move the material along. A good example is the lining of your air passage, the surface of which is covered in lots of epithelial cells. The 'beating' cilia move mucous and any particles from air trapped on the surface up the throat and away from your delicate lungs. This allows the mucous to be swallowed or blow out through your nose.

 

Muscle cells

Muscle cells form soft tissue found in most animals, they are relatively long and must be able to contract quickly.

They contain protein filaments of actin and myosin that slide past one another.

This adaptation produces a contraction that changes both the length and the shape of the cell.

The contraction can be reversed and allows muscle tissue cells to function in such a way as to produce force and motion.

Muscle cells contain lots of mitochondria to supply the larger amounts of energy needed to work the muscles.

 

Nerve cells

Neurones are nerve cells that carry information as tiny electrical signals from one part of the body to anther e.g. between the brain and muscles.

These sensory neurone cells are elongated and adapted to cover the relatively 'long' distances over which they have to carry the nerve signals from receptors to the spinal cord and brain.

Nerve cells have branched connections at their ends to form a network of connections throughout the body.

See An introduction to the nervous system including the reflex arc  gcse biology revision notes

 

Red blood cells

Red blood cells are adapted to carry oxygen via the haemoglobin molecules inside them. Without this adapted cell transportation of oxygen you could not have efficient energy releasing respiration in mitochondria.

 

White blood cells

White blood cells are part of the immune system. White blood cell can ingest disease-causing bacteria to destroy them produce antibodies to destroy pathogens. White cells can change shape to engulf a microbe.

 


Specialised Plant cells

Root hair cells - are adapted to absorb water and minerals from soil and then through the root system to transport these minerals around the plant. Root hair cells are long and cover the surface of plant roots to create a large surface area to absorb water and minerals.

Xylem cells - are not living cells, but rod-like cells that form hollow tubes that can move water and dissolved minerals from the roots around the plant.

Phloem cells - phloem vessels (columns of living cells) move dissolved sugars, produced during photosynthesis, and other soluble food molecules from the leaves to growing tissues (e.g. the tips of roots and shoots) and storage tissues (e.g. in the roots).

Palisade leaf cells - their structure is adapted to support the sites of photosynthesis. They allow light to be absorbed and contain chloroplasts, subcellular structures that contain the chlorophyll needed for photosynthesis.

Guard leaf cells - can open and close the pores (stomata) in leaves and allow oxygen and carbon dioxide to pass in and out.

For details see Transport and gas exchange in plants, transpiration, absorption of nutrients, leaf and root structure


Stem cells

Most types of animal cells differentiate at an early stage whereas many plant cells retain the ability to differentiate throughout life.

In mature animals, cell division is mainly restricted to repair and replacement.

Most differentiation occurs as an organism develops - stem cells are found in early human embryos.

Initially, the cells in an embryo are all the same and referred to as embryonic stem cells.

Stem cells are undifferentiated and have not changed into a specialised cells in the developing embryo.

Since they are unspecialised, they are able to divide and produce any type of specialised cell (like those described so far on this page), but stem cells lose this ability as the animal matures.

In human embryos the cells are unspecialised as far as the eight cell stage after three cell divisions by mitosis.

The process of stem cells becoming specialised is called differentiation.

Cell differentiation enables the embryo to grow and develop tissues - groups of specialised cells working together to perform a particular function e.g. skin, muscle, organs etc.

Adults have stem cells in their bone marrow but these can only be converted into a few specific type of cells - so only quite limited specialisation is possible.

The stem cells in the bone marrow are important in replacing dead cells e.g. producing new red blood cells.

All body cells contain the same genes, this differs from specialised cells in which most genes are not active.

This means specialised cells only produce the specific proteins they need.

Stem cells can switch any gene 'on' or 'off' during their development.

Genes which are switched on ('active') facilitate the production of proteins that will determine the type of specialised cell a stem cell becomes.


The potential uses of stem cells

Cells from early human embryos and adult bone marrow, called stem cells, can be made to differentiate into many different types of cells, e.g .nerve cells - so have the potential to be converted into any type of cell found in the human body.

However, whereas early embryonic stem cells can differentiate into any type of specialised cell, adults only have stem cells in a few places like bone marrow and these can only turn into a few types of cell e.g. blood cells.

Since human stem cells have the ability to develop into any kind of human cell, they have the potential use for treating certain medical conditions.

As the embryo develops, the stem cells divide, producing more stem cells, but also differentiated cells - the process of differentiation in which cells for a specific specialised function are produced e.g. cells for skin, organ tissue, blood cells etc.

It is possible to extract stem cells from early human embryos and adult bone marrow and reproduce them under particular conditions so that they differentiate into particular types of specialised cells.

It is possible to grow the embryonic stem cells and stimulate them to differentiate into specialised cells for use in further research or medical applications.

In the laboratory you can produce clones of stem cells - genetically engineered identical cells.

These cells could be used to replace diseased damaged tissue or tissue damage from injury or disease e.g. initiate new nerve connections, transplant new cardiac muscle cells to replace cardiac tissue of people suffering from heart disease.

Quite simply, there is huge potential from stem cell research and application to alleviate many medical conditions, which up to now, have been very difficult to treat - hence the huge scientific interest in the potential for new cures - but such strategies are not without risks and hot debates on the ethical issues involved.

e.g. treatment with stem cells may be able to help conditions such as paralysis and it is hoped to be able to grow nerve cells for people disabled by a spinal injury.

Adults have stem cells in their bone marrow but these can only be converted into a few specific type of cells - so only quite limited specialisation is possible.

The stem cells in the bone marrow are important in replacing dead cells e.g. producing new red blood cells.

You may have heard the phrase 'bone marrow transplant' - this involves treating a patient with a supply of healthy stem cells to differentiate into specific healthy cells to replace damaged or faulty cells e.g. blood cells.

A bone marrow transplant is a gene therapy procedure that involves replacing damaged bone marrow with healthy bone marrow stem cells.

Stem cells in bone marrow produce three important types of blood cells : red blood cells which carry oxygen around the body, white blood cells which help fight infection and platelets which help stop bleeding.

Bone marrow transplants are used to treat sufferers of leukaemia, non-Hodgkin's lymphoma and sickle cell anaemia.

One of the latest developments is called therapeutic cloning. An embryo can be modified to have the same genetic information as the patient. This means these stem cells have the same genes and less likely to be rejected by the patient's body/

Some risks

Stem cells divide very quickly and if the speed of cell division cannot be controlled inside a patient a tumor may develop.

Inside cells viruses can co-exist with their host. If, unknowingly, donor stem cells are infected with a virus, this virus can be passed on to the patient under treatment worsening their condition.

Stem cell research and its applications are very controversial, despite the obvious great medical benefit to individual patients.

The ethical issue of using embryos for medical purposes is abhorrent to some people who would argue that human embryos shouldn't be used to provide stem cells because the embryo is destroyed in the process - removing one that had the potential for human life.

This is the argument of 'potential life' versus help for seriously ill 'living people' i.e. each embryo has the potential to develop into a human being, but equally potently, using embryonic stem cells might save a life. In other words the rights of suffering patients overrides the rights of the embryo.

It is possible to use unwanted embryos from fertility clinics because there is no other source of universal stem cells and these unwanted embryos would be destroyed. The unwanted embryos often come from fertility clinics and would be destroyed if not used for research purposes - but this argument would not satisfy campaigners want to completely ban the use of human embryos.

Many campaigners believe scientific research should be directed towards finding and developing other sources of stem cells and avoid the use of human embryos.

Stem cell research is allowed in some countries like the UK, but there are very strict rules and guideline as to how it can be carried out. Stem cell research is completely banned in some countries.

 


Plant stem cells - the production of identical plants

In plants, the only cells that divide by mitosis are found in plant tissues called meristems.

Meristem tissue is found in the tips of roots and shoots - the parts of plants that are growing.

Meristems make unspecialised cells that can divide and form any type of cell.

These unspecialised cells are effectively acts as 'embryonic' stem cells.

However, unlike human stem cells, they can differentiate to form any type of cell for the lifetime of the plant.

These unspecialised cells can form the specialised tissue cells of the xylem and phloem.

Plant stem cells can be used to make clones - so identical genetic copies of a plant can be grown quickly and cheaply.

This means plant growers can grow crops of identical plants that have been genetically engineered to have more desirable features for farmers e.g. increased size of wheat grain, more disease resistant - but GM crops are also a controversial topic too!

Cloning has an important application in preserving rare species of plants - to grow more of those in danger of becoming extinct.

See Transport and gas exchange in plants, transpiration


Pre-reading: Introduction to plant and animal cell structure and function gcse biology revision notes

These notes about cell specialisation and specialised cell function are developed in detail on other pages.


GCSE Cell biology revision notes index

Stem cells and an introduction to cell specialisation gcse biology revision notes

Stem cells and an introduction to cell specialisation gcse biology revision notes

Microscopy - the development and use of microscopes in biology  gcse biology revision notes

Diffusion, osmosis, active transport, exchange of substances - examples fully explained

RESPIRATION - aerobic and anaerobic in plants, fungi and animals, conditions, substrates etc.  gcse biology

ENZYMES - structure, function, optimum conditions, investigation experiments  gcse biology revision notes

CELL DIVISION - cell cycle - mitosis and meiosis in sexual reproduction    gcse biology revision notes

Genetics - from DNA to GM and lots in between!

DNA and RNA structure and Protein Synthesis  gcse biology revision notes

GENOME - gene expression - considering chromosomes, alleles, genotype, phenotype, variations

An introduction to genetic variation and the formation and consequence of mutations  gcse biology

Introduction to the inheritance of characteristics and genetic diagrams (including Punnett squares) 


Edexcel gcse 9-1 biology: Be able to describe how specialised cells are adapted to their function, including: (a) sperm cells acrosome, haploid nucleus, mitochondria and tail (b) egg cells nutrients in the cytoplasm, haploid nucleus and changes in the cell membrane after fertilisation (c) ciliated epithelial cells


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