Differentiation,Cell differentiation,Types of differentiation,Mechanism,Factors,Controls

1. Shri Shankaracharya Mahavidyalaya, Junwani , Bhilai
Differentiation
Dr. Sonia Bajaj
(Head of Department)

2. Cell Differentiation
“It is a biological process where in cells gain specialised roles and switch from one cell
type to another in an entity”
• All cells contain the same DNA so cells initially have the potential to become any type of cell.
• Cell Differentiation is irreversible.
• A cell that is able to differentiate into all cell types of the adult organism is known as pluripotent.
• A cell that is able to differentiate into a total organism with all cell types, including the placental
tissue, is known as totipotent.
• All cells in multicellular organism have the same number of chromosomes and DNA.
• Different parts of the genetic instructions are used in different types of cells.
• influenced by the cell's environment.
• Chemical signals may be released by one cell to influence the development and activity of another
cell.
Examples in Animals
Process of fertilization in animals produces zygote which is totipotent (Totipotent cells are those cells
that can be differentiated into any other cell type). All the complex tissues found in advanced animals
arise from the zygote. In entities, cell differentiation commences early on.
In mammals, totipotent cells, which can differentiate into any types of cells are present in the zygote and
blastomeres (cells after a few divisions). Slowly they start differentiating and giving rise to multicellular
organisms.
In higher plants, meristematic cells are pluripotent cells, they can differentiate into many types of cells,
e.g. root and shoot apical meristem.
In animals, stem cells are pluripotent cells.

3. What is the purpose of cell differentiation?
All cells of multicellular organisms derive from a single cell. By the process of cell
differentiation, cells gain their specific phenotype and functionality at maturity. Cell
differentiation leads to various different types of cells, which perform vital functions.

4. Cellular Differentiation :
• It is the process by which a cell acquires or develops certain properties and functions or
capabilities and becomes a more specialized cell type.
• Differentiation occurs when a simple zygote turns in to a complex system of tissues and
cell types.
• Adult stem cells divide and create fully differentiated daughter cells during tissue repair
and during normal cell turnover.
• Size, shape, membrane potential, metabolic activity, and responsiveness to signals of a
cell change drastically during differentiation .
• These changes are mostly because of highly controlled gene expression .
• With a few exceptions, cellular differentiation almost never involves a change in the
DNA sequence itself.
• Thus, different cells can have very different physical characteristics despite having the
same genome.

5. Specialized Cells-
• Nerve Cells communicate information either by using electric signals
(within a cell) or chemical signals (between cells).
• Muscle cells contain protein filaments that side past one another,
producing a contraction that changes both the length and the shape of
the cell.
• Blood cells are the most common type of blood cell and the vertebrate
organism's principal means of delivering oxygen to the body tissues.
• The process of development involves the division of the fertilization egg into many
cells which assume different size, shape, structure and function.
• The various cells constitute tissues and organs that together form the complete
animal .
• This whole process is called The process of differentiation is irreversible because the
differentiated or specialized cells cannot revert back to undifferentiated stem cells.
• Differentiation is divided into intracellular differentiation and intercellular
differentiation. intracellular differentiation is found in protozoa, in spermatogenesis
and in oogenesis. intercellular differentiation is met in multicellular animals and
plants.

6. Type of differentiation :
Differentiating cells show unique characters in the form of shape, structure, chemical
nature and behaviour. The process of differentiation can be divided into four kinds on the
basis of these differences:
1. Morphological differentiation : The differences acquired in shape and size of cells
during the course of development is called morphological differentiation.
2. Physiological differentiation : This includes the difference in the functional activities
of the cells.
3. Behavioural differentiation: The difference in the behavioural pattern of certain cells
like the nerve cells to be able to transmit impulses and secretion of bile by hepatic cells.
4. Biochemical differentiation : This is the most important kind of differentiation. During
the whole process of cleavage and gastrulation the different areas of the original egg
become biochemically from each other. This biochemical difference results in the
formation of different cells, tissues, etc.

7. Cell Differentiation – Mechanism
Transcription factors are key to the cell differentiation process. The chemicals and
hormones involved determine the course of action revolving around the DNA,
deciding the transcription. The body and cells in the proximity decide the factors
found in cells right from the fetal developmental stage to death. Both the DNA
constituted in a cell and the location of expression of DNA is pivotal.
The cell differentiation process has a range of the transcription factor has a direct
influence on the proteins transcribing the DNA transforming it gradually to operating
proteins and other cells. But, cells signal each other when they start to compress
together indicating the action can no longer proceed.

8. Process involved in differentiation :
1. Totipotency of Nucleus –Thus, countless cells making up the embryo or adult body contain
the same genetic information as possessed by the original zygote. This had been proved by a
member of experiments .
(A) Hans Spemann (1928) Contricted the newt, triturus egg lengthwise with a loop of fine
hair .The construction was not carried out completely, so that the two halves still had a
narrow of cytoplasm connection. The nucleated out of the zygote cleavage, the non-
nucleated half did not after 16 cell stage, a nucleus happened to pass through the cytoplasmic
bridge into non nucleated half .Hence forth, this half also showed cleavage and further
development. Spemann tightened the noose, cutting the two halves of the egg. Each half now
developed into a full fledged larva except that the half with delayed nucleus was delayed in it
development, Spemann concluded that the nuclei remained totipotent.

9. This process takes place in several steps:
i. Fertilization: Fusion of sperm and egg.
ii. Cleavage: Development of zygote to form blastula (group of undifferentiated cells).
iii. Gastrulation:Differentiation and movement of cells to form specialised cell layers.
iv. Differentiation: Development of specialised cells into tissues, organs and growth of the
embryo.
v. Growth, Maintenance and Regeneration of some cells.
These are used as a model in the developmental genetics for the following
reasons:
i. Short life cycle (3 days).
ii. Ease of maintenance like E. coli.
iii. Can reproduce by self or by cross- fertilization.
iv. Hermaphrodite (XX)—contains 5 pairs of autosomes and 1 pair of X chromosomes.
v. The male (XO) contains 5 pairs of autosomes and a single X chromosome.
vi. Haploid genome is about 8 x 107 bp.
vii. More than 600 genes have been identified.
viii. Easy to obtain homozygous populations, as self-fertilization is possible.
ix. In-breeding is automatic in hermaphrodite population.
x. DNA transformation through microinjection at the selected stage of development is
possible in this animal.

10. Level of differentiation:
Rutter, Wesself and other (1976), shows that the both exocrine and endocrine cells
recognize four levels of differentiation. They are :
(i) The undifferentiated state,
(ii) The prodifferentiated state,
(iii) Differentiated state,
(iv) Modulation
For example- cancer cells can be
graded differently depending on their
level of differentiation as poorly
differentiated, moderately
differentiated and well differentiated.

11. Factors causing differentiation:
1. Induction
2. Competence
3. Determination
Control of Differentiation:
1. Genetic amplification
2. Gene regulation by histones
3. Gene expression
Gene expression regulates differentiation :
• Gene expression is regulated by factors both extrinsic and intrinsic to the cell.
• Cell-extrinsic factors include environmental causes, such as small molecules, secreted
proteins, temperature and oxygen.
• Intrinsic factors include growth factors, morphogens, cytokines or signaling molecules.
• Signaling causes changes in transcription or expression of genes that may include turning
off or on of some genes. This can influence the cell fate and cell functions.
• In addition, gene expression changes can lead to changes in an entire organism, such as
molting in insects.

12. Gene expression is said to be the reason. Gene set in all entities is identical as the
genetic code is replicated from the actual egg cell that is fertilized by the sperm cell. To
undertake a specific role, a cell from its genetic code only uses a few of the genes,
ignoring the remaining.
Cell differentiation is primarily influenced by:
Structure of the gene – it is the prime factor for cell differentiation. Every viable gene
possesses crucial instructions which decide the cell type and physical traits of the host.
Any mistake here will influence the cell differentiation process and host-development.
Environmental determinants – temperature-change, oxygen supply and many other
environmental factors have an impact on the working of hormones because of the
different proteins dedicated to transforming information and stimulation of hormones.
Any impact on these molecules will cause the cell differentiation and development
process to get affected.
There are few instances leading to cell differentiation:
• Regular turnover of cells (blood cells in mature entities)
• Immature entity growing into an adult
• Damaged tissues undergoing repair when special cells are to be substituted
• Influence of cytoplasm
• Interaction between cells
• Hormones

13. References-
Modern text book – R.L.Kotpal
Jantu Vigyan- S.M. Sexsena
Jantu Vigyan- Dr.H.N. Baijal