Part # 1. Findings of Different Structures in the Cell:
The exploration of cell was always accompanied by the improvement and advancement of different tools and techniques. The study of architecture of living body started with unaided eye and came to a level beyond which it was not possible to advance. The need was met with the invention of microscope, an instrument which was meant for magnification.
The quality of the microscope was gradually improved and in course of time phase-contrast, interference, and electron microscopes came, with only one purpose—to see and to know about the cells in more details.
The methods of fixing and staining developed and attained such a level that today many specific compounds can be traced within a cell. Techniques were also developed to isolate different components of the cell and to analyse them.
A short history of the progress is given below:
Bichat, Fernel and others studied the living body with the naked eye and very rightly came to a level beyond which nothing could be seen. Bichat examined tissues and concluded that different tissues have different architectures which depend upon their functions.
Fernel studied muscle cells and inferred that muscle is made up of small muscles, which in turn are made up of smaller muscles. His conclusion was that it goes ad infinitum.
No further advancement was possible till 1665, when Robert Hooke, a well-known British scientist, described a thin slice of cork and other plant parts under a microscope. He described the presence of honey comb like structure and called each box a cell.
This was the beginning of the study of cell and many new discoveries were to follow. In the seventeenth century, Malpighi and Grew, independently examined cells in detail. But they too, like Hooke, considered the cell-wall as the all-important parts and ignored its inner content. Corti (1772) and Fontana (1781) saw living substance in plant cells.
Robert Brown (1883) identified the round body inside the cell as the nucleus. In 1885, Felix Dujardin named the jelly-like inner content of protozoans as sarcode. It was Purkinje who in 1840, used the word protoplasm to replace sarcode and in 1846 Van Mohl applied the term protoplasm to the plant cells.
Rapid development of staining techniques led to the finding of other structures. Claude Bernard discovered mitochondria, Golgi described the structures of Golgi apparatus within the cell. The division of cell was noticed by Virchow (1859). Strasburger (1888) described chromosomes within the nucleus and it was found out by Flemming (1892) that division of a cell involves duplication of chromosomes.
The discovery of the phenomena, that during the formation of gametes the chromosome number is halved and again after fertilisation the normal number is restored, drew the attention of embryologists to the problems of cytology.
The beginning of twentieth century was marked by an important advancement in cell studies when Morgan discovered the presence of trait-bearing bodies, called Genes in the chromosomes. This was soon followed by rapid emergence of studies. Brachet, Caspersson, Gomori and others using improved techniques traced the chemical nature of different structures within the cell.
The use of ultra-centrifuge helped in the isolation and studies of different cell organelles. The techniques of cultivating cells in vitro was devised by Harrison and was improved by Carroi, Earl, Gay, Fell and others, which provided a new scope to study the living cells.
The invention of electron microscope was successfully utilised by Porter, Fawcett, Novikoff, Sjostrand and others who added new dimensions to our knowledge of cell structure.
Part # 2. Attainment of Generalization:
The cells are always subject of study for having certain clues about the mysteries of living body. This study started to find out the ultimate architecture of living body.
The trend showed by Vesalius was followed by Bichat and Fernel, who came down to a limit beyond which nothing is visible to the naked eye. The need to see better led to the discovery of microscope and it was left to Robert Hooke to confirm the assumption that in the living body there are minute compartments called cells.
While new facts were being discovered about cells in many organisms, Schleiden and Schwann in 1839 advanced what is known as the cell theory which states:
1. Animals and plants are made up of cells which are units of structure and function.
2. The cells have independent lives within higher forms of life.
3. New cells arise by a process resembling crystal formation.
The generalisation was an important step towards the progress of biology. But speakers in the centenary celebration of cell theory in 1939, like Conklin, Karling and others severely criticised Schleiden and Schwann for not giving due credit to their predecessors like Dutrochet of France who had clearly stated that cells are the units of living body and they possess independent life.
Moreover, the third generalisation in the cell theory that new cells arise by a process similar to crystal formation, was found wrong by Virchow, who clearly demonstrated that cells always come from pre-existing cells. In spite of this criticism it must be said that the credit should be given to the proposers of the cell theory for being able to attract the attention of the biologists of their time and the impact was great.
Virchow’s enunciation provided a solid basis to the cell theory which still remains as one of the landmarks of modern biology. Later in 1861, Max Schultze of Germany observed that protoplasm is fundamentally similar in all animal and plant cells.
The protoplasmic theory of Schultze was confirmed by T. H. Huxley and the concept of universality of protoplasm as the “physical basis of life” was another important generalisation of biology.
Once this was established, it was thought that chemical analysis of protoplasm will provide an answer to the question of mysteries of life. Numerous facts were known, but the final answer remained far away.
The concept of common physical basis raised a number of questions—if protoplasm is same in all animal and plant cells what is the reason of this worldwide diversity? How do characters go from one generation to the other?
The beginning of twentieth century saw the coming of another important discovery when Morgan proved experimentally that chromosomes in the nucleus are the hereditary vehicles. Once this was known extensive search continued to understand the working of nucleus and its relation with cytoplasm. From that moment, the study of cell joined the study of development and heredity.
At the present moment we know many details about cell. This knowledge has accumulated from the joint enterprises of physics, chemistry and biology. We know that DNA is the key chemical substance which serves as the basis of heredity. It controls, through RNA, the protein synthesis in cytoplasm but even today our knowledge about the nucleo-cytoplasmic interactions is incomplete.
The cell as the unit of structure and function in all living organisms provides the underlying basis of unity in the great diversities of the living.
We have this important generalisation today that “life is an uninterrupted succession of cells. Growth, development, inheritance, evolution, disease, aging and death are… … varied aspects of cell behaviour.” But we are still far from complete understanding of the mechanism behind it.