Cell and cytoplasmic contents may become so organized as to perform specialized functions that another cell don to perform. The change by means of which such specialization is achieved is know as differentiation. The event of differentiation is accomplished by changes which occur in embryonic tissues of plants and animals. Zygote is a unicellular stage which develops as a highly organized individual after several divisions. Cleavage or divisions of zygote forms millions of cells, each of which has some kind of functional specialization resulting in the formation of specific organs and systems. Development of multicellular structures and varied tissues are established through metamorphosis.
Factors controlling differentiation:
There are two factors which determine the future of a given cell, Viz., internal-localized within the cells, and external which depends upon the influences from outside the cell.
Role of internal factors
Development and specialization of embryonic cells are dependent on internal factors. For example, the egg of a sea urchin has a distinct polarity, with the animal and vegetal poles forming the animal-vegetal axis. A definite metabolic gradient exists along the axis. The first division plane of the sea urchin egg occurs along the animal-vegetal axis. Because the cytoplasmic contents of the two resultant cells are identical, when separated and kept under optimum conditions, each cell develops into a sea urchin larva. The second division is at right angle to the first angle to the first one, forming four cells; on separating and allowing to develop in optimal conditions each cell develop into a larva. The third division is perpendicular to the animal-vegetal axis, resulting in an eight celled stage, with four cells each pole. The four cells of the animal pole are differentiated from the cells at the vegetal pole in their cytoplasmic contents on isolation. They do not develop into normal larvae. Thus, it can be concluded that the embryonic development depends upon the cytoplasmic internal factors distributed along the animal vegetal axis.
Role of external factors
External factors are influence, embryonic development. For example, the differential oxygen availability and the opportunity for waste removal which exits among cells at the surfaces of an embryo and those that lie more deeply result in different metabolic and divisional rates. Added to this there are cluster of cells which exert influence on the other cells. Such cells act as organizers which participate in differentiation of cells into specific organs.
A mass of cells produced due to zygotic repeated divisions are totipotent, i.e., each cell in its early embryogenic stage has the entire genetic potential to develop into an individual. As the embryo develops, cells differentiate along a predetermined naturally influenced by genes. When the cells of the differentiated embryo are separated and in a suitable medium under optimal conditions, each cell does not develop in to whole organism.
The cultured cell, on the other hand, develops along a predefined path, i.e., the cell programmed to develop into an ear will only develop into an ear. It can be concluded that the cell destined to develop into an ear has the potential to develop an ear. Such a cell is pluripotent and it cannot evidence totipotency.
Weisman suggested that differentiation is due to expression of certain chromosome segments into different tissues. He assumed that complete chromosome complement was retained by those cells which were supposed to give to gametes in order to provide genetic continuity. This theory cannot be accepted, since the somatic cells of all organism have the same chromosomal compliment.
Genes control differentiation
Modern biology advance techniques could prove the cell differentiation was under control of gene functions. Every tissue or cell differentiation was under control of gene function.
Every tissue or cell has a specific protein and a specific complement of enzymes which are produced by genes, though genes don not produce these proteins and enzymes directly, they are produced through an intermediate-the messenger RNA. As a result, the process of cell differentiation is caused by the cell’s ability to synthesize various types of proteins.
Levels of differentiation:
Cell differentiation can be studied both a cell level and population level. The cellular level involves progressive morphological and biochemical changes; differentiation at population level involve specialization of two or more cells in a multicellular organism, which may follow different paths. Cells of this type undergo cytoplasmic, nuclear, and metabolic alterations in order to establish distinct developmental patterns.
Differentiation has been recognized as a four-stage process:
- Non differentiated stage of the stem cell represents a condition in which the cells produce specific proteins prior to morphological changes set in.
- Pre-differentiated cell represents extensive proliferative stage to produce vast population. Some are differentiated as specialized cells and other remains as memory cells with arrested growth.
- The differentiated stage is distinct from other cells. In this the cells have cell-specific proteins.
- In the differentiated cells modulation may take place in response to extra cellular factors such as hormones, pH and temperature.
In plant cells the situation is different. A differentiated plant cell can retrace its path and dedifferentiate. In many woods’ plants, the meristematic cells of the shoot and root give rise to greater number of meristematic cells, xylem and phloem tissue and cortical cells. The cortical cells generally from the filling tissues of root and stem, but sometime take over the function of storage cells. In a two-year-old woody plant the cortical cells redifferentiate to become embryonic cells and develop into new meristematic tissue called cambium. The cork cambium may also give rise to a large number of cambium cells, which develop into cork parenchyma and cork cells.
This situation is not clear in animals. For example, if a salamander’s organ like tail or limb is lost, a mass of undifferentiated cells divides mitotically to differentiate into bone, muscle, nerve cells, epidermis, capillaries, etc. for replacing the lost part. This process is called regeneration. Higher organisms, however, do not have regenerative potential.
Mechanism of Differentiation:
The embryonic cell, zygote, has the potential to develop into may cells; some of them remain undifferentiated and persists throughout life but may be utilized when called upon to produce specialized types. Various molecular mechanism responsible for differentiation are as follows.
Constant genetic information
Cells do not lose their potential after differentiation. For example, a single cell derived form the pitch callus of carrot grows into a whole plant. The cells undergo vigorous superficial cell division resulting in a uniform parenchyma; after sometime the callus begins to differentiate. The parenchyma has various tissue forming properties. Successive differentiation results in the creation of vascular bundles and other tissues, which eventually leads to the growth of the entire plant. This experiment shows that from a single cell by repeated cell divisions, a set of tissue are formed to produce a total plant.
Partial shielding of genome to transcription
In certain in vitro experiment conducted with a thymus DNA, which RNA polymerase was added to synthesize RNA on DNA templates, analysis proved that RNA produced in vitro represented only 10 % of the transcribed DNA. This reveals that the transcription is restricted to some parts of the genome, and the kinds of mRNA produced by a cell are regulated by the amount of DNA available for transcription.
This reveals that the DNA available to transcription in each tissue is different in its polynucleotides sequence and the balance DNA is not available for the purpose.
mRNA selection for translation
An example of translational control; during fertilization, sea urchin eggs show increase in protein synthesis rate. Little quantity if messenger RNA is synthesized just after fertilization.
Gross and co-biologists suppressed the RNA synthesis by using actinomycin-D, an antibiotic to prove that increase in the rate of protein synthesis take place in the absence of RNA synthesis.
Amplification of gene in differentiation
Gene amplification is a significant phenomenon that has only recently emerged in animal development. Gene amplification should be accomplished through selective replication of essential genes, so that some genes expand proportionally while the amount of DNA remains constant. This phenomenon is very important as it contributes to adjustments of gene activity in a cell by changing the proportion of some genes.
This phenomenon has been demonstrated in the giant polytene chromosome of salivary glands of certain insect larvae at a particular developmental stage. DNA synthesis establishes at specific bands in giant chromosome, which has an ability to hybridize with labeled thymidine, expressing that H-thymidine is incorporated into DNA regions of chromosome where there is no addition in DNA content.