Division and Differentiation in Human Cells
Writing in RED indicates the SQA outcomes.
Writing in BLACK explains these outcomes in depth.
Cellular differentiation is the process by which a cell develops more specialised functions by expressing the genes
characteristic for that type of cell.
Once a cell becomes differentiated it only expresses the genes that produce the proteins characteristic for that type of
cell.
Differentiation
Process by which cells become specialised for particular functions.
Involves genes being switched on and off.
A specialised cell will have the genes producing proteins characteristic of that type of cell switched on, in
other cells these genes will be switched off.
Some genes, e.g. those that produce the enzymes that control respiration reactions are switched on in all
cells.
Stem cells are relatively unspecialised cells that can continue to divide and can differentiate into specialised cells of one
or more types.
During embryological development the unspecialised cells of the early embryo differentiate into cells with specialised
functions.
Tissue (adult) stem cells replenish differentiated cells that need to be replaced.
Stem cells
Stem cells are unspecialised cells found in animals that can divide to produce more stem cells or differentiate
into different types of specialised cells.
Stem cells divide to maintain the stem cell pool.
In the early embryo they differentiate to produce cells with specialised functions that make up the different
types of tissues in the body.
After this stage, stem cells differentiate to replace dead or damaged cells in the body.
Stem cells — embryonic and tissue (adult) stem cells.
Adult (tissue) stem cells give rise to a limited range of cell types.
Development of tissue (adult) stem cells in bone marrow into red blood cells, platelets and the various forms of
phagocytes and lymphocytes.
Types of stem cells
Blastocyst
Inner cell mass consisting of
stem cells
2 types:
Embryonic stem cells – Found in early stage of the embryo called the blastocyst.
Embryonic stem cells can differentiate into all types of body cells.
Outer layer of cells
Adult (tissue) stem cells – Adult stem cells (also called tissue stem cells) are found not only in
adults but in children and the foetus.
Adult stem cells can only differentiate into the types of cells in the
tissues they are found in, e.g. stem cells in the bone marrow can only
form different types of blood cells.
Type
Source (where found)
Embryonic stem cell
Early embryo (blastocyst)
Potential to differentiate
Can differentiate into all
cell types
Function
To form all types of body
cells during development
of the embryo
Adult (tissue) stem cell
Various sites, e.g. skin, red
bone marrow
Limited ability to
differentiate, e.g. stem cells
in red bone marrow can
only form blood cells
To replace body cells, e.g.
those of the blood and skin
Somatic cells divide by mitosis to form more somatic cells.
These differentiate to form different body tissue types: epithelial, connective, muscle and nerve.
During cell division the nucleus of a somatic cell divides by mitosis to maintain the diploid chromosome number.
Diploid cells have 23 pairs of homologous chromosomes.
Somatic cells
A somatic cell is a diploid body cell (i.e. not a sex cell or gamete (egg or sperm cell) which are haploid).
When a diploid cell divides by mitosis it produces two diploid cells that have the same chromosomes as the original
parent cell.
Outline of mitosis
1
3
2
Chromosomes
become visible in
the cell nucleus
Chromosomes line on
the cell equator
(Each chromosome
has two chromatids)
Spindle fibres form and
join to chromosomes
In humans diploid cells have 23 pairs of homologous chromosomes
4
Spindle fibres
contract and pull
chromatids apart
Cytoplasm
divides
The body organs are formed from a variety of these tissues.
Epithelial cells cover the body surface and line body cavities, connective tissue includes blood, bone and cartilage cells,
muscle cells form muscle tissue and nerve cells form nervous tissue.
Examples of body tissue
Epithelium
Epithelial cells cover the body surface and line the body cavities and tubular structures, e.g. the windpipe, oesophagus, blood
vessels. The epithelium can consist of one or more cell layers (e.g. skin has several layers)
Connective tissue
In this tissue cells are surrounded by extracellular material(called the matrix). The matrix can be solid (in bone), liquid (in blood) or
fibrous or gelatinous (in cartilage)
Muscle
Has cells able to contract.
There are 3 types of muscle:



Skeletal muscle – move the skeleton
Smooth muscle – in the walls of the gut and large blood vessels
Cardiac muscle – make the heart
Nervous tissue
Composed of nerve cells (neurons).
Body organs are made from a
variety of these tissues, e.g.
the skin is an organ
composed of epithelium,
blood, muscles and nerves.
Germline cells divide by mitosis to produce more germline cells or by meiosis to produce haploid gametes. .
Germline cells
A germline cell is one that can form sex cells (gametes). These cells are diploid, they can divide by mitosis to form more germline
cells or by meiosis to form haploid gametes.
Outline of meiosis
There are two cell divisions.
The first division separates the homologous chromosomes.
Each of the two cells produced from the first division divides again to produce a total of 4 haploid gametes from the original germline cell.
First meiotic division
Chromosomes
appear in the
nucleus
Homologous
chromosomes
pair
Homologous
pairs line on
the cell
equator
Spindle fibres
pull
homologous
chromosomes
to each end of
the cell
Cytoplasm
divides to
end first
division
Second meiotic division
1 cell
produced by
the first
division
Chromosomes
line on cell
equator
Spindle fibres
pull chromatids
apart to opposite
poles of the cell
Cytoplasm
divides
Each diploid germline
cells produces 4
haploid gametes when
it divides by meiosis
Mutations in germline cells are passed to offspring.
Mutations in somatic cells are not passed to offspring.
Since they divide to produce gametes, mutations in germline cells are passed to offspring.
Mutations in somatic cells are not passed to offspring
Research and therapeutic uses of stem cells by reference to the repair of damaged or diseased organs or tissues.
Stem cells can also be used as model cells to study how diseases develop or for drug testing.
The ethical issues of stem cell use and the regulation of their use.
Uses of stem cells
Stem cells can be used:
• To study the development of diseases and disorders at cellular level, e.g. Parkinson’s disease
• To test the effect of new drugs at cellular level in treating diseases and disorders
• In medical treatments and therapies, e.g. use of skin grafts in treating burns, use of bone marrow transplants to
treat, e.g. genetic blood disorders like sickle cell anaemia and thalassaemia, in repair of damaged eye corneas.
Cancer cells divide excessively to produce a mass of abnormal cells (a tumour) that do not respond to
regulatory signals and may fail to attach to each other.
If the cancer cells fail to attach to each other they can spread through the body to form secondary tumours.
. Cancer cells
A cancer is caused by excessive and uncontrolled cell division producing a mass of cancerous cells called a tumour.
Cancer cells look different under the microscope from normal cells.
They also behave differently:
• They divide uncontrollably to produce a mass of abnormal cells
• They do not respond to normal regulatory signals which would instruct them to stop
dividing when necessary;
• In a malignant tumour, they lose the molecules on their surface that normally hold
them in place; they can therefore become detached from their neighbours and
spread through the body to form secondary tumours.
A tumour is benign if the mass of cells stays in one location – most benign tumours can be removed.
A tumour is malignant if some of the cells fail to remain attached and so spread to other parts of the body where they
cause new tumours (secondary tumours).
Cancer cells have undergone mutations, these are increased by agents such as certain pollutants, chemicals in cigarette
smoke and UV radiation.