Cell Differentiation



A multicellular organism has three types of cells regarding their capacity to divide and differentiate:undifferentiated  or stem cells, differentiated  or post mitotic cells and dedifferentiated or transformed cells.

1): Undifferentiated  ( Stem) Cells:These are unspecialized cells which by mitotic divisions give rise to new cells for the formation and maintenance of tissues. 

Examples: some common examples of stem cells are given below_ 

a) Germinal Epithelium: Germinal epithelium found in the gonads consists of stem cells.These cells are unspecialized and, by mitotic divisions,  produce new cells that give rise to the gametes.

(b) Malpighian layer: Malpidghian layer of the epidermis in vertebrate skin consists of undifferentiated stem cells.These cells replace the epidermal cells lost to the environment at the surface.

(c) Meristematic Cells: Vascular plants have undifferentiated meristematic cells at the top of the stem, a little above the root tip, and in the cadmium.These cells continue to divide and mature into xylem, phlegm, companion cells and so on.

(d) Bone Marrow: Born marrow in vertebrates has unspecialized stem cells which continuously produce blood cells.

(e) Zygote: Zygote too is undifferentiated cell.By segmentation,it produces millions of cells which gradually differentiate and finally form a new organism. 

2): Differentiated ( Postmitotic Cells: These are specialized cells formed from the unspecialized cells by change in structure and function during development and growth of an organism. They have permanent form, structure, function and position in the organism's body.They ordinarily do not change further, nor divide.

Mechanism of Dufferentiation: All the cells of a developing organism receive in their  nuclei  complete set of  Genes present in the zygote through mitotic divisions. However,the cells of an early embryo lose the potential for the expression of all the Gene's.Only certain genes remain functional in certain cells.This permits the cells having the similar genes to acquire different structure and function, and this results in differentiation.Thus,only a small proportion of the total genetic information contained in the  nucleus of any cell is actually used by that cell, and the more specialized the cell,the smaller the number of genes utilized. Red blood corpuscles lose their capacity for aerobic respiration, DNA  replication and RNA synthesis after differentiation.Similarly nerve cells lose the capacity to divide.

Exactly how masking of genes is effected is not clear,but certainly the cells themselves cause differentiation of one another by passing some chemicals between them.These chemicals are called "inducers", or" organizers". 

Advantage of Differentiation: 
  Specialization of cells into tissues, organs, and organ _ systems is advantageous for the organism.It increases the operational efficiency through division of labour  which avoids duplication of work. 

3): Dedifferentiated ( Trnasformed) Cells: These are specialized cells reverted to a more generalized ( embryonic), actively dividing state.Dedifferentiation often occurs for regeneration. Regeneration is the natural ability of organisms to replace the lost parts of the body.A piece of stem cut off from a rose plant, when placed in the soil, soon sprouts new roots and finally regenerates the whole plant.A starfish regrows lost arms and a newt redevelops an excised limb.Humans cannot  replace the lost parts but can heal the wounds.

Mode of Dedifferentiation: The differentiated cells at the cut surface become dedifferentiated, regain the power of division, and produce new cells which differentiate once again into the cells of various tissues that make up new parts in place of the lost ones.

This shows that the differentiated cells retain the full genetic information. Thus, differentiation is a temporary inactivation of certain genes, and dedifferentiation  is the reactivation of these genes. 

Cell Culture:  The cell Culture experiments illustrate the concepts of Dedifferentiation and Dedifferentiation. In these experiments, mature, nondividing cells are taken from a multicellular organism and grown in a sterile nutrient medium. The cells regain the capacity to divide and produce a large population of cells.The cells aggregate in clusters, undergo differentiation, form tissues and finally new organisms 

Advantage:  The advantage of Dedifferentiation is obvious. It is a means ( 

a) of regeneration in lower animals,

 (b) of healing in higher animals including humans, and

( c) of propagation in plants.

Stomach               Pancreas

RNA                    Nucleic Acids


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