Division and Differentiation in Human Cells

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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

2 types:

Embryonic stem cells – Found in early stage of the embryo called the blastocyst. 

Blastocyst

Outer layer of cells

Inner cell mass consisting of stem cells

Embryonic stem cells can differentiate into all types of body 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 Embryonic stem cell Adult (tissue) stem cellSource (where found) Early embryo (blastocyst) Various sites, e.g. skin, red

bone marrowPotential to differentiate Can differentiate into all

cell typesLimited ability to

differentiate, e.g. stem cells in red bone marrow can

only form blood cellsFunction To form all types of body

cells during development of the embryo

To replace body cells, e.g. those of the blood and skin

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.

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.

Chromosomes become visible in the cell nucleus

(Each chromosome has two chromatids)

1Chromosomes line on the cell equator

Spindle fibres form and join to chromosomes

2Spindle fibres contract and pull chromatids apart

3

Cytoplasm divides

4

Outline of mitosis

In humans diploid cells have 23 pairs of homologous chromosomes

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 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.

Germline cells divide by mitosis to produce more germline cells or by meiosis to produce 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.

Chromosomes appear in the nucleus

Homologous chromosomes pair

Second meiotic division

Homologous pairs line on the cell equator

Spindle fibres pull homologous chromosomes to each end of the cell

Cytoplasm divides to end first division

First 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.

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.

Since they divide to produce gametes, mutations in germline cells are passed to offspring.Mutations in somatic cells are not passed to offspring

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 stopdividing when necessary;• In a malignant tumour, they lose the molecules on their surface that normally holdthem in place; they can therefore become detached from their neighbours andspread 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.