Here are your brief notes on Totipotency

Fertilization is an important biological event in the life-cycle of most plants and animals. Two haploid gametes unite and result in a diploid fertilized egg or zygote.

The unicellular zygote then undergoes repeated educational divisions forming a mass of genetically identical cells by a process, known as cleavage.

From these cells originates a complete multi-cellular plant or animal, as the case may be. The parts of a flowering plant, such as the underground root, the aerial stem, the green leaves, the beautifully coloured flowers and the fruits are structurally and functionally diverse. Logically speaking, all the cells of the plant originate from the same unicellular zygote.

Therefore, each cell receives the same genetic complement i.e. all cells, whether it is chlorophyll bearing cell of the leaf or a meristematic cell of the root tip, are genetically identical. However, the two types of cells are morphologically and functionally different i.e. they have variations.

The process of manifesting these variations is known as differentiation. This leads to a fundamental question as to whether these changes are permanent and cannot revert.

The example of Bryophyllum leaf answers this question partially. When a Bryophyllum leaf falls on the soil, it develops roots from multiple parts of it. Each part differentiates into a tiny plant .This phenomenon can also be demonstrated by keeping a leaf inside the pages of a book.

After a few days the leaf margin is seen to have regenerated plantlets. This phenomenon of regenerating a whole organism from a differentiated cell is known as totipotency. Another phenomenon, known as dedifferentiation applies to this type of development.

Dedifferentiation is the attainment of an undifferentiated condition of the differentiated cells. Dedifferentiation of cells is followed by redifferentiation into the required structure under specific stimuli.

Most of the flowering plants spell a different story. A differentiated palisade cell remains as a palisade cell and dies as a palisade cell and an epidermal cell does not revert to a meristematic cell. This suggests that differentiations are of a permanent nature.

However, the experiment of Vochting (1878) suggests another fact. He observed that all the cells of a stem cutting have the potential of forming roots and shoots. The upper portion differentiates as a shoot, while the lower as a root. The best way to investigate into this differential activity is to separate single cells from the influence of the neighbouring cells and grow them in isolation on an artificial nutrient medium.

This suggestion of Vochting gave birth to the concept of plant cell and tissue culture. We shall learn later that isolated plant cells and meristematic cuttings (explants) grow and differentiate into plant structures on artificial nutrient media. These cells and the cells constituting the meristematic parts are totipotent cells.

Among animals, there is no instance of an isolated cell regenerating a complete animal. However, in some lower group animals like sponges, Hydra and Planaria, when a part of the body is damaged or lost, the remaining part is able to regenerate the lost part.

This phenomenon is known as regeneration. There is a reservoir of undifferentiated cells in these animals, which, on demand, differentiate into several kinds of cells of the adult and replenish the lost part. These cells are known as embryonic undifferentiated cells. Archaeocytes of sponges, interstitial cells of Hydra and neoblasts of Planaria are among a few examples. The degree of regeneration capacity gradually decreases with the increasing complexity in the structural organization. There is practically no regeneration in mammals.

Alternately speaking, differentiated animal cells are more rigid unlike plant cells and their totipotency decreases through an ascending order of evolution. Even the isolated cells are very difficult to culture in isolation. Some animal cells have been successfully cultured under stringent conditions.

The requirements are more complex than those of the plant cells. However, the cells of animal embryos are undifferentiated or less differentiated than those of their adults. In higher group of animals, there are groups of undifferentiated cells, which continually form and renew a few types of cells.

These are known as embryonic stem cells. Bone marrow stem cells continually form and renew the blood cells and the worn out cells of the intestinal epithelium is replenished by another group of stem cells. These cells have a narrow range of differentiation capacity.

The stem cells are totipotent in a restricted sense. They can be made to develop under a strictly regulated set of conditions. Some stem cells have the potential to differentiate into many kinds of cells. These are known as pluripotent cells. Some have a potential to develop into a few types (multipotent) and some others into a specific type (unipotent).