Cell theory

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Cell Theory - A History

Cell theory is the progression of the understanding of the cell in the history of science. It is also a theory that states that all living things are composed of one or more cells.

An account from the early 1900s:

The Cell Prior to the Early 1900s

A brief account of the structure, physics, and chemistry of the cell will serve to give us some idea of the condition of the zygote from which the individual arises, and will help us to understand certain events in the germ-cell cycle to be discussed later.

The cell is the simplest particle of matter that is able to maintain itself and reproduce others of its kind. The term 'cell' was applied by Hooke in 1665 to the cell-like compartments in cork. Cells filled with fluid were slightly later described by Malpighi.

In 1833 Robert Brown discovered nuclei in certain plant cells. What is known now as the CELL THEORY is usually dated back to the time of the botanist Schleiden (1838) and the zoologist Schwann (1839), whose investigations of the cellular phenomena in animals and plants added greatly to the knowledge of these units of structure. At this time the cell wall was considered the important part of the cell, but continued research proved this idea to be erro neous.

Schleiden called the substance within the cells plant slime. Later (1846) von Mohl gave the term protoplasm to the same substance. The substance within the animal cell was named sarcode by Du jardin. The similarities between the protoplasm of plants and the sarcode of animals were noted by Cohn, and animal cells without cell-walls were observed by Kolliker (1845). It was not, however, until 1861 that Max Schultze finally established the fact that plant protoplasm and animal sarcode are essentially alike, and defined the cell as a mass of protoplasm containing a nucleus. Schultze's re searches serve as the starting point for modern studies of cellular phenomena, but the definition furnished by him must be modified slightly, since we now know that many cells exist without definite nuclei. These cells, however, are provided with nuclear material scattered throughout the cell body (the so-called distributed nucleus). Our definition must be changed to read, a cell is a mass of proto plasm containing nuclear material. Changes likewise have taken place in the Cell Theory; we no longer consider cells as isolated units and the multi cellular animal as equivalent to the sum of its con stituent cells, but recognize the influence of the cells upon one another, thus reaching the conclusion that the metazoon represents the sum of the individual cells plus the results of cellular interaction.

Cell Size a History

Cells vary considerably in size, ranging from those we call Bacteria, which may be no more than S^ITTO" of an inch in length, to certain egg cells which are several inches long; the latter, however, owe their enormous size to the accumulation of nutritive sub stances within them. An average cell measures about 2 sW f an mcn m diameter. Cells vary in shape as well as in size; egg cells are frequently spherical, but most cells are not, since they are sur rounded by other cells which press against them. A diagram of a typical cell is shown in Fig. 1.

FIG. 1. Diagram of a cell, as = attraction-sphere; c = centrosome; ch = chromatin reticulum; cr = chromidia; ec ectoplasm; en = en doplasm; k = karyosome; Z = linin; m = mitochondria; me = meta plasm; nm nuclear membrane; p = plastid; pi = plasmosome or nucleolus; s = spongioplasm; v = vacuole.

Authorities are not agreed as to the structure of protoplasm ; to some it appears, as shown in Fig. 1, to consist of a network of denser fibers called spon gioplasm (s) traversing a more liquid ground substance, the hyaloplasm. Others consider proto plasm to be alveolar in structure, thus resembling an emulsion, whereas another group of zoologists maintain that while protoplasm may appear to be fibrillar or alveolar, its essential basis consists of multitudes of minute granules. Wilson's view is the one usually adopted at the present time; that is, the protoplasm of the same cell may pass suc cessively "through homogeneous, alveolar, and fibrillar phases, at different periods of growth and in different conditions of physiological activity," and that "apparently homogeneous protoplasm is a complex mixture of substances which may assume various forms of visible structure according to its modes of activity."

The physical properties of protoplasm are not well known, since most of our studies have been made with fixed material. We know that protoplasm may exist as a gel or a sol, and that it is intermediate between true solids and true liquids, with many of the properties of each and a number of properties peculiar to itself. No doubt the protoplasm differs in its physical nature in different cells. In the egg of the starfish, Asterias, Kite (1913) has shown that the cytoplasm is a translucent gel of comparatively high viscosity and is only slightly elastic; pieces become spherical when separated from the rest of the egg. Scattered throughout this gel are minute granules (microsomes) about -nroTJ" mm. in diameter which cannot be entirely freed from the matrix.

What appear to be alveoli contain globules which possess many of the optical properties of oil drops ; these are suspended in the living gel. The cytoplasm of the starfish egg is not therefore alveolar in structure as usually stated, but is rather of the nature of a suspension of microsomes and globules in a very viscous gel. The nuclear membrane is a highly translucent, very tough, viscous solid, and not a delicate structure as ordinarily conceived. The nucleolus is a quite rigid, cohesive, granular gel suspended in the sol which makes up the rest of the nuclear material. Dividing male germ cells of cer tain insects (squash bugs, grasshoppers, and crickets) revealed the fact that the chromosomes are the most highly concentrated and rigid part of the nuclear gel ; that the spindle fibers are elastic, concentrated threads of nuclear gel ; and that the metaphase spindle fibers seem to be continuous with the ends of the chromosomes.

The ground substance of the nucleus is a sol termed nuclear sap or karyolymph. In the so-called 'resting' nucleus a network of fibers may be observed similar to the spongioplasm in the cytoplasm ; these consist of a substance named linin because it usually occurs in threads (Fig. 1, ).

Distributed along the linin fibers are granules of a substance which stains deeply with certain dyes, and for this reason is known as chromatin (ch). These chromatin gran ules may unite to form larger spherical masses, the karyosomes or chromatin-nucleoli (&), and during mitotic nuclear division constitute the chromosomes (Fig. 3, C). In many cells one or more bodies resembling the karyosomes somewhat, but differing from them chemically and physiologically, are pres ent; these are the true nucleoli or plasmosomes (Fig. 1, pi). Embedded in the cytoplasm near the nucleus may often be seen a granular body, the centrosome (c), which is thought to be of great importance during mitotic cell division. The pro toplasm surrounding the centrosome is usually a differentiated zone, the attraction-sphere (as), con sisting of archoplasm. The chromatin which may be seen in the cytoplasm of certain cells is as a rule in the form of granules called chromidia (cr). Cer tain other cytoplasmic inclusions that have attracted considerable attention within the past fifteen years exist as granules, chains, or threads, and are known as mitochondria, chondriosomes, plastosomes, etc. (m). Various sorts of plastids (p), such as chloroplastids and amyloplastids, may be present, besides a varying number of solid or liquid substances, collectively designated as metaplasm (me) or paraplasm, which are not supposed to form part of the living sub stance; these are pigment granules, fat globules, excretory products, vacuoles (v), etc.

It has been found possible to explain many cellular activities and even the results obtained by experimental animal breeding by studies of the physics and chemistry of protoplasm. An exhaustive ac count of the subject is impossible and even unnecessary here, but the importance assigned to the physico chemical explanation of life phenomena requires a brief statement. Kossel has separated the cellular constituents into two main groups. (1) Primary constituents are those necessary for life; these are water, certain minerals, proteins, nucleoproteins, phosphatides (lecithin), cholesterin, and perhaps others. (2) Secondary constituents are not essen tially necessary and do not occur in every cell; they are usually stored up reserve material or metabolic products representing principally what we have termed metaplasm.

Water which constitutes about two-thirds of the animal is necessary for the solution of various bodies, the dissociation of chemical compounds, the exchange of materials, the removal of metabolic products, etc. Mineral substances are present in all animal tissues, and different tissues are characterized by the presence of different minerals. The principal ones are potassium, sodium, calcium, magnesium, iron, phosphoric acid, sulphuric acid, and chlorine. The other constituents are of a colloidal nature, and its richness in colloids is one of the chief characteristics of protoplasm. To understand the activities of protoplasm we must therefore know something of the physics and chemistry of colloids.

Colloids (from colla = glue) do not diffuse, or diffuse very slowly, through animal membranes ; in this respect they differ from crystalloids, which diffuse comparatively rapidly through animal mem branes. Wolfgang Ostwald recognized two sorts of colloids : (1) suspension colloids, which are mix tures of solid and liquid phases, are non-viscous, and easily coagulated by salts, e.g. a mixture of finely divided metal and water; and (2) emulsion colloids, which are composed of two liquid phases, are viscous, and coagulated by salts with difficulty. Protoplasm is rich in emulsion colloids ; these may exist as liquid sols, or more solid gels. In either case they consist of fine colloidal particles. Accord ing to another classification colloids may be separated into reversible and irreversible ; the former may change from the sol to the gel state and back again, but the latter are unable to do this. Protoplasm is a reversible colloid, and many cellular structures appear to originate through the gelation of liquid colloids. Since protoplasm is a sol or gel due to water, it is a hydrosol or hydrogel, and because of its water content is said to be hydrophylic. It contains crystalloids and its chemical reactions take place in a dilute solution of electrolytes ; these are substances which dissociate, at least in part, into their constituent ions when in solution, and the ions are electrically charged. For example, NaCl dissociates into electro-positive Na ions (cations) and electro-negative Cl ions (anions). Colloidal particles are likewise electrically charged, those of acid colloids usually negatively and those of alkaline colloids positively. The union and separation of particles and their consequent rearrangement cause gelation, liquefaction, etc. ; it is thus evident that many physiological activities may be due to the electrical charges of ions instead of the chemical nature of the particles themselves. Cellular structures therefore depend upon the tendency of col loidal particles to form aggregates (gelation, coagulation), and more or less upon the electrically charged nature of the particles.

The most characteristic chemical constituents of protoplasm are the proteins.