Transcript The Cell Cycle

Tissue Culture and Differentiation
Monolayer cells: Most cells tend to grow as monolayers (eg fibroblasts) on
a substratum (such as a petri dish or flask). They grow up until reach 100%
confluence.
Suspension: They are floating cells in the medium (eg haemopoeitic cells,
hybridomas), cells in suspension continue to grow until all of the nutrients in
the medium are exhausted or depleted.
A practical note: A pH indicator indicates that a change in medium is
required (due to nutrient depletion or waste build up) and the cells should
be passaged or subcultured.
Cell migration is one of the important phenomenon in monolayer cell
cultures.
The Cell Cycle
• Four successive phases of standard
eukaryotic cell cycle
• Cell is either dividing or in interphase
• 12 hours or longer
Different Types of Stem Cells
• Totipotent

Pluripotent

Multipotent
Destiny of cells:
(cell growth, immoralization, differentiation, apoptosis, cell death).
Cell growth: Cells in culture usually have a finite lifespan. Most divide about
50 times before they finally stop dividing.
Transformation of cells with virus or oncogene can immortalise cells
so that they continually divide (eg HeLa cells, hybridomas).
Cells may also differentiate in medium. Either spontaneously (eg PSMB
embryonic stem cells) or under the influence of specific differentiation
conditions (eg retinoic acid induces F9 embryonic stem cell
differentiation). Again, growth characteristics may change.
Cell death: (apoptosis or necrosis)
Practical notes: It is advisable to freeze down early passage stocks of
cells (to be thawed later) to avoid this limit.
Transformed cells can become tumorigenic if used in vivo and display a
variety of altered and unique properties that may be unsuitable.
Cell cultures: Primary and transformed
Primary cultures and finite cell lines usually are anchoragedependent and contact inhibited; transformed cells typically are
not.
Primary or finite cells
Transformed cells
Cell Differentiation
• All cells carry out basic functions
– Respiration, growth, division, synthesis
• Most cells have specialized capabilities
– 200 different cells in the human body.
• Cells in their final, differentiated state
usually have very unique
characteristics, reflected in their
morphology
Embryonic stem Cell differentiation:
ES cells ---- Transient cells ---- Matured cells
1- Early differentiation
A- Cell differentiation (specialization) begins early in embryonic development –
corresponds with loss of embryo’s dependence on maternal mRNA (i.e., coincident
with turning on of the embryonic genome; usually about 4-cell+ stage) – individual
embryonic cells (blastomeres) are no longer totipotent
B- Initial recognizable differentiation event involves formation of the blastocyst A:
Inner cell mass (ICM or embryo proper, sometimes called embryonic disc) B:
Trophectoderm (also called trophoblast)
C- Subsequent differentiation/specialization involves formation of the inner germ layer
(endoderm) followed by formation of the middle germ layer (mesoderm)
D- All of the body tissues are developed from these 3 germ layers
E- Stem cells undergo self-renewing divisions (adult stem cells). Tissue-specific
stem cells are found in adult tissues including blood, skin, central nervous system,
liver, gastrointestinal tract, fat and skeletal muscle. They are responsible for
regenerating damaged tissue and maintaining tissue homeostasis.
Potentially Renewing
Cells
• Not constantly renewing
• Return to active cycling (cell
cycle/division) in response to
critical cell depletion
• Renewal by simple duplication of
existing differentiated cells
– division gives rise to daughter cells of
the same type
TRENDS in
Biotechnology
Vol.21 No.8
August 2003
2- Cell maturation/tissue specialization
A- Specificity and acquisition of tissue-specific functions
B- Hallmarks of cell differentiation:
Loss of totipotency
Loss of ability to divide
C- Cell differentiation and cell proliferation (growth) are intimately entwined
Transdifferentiation can be defined as a direct switch of
an already differentiated cell to another type of
differentiated cell.
3- Non-dividing cells
A- Q cells
Cells in G0 – thus reversibly withdrawn from the cell cycle
Can be induced to re-enter the cell cycle by growth factors, e.g.: (a) Tissue
repair (wound healing) (b) Tissue renewal (e.g. partial hepatectomy), etc.
B- Terminally differentiated cells
Fully differentiated, but can no longer divide
Eventually die by a process termed apoptosis
For example, squamous of skin, goblet and brush-border cells of intestinal
epithelium
Genes and differentiation:
Tumor Cells Can Be Induced to Differentiate Terminally by Appropriate
Agents if the Differentiation Gene Is Not Mutated; Conversely, Tumor
Cells with the Mutated Differentiation Gene Can Be Resistant to Terminal
Differentiation.
Tumor Cells Can Be Induced to Differentiate, but the Differentiated
Phenotype Is Reversible upon the Removal of the Inducing Agent Due
to Rapid Hydrolysis of Inducing Signals or to Unresponsiveness of
Regulatory Genes.
PRASAD K.N. et al. 2001
TRENDS in Biotechnology Vol.21 No.8 August 2003