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CELL DIFFERENTIATION/EMBRYONIC STEM CELLS Cell differentiation in developmental biology refers to the development of less specialized cells into

the more complex structures. Differentiation occurs numerously when a single cell zygote converts

into a complex number of cells and tissue forming a multicellular organism. Differentiation starts

from the embryonic stem cells and continued to form adult stem cells.

Embryonic stem cells (ECS) are stem cells, which are derived from the undifferentiated inner mass

cells of embryo. They are pluripotent cells as they have potential to differentiate into any of the

three germ layers i.e. endoderm, mesoderm and ectoderm.

ECS are distinguished by two major unique properties:

a) Their pluripotency

b) Ability to separate indefinitely.

Pluripotency distinguishes ESC from adult stem cells found in adults. ECS can generate all cell

types in body whereas; adult stem cells are multipotent and can produce only limited number of cell

types.

Because of their plasticity and potential of repeatedly division (self-renewal) ESC therapies have

been proposed. It helps in formation of regenerative medicines and tissue replacements and many

more.

Some important terms:

a) Totipotent: Cell, which can differentiate into any of the cell type of body plus extra

embryonic/placental cells. For e.g. embryonic cells right after first couple of cell division.

b) Pluripotent: Cells that can form all cell types of body. E.g. Embryonic stem cells.

c) Multipotent: Cells that can differentiate into more than one of the cell type (but more

limited than pluripotent): E.g. cord blood stem cells.

d) Unipotent: Cells that can differentiate into only one cell lineage. E.g Cells of skin

epithelium.

Question: Embryonic stem cells of mammals are derived from:(BHU-2014, 2016)-BT

a) Trophoectoderm b) Inner cell mass

c) Gametes d) Blastocoel

Answer: (b). Embryonic stem cells (ECS) are stem cells, which are derived from the

undifferentiated inner mass cells of embryo. They are pluripotent cells as they have potential to

differentiate into any of the three germ layers i.e. endoderm, mesoderm and ectoderm.

Cell Fate Determination

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In the field of Developmental biology it is important to understand that how does a particular cell

develops into an organism or how its fate is determined. To get the answers of this major question

one need to answer these sub-questions as follows:

a) What force affects the development of embryo?

b) How does the cells differentiate via influence of these forces?

c) How does these cells organize themselves into tissues and organs?

The embryonic environment

External factors: External factors are those, which influence the development of embryo from

outside like temperature, light, humidity, predators and competitors. Lets say an example the effect

of temperature on crocodile eggs. Incubation of eggs at 33 C produces mostly males and

temperature below that results in female. As enzyme aromatase responsible for sex determination

can be effected by temperature.

Internal factors: Internal factors are those, which influence the development of embryo from

inside the body. For e.g.: Maternal hormones like homeotic genes in case of Drosophila

Melanogaster.

Cell Differentiation:

Differentiation is the process in developmental biology, which develops less specialized cells into

the highly specialized cells. This process occurs severally during the course of development of

single zygote into a complex tissue/organ system.

Differentiation is a multi step process, which includes majorly two steps:

a) Specification/ Commitment

b) Determination

Pathway:

Undifferentiated Specification Determination Differentiation

Specified/ Commitment: In the state of specification the cells are capable of differentiating

autonomously when placed in a neutral environment. This means that cells type is not yet

determined and if placed in any bias situation the fate can be transformed to another fate. This is

reversible process.

Determination: Once the cells enter into this stage of differentiation the cell’s fate cannot be

reversed/transformed when kept in any biased situation. It is irreversible in nature. For e.g. if

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embryonic region of brain cells is changed to skin cells it won’t transform as cells become

determined.

There are three ways by which cell becomes specified they are as follows:

a) Autonomous specification

b) Conditional specification

c) Syncytial specification

Autonomous Specification:

This type of specification is the result of cell-intrinsic properties. It is specified by differential

acquisition of certain cytoplasmic determinants during the cleavage of the egg. At this stage, each

blastomere contains positional information and if they are dissociated they forms the original

structure. This gives rise to the mosaic development. It occurs mostly in invertebrates.

Weismann proposed the theory of mosaic development in which he stated that there are nuclear

determinants later called morphogens is present in cell. They are unevenly distributed to different

cells once the cleavage starts they develop into specific fate, which was determined. He performed

these experiment in tunicates and animal development was in mosaic fashion only.

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Roux experiment: He performed a series of defect experiment. He took 2 and 4 cells of frog

embryo and destroys some cells with the hot needle. He obtained half blastula and half neurula. He

therefore concluded, that frog embryo was a mosaic of self-differentiating parts and each cell

received a set of nuclear determinants and differentiated accordingly.

Conditional Specification:

This type of specification is the result if cell-extrinsic properties. It relies on cues and interactions

occurring between cells or due to the concentration gradient of morphogens. Relative positions in

this specification are important as neighboring cells interact via induction to form tissue pattern.

Conditional specification produces regulative development as embryonic cells can change the fate

to compensate other missing parts. It occurs in most of the vertebrates and in some invertebrates.

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Syncytial Specification:

This specification is a combination of both autonomous and conditional specification. Syncytium

refers to the nuclear division, which is not followed by the cell division resulting in cytoplasm with

many nuclei. As there is no cell boundaries in the syncytium the morphogens can affect nuclei in a

concentration dependent fashion. Initially there is no rigid cell fate for particular nuclei but as

cellularization occurs conditional specification comes into the picture.

It occurs mostly in insects.

Question: In an experiment, when a region of prospective cells from an early amphibian gastrula

was transplanted into an area in another gastrula where the neural tissue is normally formed, the

transplanted cells give rise to neural tissue, What kind of development do these transplanted cells

exhibit (JNU-2009)-LS

a) Autonomous b) Mosaic

c) Regulative d) Independent

Answer: (c). Relative positions in this specification are important as neighboring cells interact via

induction to form tissue pattern. Conditional specification produces regulative development as

embryonic cells can change the fate to compensate other missing parts.

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Question: During embryogenesis, differentiation of a cell involves processes like specification and

determination. In this regard, which of the following statements are correct? (JNU-2011)-LS

i) Autonomous specification leads to mosaic development.

ii) Autonomous specification leads to regulative development.

iii) Conditional specification leads to regulative development.

iv) Conditional specification leads to mosaic development.

a) (i) and (iii) b) (ii) and (iv) c) (i) and (iv) d) (ii) and (iii)

Answer: (a). Autonomous leads to the mosaic development and conditional gives rise to regulative

development.

Question: In case of turtles, the temperature at which the eggs are exposed during development is

the deciding factor in sex determination. This is because of the the temperature sensitivity of

(GATE-2007)-LS

a) Estrogen b) Testosterone

c) Aromatase enzyme d) Progesterone

Answer: (c). Incubation of eggs at 33 C produces mostly males and temperature below that results

in female. As enzyme aromatase responsible for sex determination can be effected by temperature.

Question: The ability of the cell or tissue to respond to a specific induction signal is known as

(BHU-2013)-BT

a) Competence b) Equivalence group

c) Receptor d) Potency

Answer: (a). When cells are kept in another environment it is competent enough to perceive the

signal and respond accordingly this property is called as competence.

Question: The ability of cells, to achieve their respective fate of differentiation by interaction with

other cells is known as (BHU-2015)-BT

a) Inductive determination b) Facultative differentiation

c) Autonomous specification d) Conditional specification

Answer: (a). When cells interacts to each other such that it gets induced by other cells and achieve

their respective fate is called inductive determination.

Question: The ability of cells or tissue to respond to a specific induction signal is known as

(BHU-2016)-BT

a) Determination b) Specification

c) Competence d) Differentiation

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Answer: (c). When cells are kept in another environment it is competent enough to perceive the

signal and respond accordingly this property is called as competence.

Question: The ability of cells, to achieve their respective fate by themselves without the influence

of neighboring cells is called as (BHU-2016)-BT

a) Autonomous specification b) Conditional specification

c) Syncytial specification d) Non-conditional specification

Answer: (a). Specified bydifferential acquisition of certain cytoplasmic determinants during the

cleavage of the egg. As neighboring cells do not play any role in the fate determination is called

autonomous specification.

Question: Insect embryo undergo: (BHU-2016)-BT

a) Rotational cleavage forming syncytium

b) Spiral cleavage forming cells of unequal size

c)Superficial cleavage forming syncytium

d) Gastrulation without cleavage divisions

Answer: (c). Superficial cleavage forming syncytium. Syncytium is the phenomenon in which

nucleus divides without proper cell division and one cell becomes multinucleated. This type of

cleavage is seen in insects.

Question: The blastomeres of a two-celled embryo are separated and allowed to develop

independently. Each blastomere gives rise to the young one. This type of development pattern

suggests that potential of development is (JNU-2014)-LS

a) Greater than fate b) less than fate

c) Equal to its fate d)Independent of fate

Answer: (c). Eachblastomere give rise to the young one this means that developmental potential is

equal to its fate because according to fate blastomere is destined to form an individual.

Question: Blastomeres derived from early stages of cleavage divisions of frog embryo, when

separated, can give rise to viable tadpoles. The blastomeres derived from late cleavage stages,

however, fail to develop into normal tadpoles when separated from each other. Which is the most

correct statement that explains the phenomenon (GATE-2006)-LS

a) Each early blastomere carries the entire genetic information for the development of a tadpole.

b) Each early blastomere carries the entire cytoplasmic information for the development of a

tadpole.

c)Early blastomeres are mirror images of each other.

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d) None of the above

Answer: (c). Everyblastomere carries the cytoplasmic determinants itself to develop but when

separated from each other they are not induced hence not able to develop in a normal tadpole.

PRACTICE QUESTIONS

Multiple-choice questions:

1. Sex of crocodile eggs is dependent on (a) Light (b) Temperature (c) Humidity (d) None of the above 2. In cell fate determination stage of specification is ___________ whereas determination state is ___________. (a) Irreversible, Reversible (b) Reversible, Irreversible (c) Reversible, can’t say (depends) (d) Can’t say (depends), Reversible 3. French flag analogy defines the concept of (a) Effect of paternal mRNA (b) Effect of maternal mRNA (c) Effect of morphogens depending on concentration gradient (d) Effect of morphogens depending on temperature gradient 4. Morphogenesis is concerned with (a) Shape of tissue organ and entire organisms (b) Cell growth (c) Cell differentiation (d) All of the above 5. Nuclear determinants are also called as (a) Inducers (b) Organizers (c) Morphogens (d) Maternal genes 6. Tick the correct sequence of cell-fate determination (a) Undifferentiated determined committed differentiated (b) Specification determined undifferentiationdifferentiated (c) Committed undifferentiated determined differentiation (d) Undifferentiated Specification Determined Differentiation

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7. Weismann’s theory of nuclear determinants supports? (a) Autonomous specification (b) Conditional specification (c) Syncytical specification (d) Determination theory

8. If prospective neural ectoderm of late gastrulae transplanted to prospective skin ectoderm region of other gastrula it give rise to ___________, as cells become _? (a) Neural plate, committed (b) Epidermis, committed (c) Neural plate, Determined (d) Epidermis, Determined 9. In 4 cells stage of fog embryo, one blastomere separates and allows to develop it will ___________ ? (a) Not develop as separated from other blastomeres (b) It will develop as each blastomere is capable of regulate its development. (c) Half embryo will form, as other nuclear determinants are present in other blastomeres. (d) Develop in mosaic pattern. 10. Embryonic stem cells are _________ whereas adult stem cells are (a) Unipotent; totipotent (b) Pluripotent; multipotent (c) Multipotent; totipotent (d) Pluripotent; unipotent 11. Embryonic stem cells are derived from? (a) Undifferentiated inner cellmass of embryo (b) Differentiated inner cell mass of embryo (c) Undifferentiated trophoblast cells (d) Differentiated trophoblast cells 12. Which of the following cells would be considered as differentiated cells. (a) Blastomere (b) Spemann organizer (c) Stem cell (d) Muscle cells 13. The end point of a cell’s migration in embryo development is determined by concentration of chemicals called? (a) Regulators(b) Organizers (c) Inducers(d) Morphogens 14. In induction, an inducing tissue directs the target tissue to develop along a particular fate path (a) True (b) False (c) Can’t say (depends) (d) Random 15. In autonomous specification; the blastomere contains (a) Pattern information (b) Positional information (c) Signal for concentration gradient (d) Competence to induce from neighboring cells

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Multiple Select Questions 1. Which of the statements are correct for embryonic stem cells (ECS) (a) ECS are multipotent (b) Derived from inner cell mass (c) Potential of self-renewal (d) Uses in regenerative medicines 2. If a cell in vitro experiment are placed in neutral environment it starts differentiating autonomously this defines that (a) Cell is under commitment state (b) It can transform to other fate when environment is changed (c) Cell is determined (d) It will show regulative development 3. If embryonic region of neural cells are changed to skin cells it won’t transform because-cell fate is not (a) Committed (b) Determined (c) Specified (d) None of the above 4. Autonomous specification theory is supported by (a) Spemann and Mongold (b) Weismann (c) Roux (d) All the above

5. Regarding syncytial specification which statements are true? (a) Morphogen can affect nuclei in a concentration dependent fashion. (b) After cellularization, conditional specification occurs. (c) Occurs mostly in insects. (d) It is a combination of both autonomous and conditional specification.

ANSWER KEY

MCQ

1. (b) 2. (b) 3. (c) 4. (a) 5. (c) 6. (d)

7. (a) 8. (c) 9.(b) 10. (b) 11. (a) 12.(d)

13. (d) 14. (d) 15. (b)

MSQ

1. (b, c, d) 2. (a, b) 3. (a, c) 4. (b, c) 5. (a, b, c)