Stem cells are a type of undifferentiated cells that are able to differentiate into a specialised cell.
Commonly stems cells come from either the bone marrow of an adult or the inner cell mass of an early stage embryo.
Adult Stem Cells:
Adult stem cells can be found in the brain, bone marrow, blood and blood vessels, skeletal muscles and liver of adults.
These stem cells have no hayflick limit and can divide indefinitely, however it is commonly thought that depending on what tissue they originated from, they are limited in what cells they can differentiate into. However there is some evidence to suggest that they can differentiate to become other cell types.
Embryonic Stem Cells:
Embryonic stem cells come from a four to five day old human embryo. These stem cells are usually taken from an 'extra' embryo that has been left over from in-vitro fertilisation (IVF).
After 4-6 days of dividing, the embryo is in a stage called the blastocyst stage in which some differentiation of cells has occurred.
The blastocyst stage of an embryo consists of an inner cell mass and an outer cell mass (which later becomes part of the placenta) and the inner mass of the embryo is where the stem cells are removed from.
Cells can differentiate in 3 ways:
- A cell can vary in the amount of one particular organelle.
- The shape of the differentiated cell.
- Some of the contents of the cell.
The following are different cells and their adaptations:
Sperm Cells (Spermatozoa)
There are a number of ways in which a sperm cell is specialised for it's function, these include:
1) The cells are thin and small in order to move more efficiently.
2) There are many mitochondria present in the cell to produce the ATP that the cell needs for movement.
3) The cell has a flagellum called a Undulipodium which used the ATP produced by the mitochondria to propel the cell.
4) The head of the cell contains specialised lysosomes called acrosomes which release enzymes and digest the outside of the female egg cell in order to fertilise it.
Red Blood Cells (Erythrocytes)
1) Red blood cells have a biconcave shape. This shape increases the surface area of the cell and thus increases the efficiency of the diffusion of oxygen into the cell.
2) The cell has no nucleus, Golgi apparatus, mitochondria or rough endoplasmic reticulum. This lack of organelles makes more room for the protein 'haemoglobin' (which the cell needs lots of).
Neutrophils (Type of White Blood Cell)
1) The neutrophil cell contains lots of lysosomes which release enzymes to ingest and destroy invading organisms.
2) The cell has a lobed shaped nucleus for greater flexibility. This flexibility allows them to change shape easily so that they can effectively surround and engulf the organism.
Ciliated Epithelium is found in the trachea, uterus and bronchi.
1) The cells of the ciliated epithelim tissue are column shaped.
2) The part of the cell surface that is exposed have tiny projections called 'cilia' which move in a synchronised manner in order to move mucus (with bacteria and particles in). For example, in the trachea the cilia wave mucus to the back of the throat so that the bacteria and particles can be swallowed and killed by the stomach acid.
Squamous Epithelium lines the alveoli and blood vessels.
1) They are very thin which makes diffusion more efficient and quick.
2) They are very smooth which makes it easier for fluid to pass over them (such as blood in the blood vessels).
Root Hair Cells
Root hair cells are found in the roots of plants and their function is to absorb water and minerals.
1) Each root hair cell has a large projection which increases the surface area of the cell and therefore increases how much water it can take in.
2) Each cell has lots of mitochondria that produce ATP which fuel the active transport of water into the root hair cell.
Palisade cells are found in the mesophyll in leaves of dicotyledonous plants.
1) They contain lots of closely packed chloroplasts which aid photosynthesis.
2) Their cylindrical shape allows large amounts of liquid to be absorbed.
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