Advances in Cell Theory
Advances in Cell Theory
In the 1830s Matthais Jakob Schleiden (1804-1881) and Theodor Schwann (1810-1882) established the basic principles of modern cell theory. Cell theory allowed scientists to see the cell as the fundamental unit of life. Schleiden and Schwann realized that the entity Robert Brown (1773-1858) had named the "nucleus" was an essential feature of cells in animals as well as plants. As cell theory developed it provided a powerful new framework for understanding the structure of the body, the mechanism of inheritance, the development and differentiation of the embryo, the unity of life from simple to complex organisms, and evolutionary theory. According to cell theory, the body is composed of cells and cell products.
Tissue doctrine, elaborated by Marie François Xavier Bichat (1771-1802), represents an ambitious attempt to analyze the fundamental structural and vital elements of the body. According to Bichat, the human body could be classified into 21 different kinds of tissue. Obviously, Bichat's simple tissues were themselves complex combinations of vessels and fibers rather than the most basic unit of structure and function. Many of Bichat's followers, however, thought of the tissue as the body's ultimate level of resolution even after the microscope revealed a world of previously invisible entities. When microscopic investigations began in the seventeenth century, naturalists reported seeing various corpuscles, vesicles, sacs, and globules. In 1665 the word "cell" appeared in Robert Hooke's book Micrographia, along with pictures of little boxes seen under the microscope in freshly cut sections of cork. While the cells in cork appeared to be empty, Hooke (1635-1703) noted that living plants contained cells full of streaming green juices.
By the beginning of the nineteenth century, various theories had been advanced about the "globules" found in biological materials. Some of these globules may have been cells, but others were probably optical illusions and artifacts. Plants clearly contained cell walls enclosing some fluid, but it was difficult to see a basic relationship between the microscopic structure of plants and that of animals. One of the keys to understanding the fundamental similarity of plant and animals cells was the discovery of the nucleus in 1831 by Scottish botanist Robert Brown. Brown is also famous for his studies of the phenomenon now called Brownian motion. Brown described the nucleus of plant cells as a dark circular area that appeared to be more opaque than the cell membrane. The discovery of the nucleus provided a focus for studies of plant cells, but it was more difficult to understand the basic structure of animals. By the end of the 1830s, however, botanists and zoologists had developed the concept of animal and plant life known as the cell theory.
Credit for the formal statement of cell theory is generally attributed to Schleiden and Schwann. Schleiden, however, noted his interest in the work of the French botanist Charles Brisseau-Mirbel (1776-1854), who believed that cells were found in all parts of the plant. In 1838 Schleiden published "Contributions to Phytogenesis" in Müller's Archives for Anatomy and Physiology. Taking Brown's work on the nucleus as a starting point, Schleiden focused his attention on the distribution of this structure. Soon he came to regard the nucleus, which he renamed the cytoblast, as a universal characteristic of all plants. Schleiden believed that all plants were aggregates of cells. Although the cells were actually independent entities, each cell also acted as an integral part of the plant. Thus, all aspects of plant physiology were fundamentally manifestations of the vital activity of cells.
Schleiden described several possible methods of cell formation in Principles of Botany. He thought that the hypothesis known as "free-cell formation" was most likely. According to this hypothesis, cell growth was rather like the process by which crystals grew. A fluid called the cytoblastema, rich in sugars and mucus, supported the formation of a nucleolus. As mucus particles aggregated around the nucleus, the cytoblast formed and grew into the young cell. Eventually, a complete cell developed within a rigid cell wall. Schleiden thought that plants could also grow by the formation of cells within cells so that the contents of a cell were divided into two or more parts. Although the process of cell formation was unclear, Schleiden rejected the possibility that life arose through spontaneous generation.
By clearly expressing the idea that a plant was a community of cells, Schleiden provided the key principle of the cell theory. His description of this hypothesis led Schwann to consider the possibility that animals might also be composed of cells. Animals appeared to be more complicated than plants, even at the microscopic level. Also, animal cells were difficult to analyze under the microscope because they are usually transparent and do not have the cell walls found in plants. Schwann had noticed cell nuclei in preparations of notochord, but until he talked with Schleiden he did not grasp the importance of these observations. After Schleiden described the importance of the nucleus in plant cells, Schwann realized that the cell nucleus was the unifying factor in the most diverse forms animal tissues. In his book Microscopical Researches into the Accordance in the Structure and Growth of Animals and Plants (1839) Schwann attempted to prove the basic unity of the plant and animal worlds. Schwann provided evidence that cells are the basis of all animal tissues, and that animal tissues originated from cells which were analogous to the cells of plants. The generation of animal cells seemed to be similar to the processes described by Schleiden in plant cells. Building on the work of Brown and Schleiden, Schwann argued that true cells could be defined in terms of the presence of the nucleus. The whole animal was composed of cells and cell products. In Microscopical Researches Schwann summarized his research and explicitly stated the powerful generalization known as cell theory.
In a section called "Theory of the Cells" Schwann attempted to deal with the question of whether cellular phenomena should be considered mechanical or vital. Schwann discussed the problem of chemical changes that took place within the cell and in the surrounding cytoblastema. He called these cellular activities "metabolic phenomena." He coined the word "metabolic" from the Greek to describe "that which is liable to occasion or to suffer change." According to Schwann, metabolism was a universal property of cells and, therefore, of life. He elaborated his theory by analyzing fermentation in yeast as an example of the metabolic activities of cells. Schwann's metabolic theory led to a battle between the German chemists Justus von Liebig (1803-1873) and Friedrich Wöhler (1800-1882) and the great French chemist and microbiologist Louis Pasteur (1822-1895).
While Schwann and Schleiden had provided a powerful new framework for understanding the structure of plants and animals, their theory was different from modern cell theory in several important respects. The major problem was their concept of free-cell formation and the cytoblastema. In subsequent years various botanists, zoologists, and microscopists attacked this concept, but it was Rudolf Virchow (1821-1902) who established the principle that every cell is the product of a pre-existing cell. Virchow was, therefore, the author of cell theory in its modern form. His ideas were summarized in a series of lectures later published under the title Cellular Pathology (1858).
LOIS N. MAGNER
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