Cell Division and Mitosis
Cell Division and Mitosis
Cell theory, the cornerstone of biology, states in modern form that (1) living matter is composed of cells; (2) chemical reaction takes place within cells; (3) cells arise from other cells; and (4) cells have information that is passed from parent to daughter cells. In the nineteenth century three scientists advanced the knowledge of cell division and mitosis. German botanist Hugo von Mohl (1805-1872), using improved microscopic techniques, studied cell division in algae and determined that cells divide. German biologist Walther Flemming (1843-1905) applied aniline dyes to observe the processes of cell division in the nucleus and how chromosomes behave. Belgian zoologist Edouard van Beneden (1846-1910) first described the process of fertilization and stages of embryonic division.
At the beginning of the nineteenth century, scientists knew nothing about the existence of cells. When they looked at a whole cat or bird or human being, they thought they were looking at the basic unit. Some proposed that body parts were made of strings and tubes that grew like crystals and enlarged until the person attained adult proportions. The internal organs were pots or pipes of different shapes.
The general orderliness of living things had led to a system of thought called "vitalism." Because organs were basically structured in the same way, vitalists contended that the breath of life that God gave Adam was the "vital force of life." Absence of the vital force was death—when the force or spirit or ghost is given up. It was an ancient system of thinking, but eighteenth-century German philosopher George Stahl (1660-1734) put a stranglehold on biology for the next two centuries by applying vitalism to life sciences. His related idea, called "animism," dominated early cell biology by insisting that all body parts came from the soul. Before there was knowledge about cells, vitalism was mainstream science. Even in the early days of scientific method, it was not unusual to mix natural investigation with supernatural explanation.
Another popular precept was the idea of spontaneous generation—that living things could develop from nonliving things. Even though Francesco Redi (1626-1697) had performed his famous experiment disproving spontaneous generation in the seventeenth century, the controversy still raged in 1860. In fact, the conflict was so spirited that the Paris Academy of Science conducted a contest inviting members to shed new light on this subject. It was Louis Pasteur (1822-1895) who, in 1864, devised an experiment to show that microorganisms did not appear spontaneously. However, many stubbornly clung to the old ideas until the last few decades of the nineteenth century.
Other events had taken place to set the stage for the discovery of cell division. During the seventeenth century, Robert Hooke (1635-1703) observed cork and called the blocks that he saw cells because they reminded him of jail cells. In 1673 Anton van Leeuwenhoek (1632-1723) ground lenses to look at a small world of "cavorting beasties," but cell studies were hampered by the poor quality of magnifiers and by beliefs in spontaneous generation and other ideas of biological structure.
Another philosophical turn was developing at the end of the eighteenth and early nineteenth centuries. The rise of natural philosophy led to the proposal that a "chain of being" connects all living organisms as well as inanimate objects. This chain, it was thought, started with minerals and progressed from sponges to plants, to birds, to mammals, and, finally, to man. For these philosophers, all species were interrelated and the ideal was to return to nature and the natural man. The German poet Johann Wolfgang von Goethe (1749-1832) and philosopher Friedrich Schelling (1775-1854) were leading proponents of this movement, which has been dubbed Romantic philosophy. However, these ideas of interrelations and connectedness with nature spawned interest in zoology and botany as well as comparative development, including embryology.
During the nineteenth century, the development of the microscope became critical in determining mitosis and cell division. Lenses were improved so that higher powers could be seen with accuracy. Illumination made it possible to see intricate details.
In 1839 two Germans—Matthias Schleiden (1804-1881) and Theodor Schwann (1810-1882)—advanced a radical theory that proposed that plants and animals were made of cells. They showed all cells to have a nucleus, cell body, and cell membrane.
Mohl made his most important contributions in the field of microscopic botany. Using his knowledge of optics, he wrote a manual on the use of the microscope in 1846. His great claim to fame was the development of microscopic study of plant cells. In the 1850s German pathologist Rudolph Virchow (1821-1902) suggested that all cells are formed from existing cells by cell division. Drawing upon this insight, in 1853 Mohl published in a classical reference of the time a description of the cell as the elementary organ. Looking closely at the cell under his developed microscope, he was able to show how aligned cells fused and how cells moved within. He saw how the cell generated by dividing and understood how the cell obtained nourishment, moved, and reproduced. His meticulous studies led to a description of cell structures of the membrane, protoplasm, nucleus, and cellular structure. He was the first to delve into cytochemistry and named the intracellular contents "protoplasm."
Mohl was the first to determine that new plant cells are formed by cell division. Working with the alga Conferva glomerata, he noted how cells around the edges of the colonies began to divide into two. Although it was unique for the day, his view of division and the fibrous walls of plants cells is well confirmed today.
Flemming became a professor at the University of Kiel, Germany, where he carried out his great work on cell division. While the knowledge that cells divide into two daughter cells was becoming accepted, exactly how the division occurred was still a mystery. Several years after Mohl proposed the idea of algal cell division, Flemming used the oil immersion lens to study cells and stains to reveal their structure. He had heard of the aniline dyes developed by Paul Ehrlich (1854-1915) and used by Robert Koch (1843-1910), and he used a special class of these dyes to reveal small, thread-like structures in the nucleus. The thread-like structures would later be dubbed "chromosomes," meaning "colored bodies."
Flemming found he could apply these aniline stains to cells killed at different stages of dividing. He was able to make slides that revealed a clear and established sequence of what occurs during cell division. According to his observation, the threads shortened, then split lengthwise in two. Each of the halves went to a separate end of the cell. The cell then divided into two daughter cells. Flemming found nine stages in the processes of cell division and gave names to each stage. He also coined the term "mitosis" for the process of cell division.
Flemming insisted correctly that animal and plant cells have the same process of division. Even with the high-power electron microscopes of modern science, Flemming's descriptions of the phases are very current today, with only a few exceptions. The nine phases, which correspond to the drawings and plates produced by Flemming, are now known as interphase (when the cell is not dividing), early prophase, midprophase, early metaphase, metaphase, early anaphase, late anaphase, early telophase, and late telophase.
Flemming then focused on nuclear division in the testes and concluded that sperm are formed from cells that have divided twice. In 1887 he saw the sperm precursor cells—spermatocytes—divide and then divide again. These are now known as meiotic divisions. He also traveled to Naples to study cell division in the egg cells of echinoderms (starfish). He concluded that the egg also divided twice. At the end of the process of meiosis, the egg and sperm have only half of the chromosome information of the original cell.
The idea of sperm and egg as the mechanism of reproduction was not well developed at the beginning of the nineteenth century. Flemming had spent some time studying the formation of egg and sperm. It was Beneden, though, who in 1883 published a work on fertilization. Studying the egg of Ascaris megalocephala, a nematode or roundworm, he described the essential process of fertilization. One-half nuclei from the female joins with one-half from the male and forms pronuclei—which are now called chromosomes. In drawings and plates he accurately represented the chromosomes. He described how the nuclear membrane of the egg and sperm break down. The sets of chromosomes then line up at the equator. The variety of Ascaris Beneden studied had only two chromosomes, with each parent contributing one to the zygote, or fertilized egg.
In 1887 Beneden published a paper describing the centrosome as a permanent cell organ, which was in the cell during resting period and divided into two parts before the beginning of the next mitosis.
The work of these three scientists—Mohl, Flemming, and Beneden—had advanced the concepts of mitosis, meiosis, and fertilization. They moved cellular investigation away from spontaneous generation, romantic natural philosophy, and vitalism to an era of scientific method. By the end of the nineteenth century it was generally accepted that cells divide in phases, sperm and egg are formed in a special division process, and the union of sperm and egg make a new cell. By 1900 cell division or mitosis had become an integral part of cell theory, presenting a coherent view of how an organism—both simple and complex—arises and reproduces.
EVELYN B. KELLY
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