Book - Stoehr's Histology 1-1
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|Stoehr's Histology 1906: 1 Microscopic Anatomy | 1-1 Cytology | 1-2 General Histology | 1-3 Special Histology | 2 Preparation of Specimens | Figures | Histology | Embryology History|
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Since 1839 it has been known that all plants and animals are composed of small structural elements called cells (Latin, cellula; Greeks kOtoc). The lowest forms of animals and of plants are alike in being single cells throughout life. The more complex organisms are groups of cells which have been derived, by process of repeated division, from a single cell, the fertilized ovum. Thus the human body, which begins as one cell, becomes in the adult an aggregation of cells variously modified and adapted to special functions. Since the liver is a mass of essentially similar cells, the problems of its functional activity are the problems of the functions of a single one of its cells. The diseases of the liver are the result of changes occurring in these cells, which must be restored to a normal condition to effect a cure. As this is equally true of other organs, it is evident that cytology, the science of cells, is a basis for both physiology and pathology.
A cell may be defined as a structural element of limited dimensions which under certain conditions can perform the functions of assimilation, growth, and reproduction. Because of these possibilities a cell may be considered an elementary organism. It is described as a mass of protoplasm containing a nucleus. A third element, the centrosome, is found in the cells of animals, but not in those of the higher plants. The centrosome becomes prominent when a cell is about to divide. At other times, in many kinds of cells, it has been found as a minute granule which may be in the center of a very small clear spot in the protoplasm. Ordinarily it cannot be seen unless cell division is about to occur. Some authorities regard the centrosome as a temporary structure which forms shortly before division begins and disappears after it is completed. Others regard it as a permanent and essential part of a cell, which accordingly consists of protoplasm, nucleus, and centrosome.
Protoplasm is the living substance of which cells are composed. More specifically the term is applied to this living substance exclusive of the nucleus, or to the corresponding dead material, provided that death has not changed its physical properties. It has been proposed to substitute the name cytoplasm for protoplasm in the restricted and earlier sense of the][term, to call the nuclear substance nucleoplasm [karyoplasm], and to consider both cytoplasm and nucleoplasm as varieties of protoplasm. Although these names are often employed, the cell substance apart from the nucleus is ordinarily called protoplasm.
Fig. 1. Diagram of a Cell. Microsomes and filar mass only partly sketched.
Protoplasm is a heterogeneous mixture of substances forming a soft, viscid mass of neutral reaction. In distilled water it swells but does not disappear. It consists of water, salts and organic substances, some in solution, and some in a colloidal state. The organic bodies are classed as proteids, glycogen or some allied carbohydrates, and lipoid (fat-like) bodies. Protoplasm may exist in a numberless variety of forms.
On microscopic examination protoplasm is seen to contain small granules, microsomes. In different cells these vary in abundance and in character. They may be absent from the outer layer of protoplasm, the exoplasm, which is firmer and chemically different from the inner endoplasm, and perhaps has a separate function. The microsomes have been considered both as inert bodies and as the essential living basis of protoplasm. The simplest description of protoplasmic structure is that it consists of a fluid ground substance in which microsomes are embedded.
With high magnification it appears that the protoplasm contains a network of filaments (called mitome, or the filar mass, from the Greek fihoc and Latin filum, both meaning "a thread," â€” sppngioplasm is another s3nionym). This network is embedded in a more or less homogeneous and chemically different ground substance (paramitome, interfilar mass, or hypoplasm). Some of the filaments appear as rows of microsomes, but small particles may also be found in the ground substance between the filaments. The conception of protoplasm as fibrillar or reticular has been considered at variance with the "granular theory," yet undoubtedly both fibrils and granules occur in protoplasm.
According to a third interpretation protoplasm has the structure of foam, or of an emulsion, â€” that is, it consists of minute droplets of one substance completely surrounded by walls of a different substance. This view, which has much in its favor, is not inconsistent with the presence of granules or of fibrils scattered through the mass.
In addition to these general characteristics the protoplasm of particular cells may contain other structures of various significance. These may be grouped as follows:
I. Fibrils. Although an obscure fibrillar network may be characteristic of all cells, a high development and orderly arrangement of fibrils opcurs only in certain specialized cells, as for
example, in muscle, nerve, and connective tissue cells. These fibrils are of very different sorts and will be described more fully in the section on General Histology.
2. Granules. These are not the microsomes found in all protoplasm, but are larger bodies of definite staining reaction, which often are important secretory products elaborated by the cell. In many gland cells, and in the "granular" white blood corpuscles, these structures are conspicuous. Other granules may be excretory or waste products of the cell, and some of these, which, without being stained, are deeply colored, are called pigment granules.
Fig. 2. Fibrils in a Nerve Cell.
Fic. 3. Clumps of Granules (NissL's Bodies) in a Nerve Cell.
Fig. 4.Vacuole in Young Fat cells.
Fig 6, ReTictijLAM Nktwork iN A NsKVh CmLL, C After
the cell. It consists of a or nuclear sapi in which
3. Vacuoles. Well defined, round spaces, apparently empty, may occur in the protoplasm due to the formation of droplets of fat or of watery fluids. They vary greatly in size, and one or several of these vacuoles may be found in a single cell.
4. Canals, of two sorts* (a) Secreton^ canals, which occur in protoplasm of gland cells and empty into the gland cavity or lumen; (i) fine tubes which communicate with lymphatic spaces outside of the cell* They are found in all cells of higher physiological importance, but are lacking in most of the supporting tissues and in stratified epithehum. They presumably share in nourishing tlie cell and have been called the ** trophospongium," This name, implying a network, is due to the opinion, not established however, that the Uttle canals are occupied by cell processes extending into the protoplasm from adjoining '^capsule cells." Other investigators consider that the trophospongium canals are wholly within the cell and constitute a form of vacuole.
5. Closed networks, which do not open at the periphery of the cell. This ** reticular apparatus'* has been found in nen-e, cartilage and many gland cells. Its significance is unknown,
6. Inclusions. These are foreign bodies which have been ingested by the cell and are found in the protoplasm. Inert crystalloid substances formed within the cell are also called inclusions. The name "paranucleus" has been applied to various structures, such as a dead cell ingested by a living one, a transformation of the centrosome, or a mass of secretion. Some of the paranuclei are still obscure.
The nucleus (Latin, nuclms, '*the kernel of a nut"; Greek, ^i^jov, "a nut'*) is a well defined, refractive body of vesicular form situated within membrane enclosing a mass of ground substance, there is a fibrillar network associated with some deeply staining bodies. The ground substance and network are closely related to the corresponding structures in the protoplasm. In fact, at the time of cell division, when the nuclear membrane disappears, the network and ground substance of nucleus and protoplasm are respectively continuous with one another. The ground substance and a delicate, fibrillar portion of the network do not stain readily; therefore they are called achromatic substances. The fine achromatic fibrils of the network are further designated as linin fibrils. Since the linin fibrils cannot be isolated for chemical analysis, their composition is unknown. There are two deeply staining or chromatic substances found in the nucleus. One of these, chromatin, is its most essential and characteristic element. Chemically it is a nucleo-proteid, but undoubtedly it exists in several varieties. A portion which responds to acid dyes is called oxychromatin, in distinction from the ordinary form which takes the basic stains. Chromatin is distributed as irregular granules or coarse strands along the linin fibrils, thus tending to form a network (Fig. i, and Fig. i8, p. i6). Often a nucleus presents from one to several large clumps of chromatin, known as chromatin "knots." These are to be distinguished from the round masses of pyrenin, called nucleoliy which are found between the meshes of the nuclear network. All nuclei contain chromatin, but many are without nucleoli. The latter are present with great regularity in certain kinds of cells. Usually only one is found in a nucleus, although several may occur (Fig. i8). They difiFer from chromatin chemically, as is evident from differences in staining, and also functionally, as is seen during cell division. Pyre nin, of which the nucleolus is said to be composed, is, however, a cytological rather than a chemical term.
The nuclear membrane is usually described as formed of amphipyrenin, a term of questionable value. The membrane may consist of a distinct chemical substance as the name suggests, or it may be rather a condensation of the nuclear reticulum, in which the linin fibrils terminate. A nuclear membrane may be simulated by a thin superficial layer of chromatin.
Every cell contains a nucleus consisting, as has been shown, of nuclear membrane, ground substance, a network of linin fibrils and of chromatin, with perhaps a nucleolus. Non-nucleated bodies like the mammalian red blood corpuscles, and the dead outer cells of the skin have lost their nuclei in the course of development. Occasionally a single cell contains two nuclei, as is frequent in the liver, or even several nuclei, as in certain bone cells.
The centrosome is a minute body consisting of a homogeneous or sometimes reticular mass, the centroplasm, which contains a much smaller body, the ceniriole. Such centrosomes have been observed in the invertebrate animals. The cells of vertebrates are not regarded as favorable for investigations of the finer structure of centrosomes. In them generally both centroplasm and centriole appear as a single small granule, the centrosome. This granule is usually, but not always, surrounded by a zone of protoplasm which is so modified as to form a darker or a lighter area, the archoplasm (Fig. i). (The archoplasm of certain spermatic cells is called the idiozome,) The centrosome may be near the nucleus or distant from it, frequently being found between the nucleus and the free surface of the cell. Rarely, as in a few invertebrates and in cancer cells, the centrosome has been found within the nucleus. In many gland cells it lies where the secretion accumulates, the expulsion of which is accomplished by the contraction of the protoplasmic framework between the masses of secretion. In the intestinal epithelial cells which send out motile projections of protoplasm (pseudopodia), the centrosome lies just beneath the place of origin of these projections. If one considers also the relation of the centrosome in the spermatozoa as well as its r61e in cell division, it seems almost certain that the centrosome is the active or passive center of the motor functions. In connection with cell division, the centrosome undergoes a cycle of changes of varying duration. That stage which is continued longest is characterized by a doubling of the centrosome, following the division of the centriole in two. The double body thus formed is the diplosome. In many resting cells, or those not actually in the process of division, a diplosome is found, and this is significant as indicating the readiness of the cell for undergoing division without delay.
- Online editor - "Cell wall" is a historic term for the cell membrane and the term is now used to describe non-membrane structures in outside plant cells and outside bacterial membranes.
A cell wall or cell membrane is an independent membranous layer covering a cell and being clearly distinct from the underlying protoplasm. It is not an essential constituent of a cell. Often it is lacking, and when present it is either a modification or a secretion of the peripheral protoplasm. If the membrane surrounds the cell on all sides it is called a pellicula; if it is on only one side, covering the free surface, it is a cuiicula. (The former term is seldom used.) Cells may unite with one another by protoplasmic processes of varying length and width, thus forming cellular networks; or they may completely fuse so that their nuclei appear irregularly distributed through a single mass of protoplasm. Such a formation is a syncytium [plasmodium]. This name is applied also to such structures as the striated muscle fiber, due not to the fusion of cells but to the multipUcation of nuclei in an undivided mass of protoplasm.
Although cell membranes are usually lacking, or if present are often inconspicuous in animal cells, they are highly developed in plants. Thus cork is a mass of dead cells from which nuclei and protoplasm have disappeared, leaving only the cell walls. In describing cork, Robert Hooke introduced the name "cell," in 1667. He wrote: "I took a good clear piece of Cork and with a Pen-knife sharpened as keen as a razor, I cut a piece of it off and thereby left the surface of it exceedingly smooth, then examining it very diligently with a microscope, me thought I could perceive it to be a little porous. . . . These pores or cells were not very deep but consisted of a great many little Boxes In this way one of the briefest and most important of scientific terms was introduced.
Form and Size of Cells
Cells are regarded as typically spherical in form. Spherical cells are comparatively numerous in the embryo, and in the adult the resting white blood corpuscles which float freely in the body fluids assume this form. Such cells are circular in cross section. When spherical cells are subjected to the pressure of similar neighboring cells they become polyhedral and usually appear six-sided in cross section. Such cells, as a whole, may be cuboidal, columnar, or flat. Certain cells become fusiform (spindle-shaped) or are further elongated so as to form fibers; others send out radiating processes and are called steUate. Thus the form of cells is extremely varied. The shape of the nucleus tends to correspond with that of its cell. It is usually an elliptical body in elongated cells, and spherical in round or cuboidal cells. In stellate cells it is either spherical or somewhat elongated. Crescentic nuclei and others more deeply and irregularly lobed are found in some of the white blood corpuscles and in giant cells.
The size of cells ranges from that of the yolks of birds' eggs - which are single cells at least shortly before being laid - down to microscopic structures four thousandths of a millimeter in diameter. The thousandth of a millimeter is the unit employed in microscopic measurements. It is called a micron, and its symbol is the Greek letter /jl. The small cells referred to are therefore four microns, 4 /i, in diameter. \The size of any structure in a section of human tissue may be roughly estimated by comparing its dimensions with the diameter of a red blood corpuscle found in the same section. These red corpuscles are quite uniformly 7.5 /i in diameter.;
The vital properties of cells are more fully treated in text-books of physiology. They include the phenomena of irritability, metabolism, contractility, conductivity, and reproduction. Under irritability may be grouped the response of cells to stimuli of various sorts such as heat, light, electricity, chemical reagents, the nervous impulse, or mechanical interference. Metabolism, in a wide sense, includes the ingestion and assimilation of food, the elaboration and secretion of desirable products, together with the elimination of waste products. Contractility may be manifest in the locomotion of the entire cell, in the vibratile action of slender, hair-hke processes, the cilia, or in contraction of the cell body. Conductivity is the power of conveying impulses from one part of the cell to another. Reproduction is seen in the process of cell division. Many phases of these activities are observed in microscopic sections and as such they will be referred to in later chapters. A few which are of general occurrence will be described presently.
The imicellular animal, Amoeba, exhibits a type of motility known as amoeboid, which has been observed in many sorts of cells in the vertebrate body. In marked cases, as in certain white blood corpuscles (the leucocytes), the cell protoplasm sends out fine or coarse processes which divide or fuse with one another, causing the cell to assume a
sÂ«', 5 6 8 10 Minuses. g^eat Variety of forms. The processes may be retracted, or they may
Fig. 8. Leucocytes of a Frog. X 560.
Changes in form obsen ed duriiiR ten minutes : bCCOme attached SOmCWhcrC and o, at the beeinninj; of the observation;
half minute later, etc. draw the remaindrr of the cell body after them, the result of which is locomotion or the so-called wandering of the cell. Such wandering cells play an important part in the economy of the animal body. Their processes can flow around granules or cells and thus enclose them in protoplasm. Some of these ingested bodies may be assimilated by the cell as a result of complex chemical and osmotic reactions. Cells which feed on foreign particles and can alter or digest them are known as phagocytes. Amoeboid movements take place very slowly. In preparations from warmblooded animals they may be accelerated by gently heating the object.
Another form of motion, which, however, does not occur in living cells, consists in an oscillation of minute granules within the cell. This may be due to diffusion currents or to the Brownian phenomena. It may often be seen in salivary corpuscles.
Formation and Reproduction of Cells
In the past, two sorts of cell formation have been recognized, namely the spontaneous generation of cells, and the origin of cells through the division of pre-existing cells. According to the theory of spontaneous generation it was once thought that animals as highly organized as intestinal worms came into existence from the fermentation of the intestinal contents. After this had been disproved it was still thought that cells might be formed directly from a suitable fluid, the cytoblastema. Something of the sort may have occurred when life began, and it is the expectation of certain investigators that conditions may yet be produced which shall lead to the formation of organic bodies capable of growth and reproduction. At present, however, only one source of cells is recognized, â€” the division of existing cells. "Omnis cellula e cellula." A nucleus likewise can arise only by the division of an existing nucleus. There is no satisfactory evidence that a nucleus may be formed from nonnudeated protoplasm. In cell division the nucleus divides first and then the protoplasm, generally into two nearly equal parts. During the process a special grouping and transformation of the nuclear substance occurs in accordance with fixed laws. The ordinary mode of cell division is called mitosis or indirect division [karyokinesis] and the characteristic groups of nuclear material are commonly known as mitotic figures. Mitosis is arbitrarily but conveniently divided into three successive phases, the prophase, metaphase, and anaphase, in which respectively the nuclear material prepares for division, divides, and returns to its usual condition. (The final stages of reconstruction are often grouped as a fourth phase, the telophase.) In the details of mitosis there are considerable variations, not only in different animals but also in different kinds of cells in a single species. The account of the process which follows will apply only in a general way to a particular case of cell division which the student may be examining.
Prophase, The centrosome and nucleus approach one another until the centrosome is close to the nuclear membrane where it lies surrounded by the clear zone of archoplasm. The archoplasm contributes to the formation of radiating fibrils which extend from the centrosome in all directions, and are known collectively as the centrosphere [astrosphere]. The two parts of the centrosome which had formed in the '* resting stage" by the division of one, are in the midst of the centrosphere. They move apart, the diplosome thus separating into two centrosomes, and ^ the centrosphere becoming divided into two spheres, each of which contains a centrosome. Fig. 9.
The nucleus meanwhile enlarges and its chromatin stains much more deeply. The branching portions of the chromatin network are withdrawn, so that instead of a net, the entire chromatic material forms one convoluted thread, a monospireme, as this mitotic figure is called. The thread is at first more closely coiled than it is later. It divides transversely into a definite number of segments, called chromosomes. These bodies may be spherical or rod-like, but generally they are U- or V-shaped. The apices of all the V's may at first point toward the centrosome with their free ends directed away from it as shown in the diagram. Fig. 9. Instead of being arranged in the orderly manner of the diagram, however, the chromosomes are so massed that they can scarcely be counted.
Fig. 9. Early Prophase: Mono-spireme.
Fig.10. Later Prophase: Mono-spireme.
This is shown in Fig. 15, representing mitoses in the salamander. In man they are even harder to count and have been estimated both as sixteen and twenty-four. This is of importance, since in any one species the number of chromosomes is believed to be constant for all the cells except the sexual cells. Certain worms in which the chromosomes are only two or four in numter and hence can be followed with certainty, have furnished the strongest evidence for this. Except in the sexual cells, the number of chromosomes is always even. Since it has been found that the same number of chromosomes which entered into the formation of the chromatin network of the resting nucleus, will emerge from that net preceding mitosis, the suggestion is made that the chromosomes retain their individuality in the quiescent nucleus. They are regarded as disguised by numerous branches. In the prophase of mitosis the chromatin in many cases does not form a continuous thread but passes from the network condition directly into that of a group of chromosomes. Such a group is, however, properly called a monospireme.
The centrosomes, in moving apart from one another, travel along the nuclear membrane to points 90Â° from their original position. Thus if before division the centrosome was on one side of the nucleus, now the two centrosomes into which it has divided will be found one at either end of the nucleus. Fine fibrils extend between them as they separate, constituting the central syndic. Outside of these, there are other fibrils passing from the chromosomes to the centrosomes (Fig. 10). These fibrils, which are sometimes derived from those of the centrosphere and sometimes from the linin framework of the nucleus, are known as mantle fibrils. Toward the end of the prophase the nuclear membrane disappears, together with the nucleoli.
Metaphase. The V-shaped chromosomes become arranged about
Fig. 11. Early Metaphase: Mon-
Fig. 12. Metaphase : Division
the equator of the spindle in such a way that their apices point toward the axis of the spindle and their free ends radiate from it in all directions, Fig. II. At either apex of the spindle is the centrosome surrounded by the centrosphere, the radiating fibrils of which are now called polar radiations. K the cell at this stage is viewed from one of its ends or poles, the chromosomes together constitute a single star and this mitotic figure is accordingly called the monaster. Fig. 15 shows the monaster both in side and polar views.
In the prophase, before the chromosomes have formed, the convoluted thread of chromatin is sometimes seen to be split longitudinally into halves. During the prophase, therefore, each V-shaped chromosome may consist of parallel portions which remain together until the monaster is complete. Then, beginning at the apex of the V, the halves of each chromosome axe drawn apart as if by means of the outer spindle, or mantle fibrils. In an unusual but important form of mitosis, known as helerotypical mitosis, the partially divided chromosomes remain for some time united by their ends, in the form of rings. How such ring-shaped chromosomes may occur is shown in Fig. 12. Ordinarily the V's are completely divided, and the separate halves travel, apex forward, toward their respective poles. Two stellate groups are now observed and this stage is called the dyaster (Fig. 13). Stretching between these groups are the central fibrils of the spindle, not shown in the drawing. A development of granules in these fibrils along the equatorial plane may take part in forming a new transverse cell wall.
The metaphase is the stage of division of the chromosomes, and by some writers it is considered very brief, the monaster being counted the last of the prophase, and the dyaster being included in the anaphase.
Fig. 13. Latk Metaphase: Fig. 14. Anaphase : DiDyaster. spireme.
Anaphase. The chromosomes of either portion of the dyaster are the same in number as those of the nucleus from which they came. Each group represents half of the chromatic material. These new chromosomes unite with one another, each group forming a spireme. The mitotic figure thus produced is the dispireme (Fig. 14). The centrosphere loses its radiations, becoming reduced to a zone of archoplasm, and the centrosome often divides to form a diplosome. A nuclear membrane forms, beginning at a point opposite the centrosomes. The nucleoli reappear as the chromatin thread returns to a network by sending out branches. Thus two resting nuclei have formed. Meanwhile the protoplasm along the equator constricts, and here, sometimes aided by the granules of the central spindle, the new cell wall develops to complete the process of mitosis.
Summary. The stages described have been successively the reticular quiescent stage, the monospireme, monaster, dyaster, dispireme, and .the return to the reticular condition. These terms refer to the arrangement of the chromatic material. The achromatic structures were successively the centrosome surrounded by archoplasm; the diplosome in a centrosphere; two centrosomes connected by a spindle and each surrounded by polar radiations; the division of this amphiaster, as it is called, into two centrospheres each with its centrosome; and, finally, the reduction of the centrosphere to archoplasm. Each new cell ordinarily receives half of the protoplasm, spindle, centrosome and chromatic material of its parent, and becomes a cell of the same sort.
The process of mitosis requires probably about half an hour, but the time is variable and it may last several hours. In the blood cells of amphibia it is said to take two hours and a half. Mitoses will be found
Â». g., from the pole).
Monaster (viewed from the side).
from above). ' Division of the protoplasm (Dispiremes).
Fig. 15. Mitotic Figures from the Epithelium of the Oral Cavity of Triton Alpestris. X 560.
in all well preserved, rapidly developing tissues. They are abundant in embryos; and if numerous in tumors they furnish evidence of rapid growth and malignancy. After death, if the tissues are not hardened by cold or reagents, it is thought that mitoses may go on to completion, as they are absent from specimens which are not properly preserved.
Varieties of mitosis. In connection with the formation of sexual cells (the ova and spermatozoa) there occur two successive mitotic divisions of a unique sort. A cell which had itself been formed by ordinary mitosis, in preparing for division converts its chromatic material into one half 0} the usual number 0} chromosomes. It divides into two cells, each with the reduced number, and these divide once more in the same way. Thus four cells, each having one half the usual number of chromosomes, arise from the one which first presented this peculiarity. With some modifications but without further division they may become the mature sexual cells. The process of their formation is called maturation , and the two peculiar and final mitoses through which every mature sex cell has passed are called reduction division s. In the process of fert Uizaiion two mature sexual cells, â€¢a spermatozoon and ovum respectively, fuse, and the normal number of chromosomes is restored. Thus each parent contributes an equal number of chromosomes to the fertilized ovum and these have been considered bearers of hereditary qualities. The reduction divisions will be further considered under Testis and Ovary.
An unusual form of mitosis is that in which the centrosome divides into more than two parts and the cell correspondingly divides at once into several. These pluri- or multi-polar mitoses are said to occur normally in parts of certain higher plants; they have been induced by injecting drugs into the skin of salamanders; and are sometimes found in human cancer cells and in the rapidly growing connective tissue of scars. They may lead to an unequal distribution of the chromatic material in the cells which they produce.
Fig. 16. Mitosis in Human Cancer Cells. (From Wilson, after Galeotti.) a. Asymmetrical mitosis with unequal distribution of chromatin; bÂ» tripolar mitosis; c* quadripolar mitosis.
For further information regarding mitosis, and for definition of the many terms frequently employed but not mentioned in this account, the student is referred to Prof. E. B. Wilson's book entitled "The Cell."
Amitosis or direct cell division takes place without spindle formation or the rearrangement of nuclear material. The nucleolus, nucleus, and cell body successively divide by fission, or by elongation and constriction, into two parts. The r61e of the centrosome has not been determined. This form of divison is rare and its significance unknown. The suggestion that it is more primitive than mitosis lacks support. Generally it is regarded as a sign of cell degeneration, since it occurs in old cells - leucocytes and the superficial cells of the bladder â€” the cell bodies of which often fail to divide following the division of their nuclei. Thus cells with two or more nuclei may be produced by amitosis. It occurs in wounded
Beginning^ Completed Beginning Completed
Division of the nucleolus. Division of the nucleus.
Fig. 17. Amitosis in Epithelial Cells from the Bladder of a Mouse. X560.
tissues where it has been interpreted both as a result of injury and as evidence of activity toward repair. In the egg tubes of certain insects amitosis is a common and normal process.
Cytomorphosis is a comprehensive term for the structural modifications which cells or successive generations of cells may undergo from their origin to their final destruction. It implies that the life of a cell is limited, and that during its life it may change in structure by becoming dijferentiated^ or adapted to the performance of special functions, and that finally it will pass through regressive changes to its death. Successive generations of cells may represent stages along a certain line of diflFerentiation. The cells resulting from mitotic division begin their specialization where the parent cell left off, and the phenomena of regression are then reserved for the final generations in the series. Four successive stages of cytomorphosis have been recognized: First, the undifferentiated stage; second, that of progressive differentiation; third, the stage of regression; fourth, the removal of dead material. These may be considered in turn.
Undifferentiated cells, as can be seen in sections of young embryos, are characterized by large nuclei and relatively little protoplasm. They have great power for undergoing division. The subsequent increase of cytoplasm which makes functional differentiation possible, retards the rate of mitosis. In the adult, relatively undifferentiated cells are found in many situations, as, for example, in the deep layer of the epidermis. These cells are a source of supply to replace the outer cells as they differentiate, die, and are cast ofiF. Since they can produce only epidermal cells, they are themselves partly differentiated. The fertilized ovum which can produce all kinds of cells must be regarded, in spite of its size and great mass of yolk-laden cytoplasm, as the least diflferentiated.
The progressive specialization of cells concerns chiefly their protoplasm, yet in the case of the muscle fibers of the salamander it is accompanied also by marked nuclear changes. Typical muscle nuclei from Necturus embryos 7 mm. and 26 mm. long, respectively, are shown in Fig. 18. The significance of the differences between them is not known, as
they have been but recently detected. The cytoplasm of muscle cells differentiates its contractile function beyond all others, and becomes filled with contractile fibrils. Many kinds of cells are specially modified for producing secretions which may either be discharged, as from gland cells, or in a somewhat solid state may remain in contact with the cell, thus forming certain of the intercellular substances. Small amounts of structureless intercellular substance, such as is sometimes found between epithelial cells, are caUed cement substance^ even though it may be fluid. Between connective tissue cells the intercellular substances are formed in such quantity that they far exceed the bulk of the cells which produced them. These ground substances may be homogeneous, or permeated with fibrils and granules, formed either by the exoplasm or by the transformation of the intercellular substance. The remnant of ground substance between the fibrils is another so-called cement substance. In cartilage and bone, the cells appear scattered through the ground substance which by their differentiation they have produced.
Regression or degeneration is the manifestation of approaching death. Normally it is not seen in nerve cells and probably not in the voluntary muscle cells. Subtle and unrecognized changes may occur in them in old age, but they remain active throughout life; if destroyed, they can never be replaced. In many glands, in the blood and in the skin, however, the cells are constantly dying and new ones are being dififerentiated.
Fig. 18. Nuclei of Striated Muscle Fibers from Young Salamanders (Necturus). (Eycleshymer.)
A, From a 7 mm. embryo ; B, from one of 26 mm.; ch., chromatin knot ; g. s., ground substance ; I, linin fibril; n., nucleolus ; n. m., nuclear membrane.
In a few organs the cells perish, but no new ones form, so that the organ to which they belong atrophies. Thus the mesonephros (Wolffian body) largely disappears during fetal life; the thymus becomes vestigial in the adult; and the ovary in later years loses its chief function through the degeneration of its cells.
The optical effects of regression cannot at present be properly classified. In a characteristic form, known as "cloudy swelling," the cell enlarges, becoming pale and opaque. In another form the cjbII shrinks and stains deeply, becoming either irregularly granular or homogeneous and hyaline. The nucleus may disappear as if in solution (karyolysis, chromatolysis), or it may fragment and be scattered through the protoplasm (karyorhexis). If the process of degeneration is slow, the cell may divide by amitosis. It may be able to receive nutriment which it cannot assimilate, and thus its protoplasm may be infiltrated with fat and appear vacuolated. It may form abnormal intercellular substances, for example, amyloid, or the existing intercellular substances may become changed to mucoid masses or have lime salts deposited in them. In short, together with optical changes in the cell substance there is often an impairment or perversion of function.
The removal of dead cells is accomplished in several ways. Those near the external or internal surfaces of the body are usually shed or desquamated, and such cells may be found in the saliva and urine. Those which are within the body may be dissolved by chemical action or devoured by phagocytes.
Every specimen of human tissue exhibits some phase of cytomorphosis. In some sections a series of cells may be observed from those but slightly differentiated, to the dead in process of removal. Because of the similarity and possible identity of this normal, "physiological" regression, with that found in diseased tissues, such specimens should be studied with particular care.
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Cite this page: Hill, M.A. (2021, April 11) Embryology Book - Stoehr's Histology 1-1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Stoehr%27s_Histology_1-1
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