cell division, cell diversity and cellular organisation

Question 1

the cell cycle

Answer 1

Mitosis is part of a precisely controlled process known as the cell cycle

The cell cycle is the regulated sequence of events that occurs between one cell division and the next

The cell cycle has three phases:
interphase
nuclear division (mitosis)
cell division (cytokinesis)

The length of the cell cycle is very variable depending on environmental conditions, the cell type and the organism
For example, onion root tip cells divide once every 20 hours (roughly) but human intestine epithelial cells divide once every 10 hours (roughly)

The movement from one phase to another is triggered by chemical signals called cyclins

Question 2

interphase

Answer 2

During Interphase the cell increases in mass and size and carries out its normal cellular functions (eg. synthesising proteins and replicating its DNA ready for mitosis)

Interphase consists of three phases:
G1 phase
S phase
G2 phase

It is at some point during the G1 phase a signal is received telling the cell to divide again
The DNA in the nucleus replicates (resulting in each chromosome consisting of two identical sister chromatids)

This phase of the interphase stage of the cell cycle is called the S phase – S stands for synthesis (of DNA)
The S phase is relatively short
The gap between the previous cell division and the S phase is called the G1 phase – G stands for growth

Cells make the RNA, enzymes and other proteins required for growth during the G1 phase
Between the S phase and next cell division event the G2 phase occurs

During the G2 phase, the cell continues to grow and the new DNA that has been synthesised is checked and any errors are usually repaired

Other preparations for cell division are made (eg. production of tubulin protein, which is used to make microtubules for the mitotic spindle)
Interphase = G1 + S + G2

Question 3

m phase

Answer 3

Follows interphase
Referred to as the M phase – M stands for mitosis
Cell growth stops during the M phase
nucleus division

Question 4

cytokinesis

Answer 4

Follows M phase
Once the nucleus has divided into two genetically identical nuclei, the whole cell divides and one nucleus moves into each cell to create two genetically identical daughter cells
In animal cells, cytokinesis involves constriction of the cytoplasm between the two nuclei and in plant cells, a new cell wall is formed

Question 5

regulation of the cell cycle

Answer 5

It is essential that the DNA within new cells is accurate in order for them to carry out their function
When the DNA is replicated (during the S phase) errors can occur

There are several checkpoints throughout the cell cycle where the genetic information contained within the replicated DNA is checked for any possible errors

Specific proof-reading enzymes and repair enzymes are involved in this checking process

When possible enzymes will repair the error but in some cases the cell may destroy itself to prevent passing on harmful mutations
There are four checkpoints in the cell cycle:

During G1 phase - chromosomes are checked for damage. If damage is detected then the cell does not advance into the S phase until repairs have been made

During S phase - chromosomes are checked to ensure they have been replicated. If all the chromosomes haven’t been successfully replicated then the cell cycle stops

During G2 phase - an additional check for DNA damage occurs after the DNA has been replicated. The cell cycle will be delayed until any necessary repairs are made

During metaphase - the final check determines whether the chromosomes are correctly attached to the spindle fibres prior to anaphase

Question 6

stages of mitosis

Answer 6

Mitosis is the process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent cell nucleus (they have the same number of chromosomes as the parent cell)

Although mitosis is, in reality, one continuous process, it can be divided into four main stages

These stages are:
Prophase
Metaphase
Anaphase
Telophase

Most organisms contain many chromosomes in the nuclei of their cells (eg. humans have 46) but the diagrams below show mitosis of an animal cell with only four chromosomes, for simplicity

The different colours of the chromosomes are just to show that half are from the female parent and half from the male parent

Question 7

prophase

Answer 7

Chromosomes condense and are now visible when stained

The chromosomes consist of two identical chromatids called sister chromatids (each containing one DNA molecule) that are joined together at the centromere

The two centrosomes (replicated in the G2 phase just before prophase) move towards opposite poles (opposite ends of the nucleus)

Spindle fibres (protein microtubules) begin to emerge from the centrosomes (consists of two centrioles in animal cells)

The nuclear envelope (nuclear membrane) breaks down into small vesicles
The nucleolus disappears

Question 8

metaphase

Answer 8

Centrosomes reach opposite poles

Spindle fibres (protein microtubules) continue to extend from centrosomes

Chromosomes line up at the equator of the spindle (also known as the metaphase plate) so they are equidistant to the two centrosome poles

Spindle fibres (protein microtubules) reach the chromosomes and attach to the centromeres
This attachment involves specific proteins called kinetochores

Each sister chromatid is attached to a spindle fibre originating from opposite poles

Question 9

anaphase

Answer 9

The sister chromatids separate at the centromere (the centromere divides in two)

Spindle fibres (protein microtubules) begin to shorten

The separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibres (protein microtubules)

Question 10

telophase

Answer 10

Chromosomes arrive at opposite poles and begin to decondense

Nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes

The spindle fibres break down
New nucleoli form within each nucleus

Question 11

significance of mitosis

Answer 11

Mitosis is the process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent nucleus

The process of mitosis is of great biological significance and is fundamental to many biological processes:

Question 12

mitosis significance in growth of organisms

Answer 12

The two daughter cells produced are genetically identical to one another (clones) and have the same number of chromosomes as the parent cell

This enables unicellular zygotes (as the zygote divides by mitosis) to grow into multicellular organisms

Growth may occur across the whole body of the organism or be confined to certain regions, such as in the meristems (growing points) of plants

Question 13

mitosis in replacement of cells

Answer 13

Damaged tissues can be repaired by mitosis followed by cell division

As cells are constantly dying they need to be continually replaced by genetically identical cells

In humans, for example, cell replacement occurs particularly rapidly in the skin and the lining of the gut

Some animals can regenerate body parts, for example, zebrafish can regenerate fins and axolotls regenerate legs and their tail amongst other parts

Question 14

mitosis in asexual reproduction

Answer 14

Asexual reproduction is the production of new individuals of a species by a single parent organism – the offspring are genetically identical to the parent

For unicellular organisms such as Amoeba, cell division results in the reproduction of a genetically identical offspring

For multicellular organisms, new individuals grow from the parent organism (by cell division) and then detach (‘bud off’) from the parent in different ways

This type of reproduction can be observed in different plant, fungi and animal species
Some examples of these are budding in Hydra and yeast and runners from strawberries

Question 15

meiosis

Answer 15

Meiosis is a form of nuclear division that results in the production of haploid cells from diploid cells
It produces gametes in plants and animals that are used in sexual reproduction

It has many similarities to mitosis however it has two divisions: meiosis I and meiosis II

Within each division there are the following stages: prophase, metaphase, anaphase and telophase

Question 16

prophase 1

Answer 16

DNA condenses and becomes visible as chromosomes

DNA replication has already occurred so each chromosome consists of two sister chromatids joined together by a centromere

The chromosomes are arranged side by side in homologous pairs
A pair of homologous chromosomes is called a bivalent

As the homologous chromosomes are very close together the crossing over of non-sister chromatids may occur. The point at which the crossing over occurs is called the chiasma (chiasmata; plural)
In this stage centrioles migrate to opposite poles and the spindle is formed

The nuclear envelope breaks down and the nucleolus disintegrates

Question 17

metaphase 1

Answer 17

The bivalents line up along the equator of the spindle, with the spindle fibres attached to the centromeres

The maternal and paternal chromosomes in each pair position themselves independently of the others; this is independent assortment

This means that the proportion of paternal or maternal chromosomes that end up on each side of the equator is due to chance

Question 18

anaphase 1

Answer 18

The homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends of the spindle
The centromeres do not divide

Question 19

telophase 1

Answer 19

The chromosomes arrive at opposite poles
Spindle fibres start to break down

Nuclear envelopes form around the two groups of chromosomes and nucleoli reform

Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I

Question 20

cytokinesis

Answer 20

This is when the division of the cytoplasm occurs
Cell organelles also get distributed between the two developing cells

In animal cells: the cell surface membrane pinches inwards creating a cleavage furrow in the middle of the cell which contracts, dividing the cytoplasm in half

In plant cells, vesicles from the Golgi apparatus gather along the equator of the spindle (the cell plate). The vesicles merge with each other to form the new cell surface membrane and also secrete a layer of calcium pectate which becomes the middle lamella.

Layers of cellulose are laid upon the middle lamella to form the primary and secondary walls of the cell
The end product of cytokinesis in meiosis I is two haploid cells

Question 21

second division of meiosis (11)

Answer 21

There is no interphase between meiosis I and meiosis II so the DNA is not replicated
The second division of meiosis is almost identical to the stages of mitosis

Prophase II
The nuclear envelope breaks down and chromosomes condense
A spindle forms at a right angle to the old one

Metaphase II
Chromosomes line up in a single file along the equator of the spindle

Anaphase II
Centromeres divide and individual chromatids are pulled to opposite poles
This creates four groups of chromosomes that have half the number of chromosomes compared to the original parent cell

Telophase II
Nuclear membranes form around each group of chromosomes

Cytokinesis
Cytoplasm divides as new cell surface membranes are formed creating four haploid cells

Question 22

significance of meiosis

Answer 22

Having genetically different offspring can be advantageous for natural selection
Meiosis has several mechanisms that increase the genetic diversity of gametes produced

Both crossing over and independent assortment (random orientation) result in different combinations of alleles in gametes

Question 23

meiosis crossing over

Answer 23

Crossing over is the process by which non-sister chromatids exchange alleles

Process:
During meiosis I homologous chromosomes pair up and are in very close proximity to each other
The non-sister chromatids can cross over and get entangled
These crossing points are called chiasmata

The entanglement places stress on the DNA molecules

As a result of this a section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome

This swapping of alleles is significant as it can result in a new combination of alleles on the two chromosomes

There is usually at least one, if not more, chiasmata present in each bivalent during meiosis

Crossing over is more likely to occur further down the chromosome away from the centromere

Question 24

independent assortment in meiosis

Answer 24

Independent assortment is the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I

The different combinations of chromosomes in daughter cells increases genetic variation between gametes
In prophase I homologous chromosomes pair up and in metaphase I they are pulled towards the equator of the spindle

Each pair can be arranged with either chromosome on top, this is completely random
The orientation of one homologous pair is independent / unaffected by the orientation of any other pair

The homologous chromosomes are then separated and pulled apart to different poles
The combination of alleles that end up in each daughter cell depends on how the pairs of homologous chromosomes were lined up

To work out the number of different possible chromosome combinations the formula 2n can be used, where n corresponds to the number of chromosomes in a haploid cell
For humans this is 223 which calculates as 8 324 608 different combinations

Question 25

random fusion of gametes in meiosis

Answer 25

Meiosis creates genetic variation between the gametes produced by an individual through crossing over and independent assortment
This means each gamete carries substantially different alleles

During fertilization, any male gamete can fuse with any female gamete to form a zygote

This random fusion of gametes at fertilization creates genetic variation between zygotes as each will have a unique combination of alleles

There is an almost zero chance of individual organisms resulting from successive sexual reproduction being genetically identical

Question 26

specialised cells

Answer 26

In complex multicellular organisms, eukaryotic cells become specialised for specific functions

These specialised eukaryotic cells have specific adaptations to help them carry out their functions
For example, the structure of a cell is adapted to help it carry out its function (this is why specialised eukaryotic cells can look extremely different from each other)

Structural adaptations include:
The shape of the cell
The organelles the cell contains (or doesn’t contain)
For example:
Cells that make large amounts of proteins will be adapted for this function by containing many ribosomes (the organelle responsible for protein production)

Question 27

erythrocytes

Answer 27

Function: transport oxygen around the body and carbon dioxide to the lungs

Adaptations:
They are biconcave in shape which increases the surface area over which oxygen can be absorbed

The cytoplasm contains high amounts of the pigment haemoglobin which can readily bind to oxygen

No nucleus is present which makes more space inside the cell for haemoglobin molecules for maximum oxygen-carrying capacity

Elastic membrane allows the cell to be flexible and change shape as it squeezes through narrow capillaries

Question 28

neutrophils

Answer 28

Function: destroy pathogens by phagocytosis and the secretion of enzymes

Adaptations:
Neutrophils have a very flexible shape that allows them to squeeze through cell junctions in the capillary wall
Their flexibility also enables them to form pseudopodia (cytoplasmic projections) that engulf microorganisms

There is a large number of lysosomes present in the cell. These digestive enzymes help to digest and destroy invading cells

A flexible nuclear membrane further helps the cell to penetrate cell junctions. It is thought that this flexibility is what causes the characteristic lobed nucleus

Question 29

sperm cells

Answer 29

Function: reproduction - to fuse with an egg, initiate the development of an embryo and pass on fathers genes

Adaptations:
The head contains a nucleus that contains half the normal number of chromosomes (haploid, no chromosome pairs)

The acrosome in the head contains digestive enzymes that can break down the outer layer of an egg cell so that the haploid nucleus can enter to fuse with the egg’s nucleus

The mid-piece is packed with mitochondria to release energy (via respiration) for the tail movement

The tail rotates, propelling the sperm cell forwards and allowing it to move towards the egg

Question 30

root hair cells

Answer 30

Function: absorption of water and mineral ions from soil

Adaptations:
Root hair to increase surface area (SA) so the rate of water uptake by osmosis is greater (can absorb more water and ions than if SA were lower)

Thinner walls than other plant
cells so that water can move through easily (due to shorter diffusion distance)

Permanent vacuole contains cell sap which is more concentrated than soil water, maintaining a water potential gradient

Mitochondria for active transport of mineral ions
Remember that chloroplasts are not found in these cells – there’s no light for photosynthesis underground!

Question 31

ciliates epithelium

Answer 31

Function: moving substances across the surface of a tissue

Adaptations:
Have cilia (hair-like structures), which beat in a coordinated way to shift material along the surface of the epithelium tissue

Goblet cells secrete mucus which helps to trap dust, dirt and microorganisms - preventing them from entering vital organs where they may cause infection

Question 32

squamous epithelium

Answer 32

Function: provide a surface covering or outer layer. Found on a variety of organs and structures e.g. blood vessels and alveoli

Adaptations:
Squamous epithelium consists of a single layer of flattened cells on a basement membrane

The layer of cells forms a thin cross-section which reduces the distance that substances have to move to pass through - it shortens the diffusion pathway

It is permeable, allowing for the easy diffusion of gases

Question 33

palisade cells

Answer 33

Function: provide a surface covering or outer layer. Found on a variety of organs and structures e.g. blood vessels and alveoli

Adaptations:
Squamous epithelium consists of a single layer of flattened cells on a basement membrane

The layer of cells forms a thin cross-section which reduces the distance that substances have to move to pass through - it shortens the diffusion pathway

It is permeable, allowing for the easy diffusion of gases

Question 34

guard cells

Answer 34

Function: control the opening of the stomata to regulate water loss and gas exchange

Adaptations:
Inner cell walls are thicker (those facing the air outside the leaf) while the outer cell walls are thinner (those facing adjacent epidermal cells). The difference in the thickness of the cell walls allows the cell to bend when turgid

The cytoplasm has a high density of chloroplasts and mitochondria. Scientists think that these organelles may play a role in the opening of the stomata

Question 35

the organisation of cells

Answer 35

In multicellular organisms, specialised cells of the same type group together to form tissues

A tissue is a group of cells that work together to perform a particular function. For example:
Epithelial cells group together to form epithelial tissue (the function of which, in the small intestine, is to absorb food)
Muscle cells (another type of specialised cell) group together to form muscle tissue (the function of which is to contract in order to move parts of the body)

Different tissues work together to form organs. For example:
The heart is made up of many different tissues (including cardiac muscle tissue, blood vessel tissues and connective tissue, as well as many others)
Different organs work together to form organ systems

Question 36

xylem vessel cells

Answer 36

Function: transport tissue for water and dissolved ions

Adaptations:
No top and bottom walls between cells to form continuous hollow tubes through which water is drawn upwards towards the leaves by transpiration

Cells are essentially dead, without organelles or cytoplasm, to allow free movement of water

Outer walls are thickened with a substance called lignin, strengthening the tubes, which helps support the plant

Question 37

phloem vessel cells

Answer 37

Function: transport of dissolved sugars and amino acids

Adaptations:
Made of living cells (as opposed to xylem vessels which are made of dead cells) which are supported by companion cells

Cells are joined end-to-end and contain holes in the end cell walls (sieve plates) forming tubes that allow sugars and amino acids to flow easily through (by translocation)

Cells also have very few subcellular structures to aid the flow of materials

Question 38

muscle cells

Answer 38

Function: contraction for movement

Adaptations:
There are three different types of muscle in animals: skeletal, smooth and cardiac (heart)

All muscle cells have layers of protein filaments in them, these layers can slide over each other causing muscle contraction

Muscle cells have a high density of mitochondria to provide sufficient energy (via respiration) for muscle contraction

Skeletal muscle cells fuse together during development to form multinucleated cells that contract in unison

Question 39

ciliated epithelium

Answer 39

Function: moving substances across the surface of a tissue

Adaptations:
Have cilia (hair-like structures), which beat in a coordinated way to shift material along the surface of the epithelium tissue

Goblet cells secrete mucus which helps to trap dust, dirt and microorganisms - preventing them from entering vital organs where they may cause infection

Question 40

squamous epithelium

Answer 40

Function: provide a surface covering or outer layer. Found on a variety of organs and structures e.g. blood vessels and alveoli

Adaptations:
Squamous epithelium consists of a single layer of flattened cells on a basement membrane

The layer of cells forms a thin cross-section which reduces the distance that substances have to move to pass through - it shortens the diffusion pathway

It is permeable, allowing for the easy diffusion of gases

Question 41

cartilage

Answer 41

Function: to provide support

Cartilage is a strong and flexible tissue found in various places around the body

One place is in rings along the trachea, called Tracheal rings
These rings help to support the trachea and ensure it stays open while allowing it to move and flex while we breathe

Question 42

features of a stem cell

Answer 42

A stem cell is a cell that can divide (by mitosis) an unlimited number of times

Each new cell (produced when a stem cell divides) has the potential to remain a stem cell or to develop into a specialised cell such as a blood cell or a muscle cell (by a process known as differentiation)

This ability of stem cells to differentiate into more specialised cell types is known as potency

There are three types of potency:
Totipotency – totipotent stem cells are stem cells that can differentiate into any cell type found in an embryo, as well as extra-embryonic cells (the cells that make up the placenta). The zygote formed when a sperm cell fertilises an egg cell is totipotent, as are the embryonic cells up to the 16-cell stage of human embryo development

Pluripotency – pluripotent stem cells are embryonic stem cells that can differentiate into any cell type found in an embryo but are not able to differentiate into extra-embryonic cells (the cells that make up the placenta)

Multipotency – multipotent stem cells are adult stem cells that have lost some of the potency associated with embryonic stem cells and are no longer pluripotent

Question 43

multi potent adult stem cells

Answer 43

As tissues, organs and organ systems develop, cells become more and more specialised

Having differentiated and specialised to fulfil particular roles, most adult cells gradually lose the ability to divide until, eventually, they are no longer able to divide

However, small numbers of stem cells (known as adult stem cells) remain to produce new cells for the essential processes of growth, cell replacement and tissue repair

Although these adult stem cells can divide (by mitosis) an unlimited number of times, they are only able to produce a limited range of cell types – they are multipotent
For example, the stem cells found in bone marrow are multipotent adult stem cells – they can only differentiate into blood cells (red blood cells, monocytes, neutrophils and lymphocytes)

In adults, multipotent stem cells can be found throughout the body (eg. in the bone marrow, skin, gut, heart and brain)

Research is being carried out on stem cell therapy, which is the introduction of adult stem cells into damaged tissue to treat diseases (eg. leukemia) and injuries (eg. skin burns)

Question 44

stem cells in the bone marrow

Answer 44

The stem cells found in bone marrow are multipotent adult stem cells
This means they can only differentiate into erythrocytes (red blood cells), monocytes, neutrophils and lymphocytes

Question 45

erythrocytes

Answer 45

Erythrocytes are red blood cells, the main function of which is the transport of oxygen around the body (and also the transport of carbon dioxide)

As red blood cells lack a nucleus, they cannot divide, meaning that new erythrocytes are constantly being formed from bone marrow stem cells in order to maintain the red blood cell count in the blood

This process is known as erythropoiesis

Question 46

structure and function of a erythrocyte

Answer 46

During erythropoiesis, changes occur that adapt the structure of the original stem cell to enable it to function as an erythrocyte

These adaptations include:
The changing of the cell into a biconcave shape: this shape has a larger surface area, allowing for more oxygen to be absorbed through the cell surface

The building up of haemoglobin in the cytoplasm: haemoglobin is the pigment that binds with oxygen and only releases it when oxygen concentrations decrease below a certain level

The ejection of the nucleus (and other organelles including mitochondria, endoplasmic reticulum and Golgi apparatus): creates more room in the cytoplasm for haemoglobin, increasing the oxygen-carrying capacity

An elastic membrane: this allows erythrocytes to change shape and therefore squeeze through narrow capillaries

Question 47

neutrophils

Answer 47

The same stem cells that form erythrocytes also form neutrophils (a type of white blood cell)
As the stem cells differentiate into neutrophils, the main changes that occur include:

Indentations form in the nucleus, giving it a lobed structure

Granules accumulate (these are lysosomes that contain hydrolytic enzymes)

Question 48

structure and function of neutrophils

Answer 48

Neutrophils are the first white blood cells to arrive at an infection site on or in the body

They exit the blood through the tiny gaps in capillary walls and collect around foreign bodies (e.g. pathogens)

They then destroy these by engulfing them (phagocytosis) and digesting them using their hydrolytic enzymes

The adaptations of neutrophils include:

A flexible shape and a flexible nuclear membrane: this allows neutrophils to fit between capillary wall cells and to form pseudopodia (the extensions of the cytoplasm that engulf foreign bodies during phagocytosis)

Containing many lysosomes: these contain digestive enzymes that destroy invading cells

Question 49

meristems

Answer 49

Xylem vessels and phloem sieve tubes form the transport systems of plants and are found throughout their roots and stems

The xylem and phloem are formed from stem cells that are found in the tissue between them
This tissue is known as the cambium

The cambium is a meristem, which is the term given to any undifferentiated tissue in a plant that has the ability to give rise to new cells
For example, there are also meristems located at the tips of shoots and roots that provide new cells to these growing parts of the plant

Question 50

the cambium

Answer 50

In the roots and stems of plants, the stem cells at the inner edge of the cambium differentiate into xylem cells and the stem cells at the outer edge of the cambium differentiate into phloem cells

Cambium cells that differentiate to form the xylem lose their cytoplasm, deposit lignin in their cell walls and lose their end cell walls

Cambium cells that differentiate to form the phloem lose some of their cytoplasm and organelles, and develop sieve plates (located at ends of the cells)

This cell differentiation is stimulated by hormones (the balance of different hormones can determine whether xylem or phloem tissue is produced)

Question 51

the use of embryonic stem cells

Answer 51

Due to their ability to differentiate into multiple cell types, stem cells have huge potential in the therapeutic treatment of disease

For many countries, such as the USA and some countries within the EU, the use of embryonic stem cells is banned, even for research
In other countries, such as the UK, the use of embryonic stem cells is allowed for research but is very tightly regulated

Embryonic stem cells can be one of two potencies:

Totipotent if taken in the first 3-4 days after fertilisation

Pluripotent if taken on day 5
The embryos used for research are often the waste (fertilised) embryos from in vitro fertilisation treatment
This means these embryos have the potential to develop into human beings

This is why many people have ethical objections to using them in research or medicine

Question 52

the use of multi potent adult stem cells

Answer 52

Adult stem cells can divide (by mitosis) an unlimited number of times but they are only able to produce a limited range of cell types

A small number of adult stem cells are found in certain tissues within the body such as:
Bone marrow - used to produce different types of blood cell
Brain - used to produce different types of neural and glial cells

These small numbers of stem cells remain to produce new cells for the essential processes of growth, cell replacement and tissue repair

Research is being carried out on stem cell therapy, which is the introduction of adult stem cells into damaged tissue to treat diseases (e.g. leukaemia) and injuries (e.g. skin burns)

The use of adult stem cells is less controversial than embryonic stem cells because the donor is able to give permission

For example, many people donate bone marrow to help treat leukaemia patients

However, if multipotent stem cells are being donated from one person to another they need to be a close match in terms of blood type and other body antigens

There is a chance that the cells used are rejected by the patient’s immune system
Ideally, the patient’s own adult stem cells are used to treat them, as there is a much lower chance of rejection