Histories of Cells Discoveries:
Subsequent to the first observations of life beneath the microscope, it took two centuries of research prior to the 'cell theory'; the idea that all the living things are comprised of cells or their products were originated. It confirmed even harder to admit that individual cells as well make up nervous tissue.
The Jesuit priest Athanasius Kircher (1601-1680) exhibited, in the year 1658, that maggots and other living creatures build up in decaying tissues. In the similar period, oval red-blood corpuscles were explained by the Dutch naturalist Jan Swammerdam (1637 - 1680), who as well discovered that a frog embryo comprises of the globular particles.
The other new world of extraordinary variety that of microorganisms was revealed through the exciting investigations of the other Dutchman, Antoni van Leeuwenhoek (1632 - 1723). The particles which he observed under his microscope were motile and, supposing that motility associates to life, he went on to conclude, in a letter of 9 October 1676 to the Royal Society that such particles were certainly living organisms. In an extended series of papers van Leeuwenhoek then explained lots of specific forms of such microorganisms (which he termed as animalcules), comprising protozoa and other unicellular organisms.
In the microscope: Drawings of the instruments employed by Robert Hooke (left) and the cellular structure of cork according to Hooke (right) (reproduced from Micrographia, in the year 1665).
However the first explanation of the cell is usually attributed to Robert Hooke (1635-1702), an English physicist who was as well a well-known microscopist. In the year 1665 Hooke published Micrographia, the first significant work devoted to microscopical examination, and exhibited what the microscope could mean for naturalists. He explained the microscopic units that made up the structure of a slice of cork and coined the word 'cells' or 'pores' to refer to such units. Cella is a Latin term meaning 'a small room' and Latin-speaking people applied the word Cellulae to the six-sided cells of the honeycomb. Through analogy, Hooke applied the word 'cells' to the thickened walls of the dead cells of the cork. However Hooke employed the word differently to later cytologists (he consider of the cork cells as passages for fluids comprised in plant growth), the modern word 'cell' comes directly from his book.
It must be notes that the given scientists have contributed to the knowledge of the discoveries of cells. They are:
Robert Hooke (1665):
Anton van Leeuwenhoek (1674):
Matthias Schleiden (1838):
Theodor Schwann (1839):
Rudolph Virchow (1855):
Bridge between Life and Non-life:
The existence of the whole world of microscopic living things (that is, microbes) was seen as a bridge among inanimate matter and living organisms which are visible to the naked eye. This looked like to support the old Aristotelian doctrine of 'spontaneous generation', according to which water or land bears the prospective to produce, 'spontaneously', different types of organism. This theory, which involved continuity among living and non-living matter, natura non facit saltus, was challenged by the masterful experiments of the Italian naturalist Lazzaro Spallanzani (1729-1799). He and other researchers exhibited that an organism derives from other organism(s) and that a gap exists among inanimate matter and life. (However it was a century later before the idea of spontaneous generation was definitively disproved, by Louis Pasteur (1822-1895). As a result, the search for the first elementary steps in the scala naturae was a pattern in early 19th-century biological thought: what could be the least unit carrying the potential for life?
After Swann and Schleiden's formulation of cell theory, the fundamental constituents of the cell were considered to be a wall or a membrane and the nucleus. This simple membrane termed as 'protoplasm is a viscous substance. It quickly became evident that the protoplasm wasn't a homogeneous fluid. A few biologists regarded its fine structure as fibrillary, while others explained it as a reticular, alveolar or granular protoplasmic architecture. This discrepancy resulted partially from artefactual and illusory images due to the fixation and staining methods which caused a non-homogeneous precipitation of the colloidal complexes.
Afterward, some staining of real cellular components led to the explanation of differentiated cellular elements that were then recognized. The introduction of the oil-immersion lens in the year 1870, the growth of the microtome method and the utilization of new fixing techniques and dyes greatly enhanced the identities of the cellular components. Towards the end of the 19th century, the principal organelles which are now considered to be portions of the cell were recognized. The word 'ergastoplasm' (that is, endoplasmic reticulum) was introduced in the year 1897; mitochondria were observed by some authors and named by Carl Benda (1857 - 1933) in year 1898. Camillo Golgi (1843-1926) discovered the intracellular apparatus, the Golgi bodies in year 1898.
The protoplasm wasn't the single structure to encompass a heterogeneous appearance. In the nucleus, the nucleolus and a stainable substance could be noticed. Furthermore, a number of structures (that is, ribbons, bands and threads) appeared throughout cell division. As such structures could be heavily stained; they were termed as 'chromatin' by Walther Fleming (1843-1905), who as well introduced the word 'mitosis' in year 1882 and gave a superb explanation of its different processes. Fleming observed the longitudinal splitting of salamander chromosomes throughout metaphase and established that each and every half-chromosome moves to the opposite pole of the mitotic nucleus.
This method was as well observed in plants, giving further evidence of the deep unity of the living world.
The Neuron Theory:
There was, though, a tissue which seemed to disprove the cell theory, the nervous tissue. Because of its fragility and softness, it was hard to handle and vulnerable to deterioration. However it was its structural complexity that prevented a simple reduction to models derived from the cell theory.
Nerve-cell bodies, nervous prolongations and nervous fibers were viewed in the first half of the 19th century. Though, attempts at reconstructing a three-dimensional structure of the nervous system were frustrated by the unfeasibility of finding out the exact relationships among cell bodies (somas), neuronal protoplasmic processes (dendrites) and nervous fibers.
In year 1865, Karl Deiters posthumously published work includes beautiful explanations and drawings of nerve cells studied by employing histological techniques and micro dissections made with thin needles under the microscope. Such nerve cells were characterized by a soma, dendrites and a nerve prolongation (axon) that exhibited no branching. Kolliker, in the 5th edition of his main book on histology, published in the year1867, stated that sensory and motor cells of the right and left halves of the spinal cord were connected by anastomoses (that is, direct fusion).
In the year 1872, the German histologist, Joseph Gerlach (1820 - 1896) expanded Kolliker's view and stated that, in all of the central nervous system, nerve cells established anastomoses with one other via a network made by the minute branching of their dendrites. According to this theory, the network or reticulum was a vital element of grey matter which provided a system for functional and anatomical communications, a protoplasmic continuum from which the nerve fibers are originated.
The most significant breakthrough in neurocytology and neuroanatomy came in the year 1873 when Golgi builds up the 'black reaction', which he declared to a friend with such few words, 'I am thrilled that I have discover a new reaction to explain, even to the blind, the structure of the interstitial stroma of the cerebral cortex. I let the silver nitrate react with the pieces of brain hardened in potassium dichromate. I have got superb outcomes and hope to do even better in the future'. This reaction for the first time provide a complete view of a single nerve cell and its methods, which could be followed and analyzed even when they were at a large distance from the cell body. The great benefit of this method is that, for reasons that are still unidentified, a precipitate of silver chromate arbitrarily stains black only a few cells (generally from 1 to 5%), and totally spares the others, permitting individual elements to emerge from the nervous puzzle.
By using the black reaction, Golgi discovered the branching of axon and discovered that, on contrary to Gerlach's theory, dendrites are not fused in a network. Golgi, though, failed to go away from the 'reticularistic paradigm'.
He considered that the branched axons stained by his black reaction made a gigantic continuous network all along which the nervous impulse propagated. However, he was misleading by a deceptive network made by the superimposition and the interlocking of axons of separate cells. Golgi's network theory was, though, a substantial step forward as it emphasized, for the primary time, the purpose of branched axons in connecting the nerve cells.
According to Golgi and Gerlach, the nervous system symbolized an exemption to cell theory, being formed not by independent cells however instead by a gigantic syncytium. The unique structure and functions of the nerve cell could well validate a breach of the general rule.
The issue varied rapidly in the second half of the year 1880. In October 1886, the Swiss embryologist Wilhelm His (1831- 1904) put forward the idea that the nerve-cell body and its prolongations form an independent unit. In conversation how the axons terminate at the motor plate and how sensory fibers originate at peripheral receptors like the Pacinian corpuscles, he recommended that a separation of cell units may be true of the central nervous system. The nervous tissue start to be considered, similar to any other tissue, as a sum of anatomically and functionally independent cells that interact by contiguity instead of by continuity.
The same conclusions were reached, at the starting of year 1887, by the other Swiss scientist, the psychiatrist August Forel (1848- 1931), and, in 1891, Waldeyer introduced the word 'neurons' to point out independent nerve cells. After that, cell theory as applied to the nervous system became acknowledged as the 'neuron theory'.
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