Units of Structure (Cytology)
The Cell Theory
It is quite essential in constructing any house of knowledge to know the units out of which the intellectual edifice is built. Thus, the chemist must know the elements from which his compounds are made, the writer must be acquainted with the alphabet, the mathematician with numerals, while the biologist must know the units that combine to form the diversities of living bodies.
The structural units with which the biologist deals are termed cells, and the science of cells is called Cytology.
Robert Hooke first suggested the word “cell” in 1665, when he described “little boxes or cells distinguished from one another” that he saw in thin slices of cork. The word is not a fortunate choice, however, suggesting as it does prison walls, for although “walls do not a prison make” neither do they adequately describe the biological unit. Nevertheless the term has come to stay and its use is now extended to indicate units of biological structure, regardless of whether the walls of a cell are in evidence or not.
The conception that all living creatures are made up of organic units, or cells, dates from 1838—1839, when Schleiden and Schwann, botanist and zoologist respectively, published important investigations on the subject. The essential conclusions of the “cell theory” propounded by them, as now modified, are:
1. Every living thing is composed of organic units (cells), or products of their activity;
2. Every living thing begins life as a single cell;
3. Every cell is derived by a process of division from some preceding cell.
A Typical Cell
A generalized diagram of a cell is represented in Figure 91. Within the cell is the nucleus, surrounded by a nuclear membrane. Outside of the nucleus is the cytosome, or body of the cell, enclosed in a cell membrane. Within the cytosome may be embedded pigment granules, chondriosomes, crystals, oil-droplets, vacuoles, plastids, or other substances. Frequently there may also be identified in the cytosome a tiny body called the centrosome, from which delicate lines radiate in every direction.
The nucleus is the headquarters of the whole organized unit, since changes which the cell undergoes are initiated there. It is made up of more than one substance, a fact that is revealed by applying certain stains which, through chemical union, affect parts but not the whole of the nuclear substance equally. That part most readily stained by certain dyes is called chromatin, or “colored material,” and during certain phases of cell life the chromatin material masses together into visibly definite structures called chromosomes.
When cells crowd together during the formation of tissues, they become subjected to mutual pressure, so that their typical spherical shape becomes modified. Any of the several parts of a cell may undergo extreme modification, but most of the fundamental features outlined above, which make a cell an organized unit of living substance, characterize every cell.
Cells are subject to the inevitable laws of change common to living things generally. The succession of changes through which each individual cell passes during its lifetime is termed cytomorphosis.
The term may be extended to include also the transformations through which successive generations of cells pass in the process of differentiation. Thus, in Lewis and Stohr’s Histology the following definition is given: “Cytomorphosis is a comparative term for the structural modification which cells, or successive generations of cells, undergo from their origin to their final dissolution. In the course of their transformation, cells divide repeatedly, but the new cells begin development where the parent cells left off.”
The initial phase of cytomorphosis is characterized by a lack of specialization. This is followed by a series of progressive changes in which the cell becomes finally fitted for its life work, whatever it may be. After a varying period of usefulness, signs of old age appear; eventually the cell goes the way of all flesh and its dead remains are removed from the society of its fellows.
Some kinds of cells, like red blood corpuscles or epidermal cells, live a strenuous life, completing their entire cytomorphosis in a comparatively brief time, while others, like germ cells, may remain dormant for years in an undifferentiated embryonic condition before they finally move forward to fulfil their destiny.
There is much similarity between the life of a cell and of an individual. Both begin with a primitive generalized stage; both pass through expanding infancy and differentiating youth; both arrive at specialized maturity and usefulness; both wear out and die. In one particular, however, there is a striking difference. An individual reproduces its successors only when mature, that is, after it has become differentiated or specialized. A cell, on the other hand, reproduces its kind only while it is still in the comparatively undifferentiated embryonic phase of its life cyle, losing almost entirely the power to do this after it has attained specialization. The result is that many cell units of the body, for example nerve cells, having once gone through to the extreme end chases of their differentiation, lose the power to replace themselves with daughter cells. They have passed beyond the embryonic stage of cytomorphosis when replacement is possible, and after wearing out are unable.to leave successors.
The path of specialization that a cell follows in cytomorphosis is dependent upon the work it has to perform in the cell community of which it is a part.
A typical embryonic cell, uncrowded by neighbors, tends to be spherical in form but rarely has opportunity to remain so. Red blood cells of the lower vertebrates perhaps come nearest to retaining the original form of ernbryonic cells, because the work they do of rolling about through the blood channels of the body is facilitated by their round shape. Leucocytes, or white blood cells, frequently become irregular or amoeboid in form and have the ability upon occasion to change their shape. They are thus able to escape from the blood vessels by squeezing through between the cells of the capillary walls into surrounding tissues, where they nose their way between other cells to remove dead ones or to devour invading bacteria in their mission of sanitation.
Squamous epithelial cells, whose business it is to cover surfaces, become flattened like shingles, while muscle cells, specialists in contraction, assume a much elongated form by which their function of shortening is best accomplished.
Detached cells like spermatozoa, that need to acquire the ability to travel in a fluid without having the propelling power of the heart back of them, as do blood cells, differentiate from the embryonic spherical form into a tadpole-like shape, becoming equipped with a powerful locomotor tail that drives them forward to their destiny.
Skeletal cells, the service of which is to furnish support or protection, develop an interstitial substance; while some secreting cells exhaust themselves at the expense of the cytoplasm in the production of their secretions. Extreme modification in form is seen in nerve cells where specialization has gone so far that it is hopeless to expect such cells ever to perform any other function than that to which they have become committed.
These examples of differentiation are only a few of the many guises in which the building blocks of organic structure appear. Some type forms are diagrammatically indicated in Figure 92.
With the development of the compound microscope, the invention and utilization of aniline dyes and improved cytological technic generally, the presence of chromosomes within the cell nuclei became known, and the far-reaching importance of their peculiar structure unquestionably established.
It has been found (1) that chromosomes are probably always constant in number and shape in every cell of every individual of any particular species; (2) that they behave in a predictable way throughout all the vicissitudes of Cytomorphosis; (3) that every cell not only comes from a preceding cell but every chromosome in the nucleus comes from a preceding chromosome like itself; (4) that during the changes of cytomorphosis when chromosomes apparently break up into indefinite masses, losing their characteristic appearance, they later reappear in the identical size, shape, and sequence of units that they formerly had; (5) that this behavior is evidence that their individuality is maintained and that they are not simply chance masses of unorganized stuff; and (6) that the various chromosomes of any cell not only assume characteristic shapes and sizes, but that they also occur in pairs, two of each kind in every cell excepting the sex chromosomes of many species.
The usual behavior of a cell during the period of increase when two cells form out of one, is called mitosis. The astonishing and intricate details of mitosis are generally unsuspected and quite strange to the uninitiated, although the process is occurring continuously in all living creatures, with countless repetitions. Upon its orderly performance depends every step that serves to differentiate any animal or plant, starting from the fertilized egg and including all growth and organic repairs. The founders of the cell theory surely could not have imagined the extent of the vistas which their preliminary generalizations were destined to open up, for mitosis is a far more detailed and complicated performance than simply pinching the original cell in two.
The essential thing in cell division seems to be the equipment of each new cell unit with a complete set of chromosomes in its nucleus, duplicating in number, form, and size those of the parent cell. In the process of cell formation, which consists of a parent cell dividing into two daughter cells, the chromosomes play the leading part. A very brief and general description of the phases of a typical mitosis, with a series of explanatory diagrams follows, but it should be kept in mind that the stages described as distinct really merge into each other continuously, like a moving picture, and that the actual process of mitosis from start to finish will have repeatedly taken place somewhere within the bodily frame of the reader many times over during the studious reading of this paragraph.
Four general phases of mitosis are recognized as sufficiently distinct to mark well-defined changes from the “resting cell.” These are known as the prophase, metaphase, anaphase, and telophase.
The resting cell (Fig. 93a) is characterized by the presence of a nuclear membrane and a chromatin network within the nucleus, as well as oftentimes also by a pair of centrosomes. In the beginning of the prophase (Fig. 93b-d) delicate spindle fibers appear between the centrosomes, and the chromatin is in the form of a threadlike spireme. As the prophase progresses the centrosomes move apart and the chromatin thread gives rise to separate chromosomes each more or less completely split into two.
In the metaphase (Fig. 93e) the chromosomes lie at the equator of the cell, being connected by spindle fibers with the centrosomes, each of which now occupies a polar position.
In the anaphase (Fig. 93f) these split chromosomes, each containing a sample of every different kind of substance that was distributed along the length of the parent chromosome, begin to separate from each other and to move towards the poles.
During the telophase (Fig. 93g) the elongated cell body begins to pinch in two, while the migration of the chromosomes towards the poles is completed.
Finally the division of the cell body into two parts enclosed in separate cell membranes becomes complete. The spindle fibers have disappeared and a nuclear membrane has reformed around the chromatin network, derived from the chromosomes. Two resting cells take the place of the single one with which the quadrille of mitosis began (Fig. 93h).
Mitosis makes clear how cells are derived from preceding cells. It does not account, however, for the process of vertebrate reproduction involved in the formation of a new individual from two parents.
Sexual reproduction, which is the prevailing means of numerical increase among higher animals and plants, involves something more than an unbroken succession of mitoses. The essential feature of it is the combination of the chromosomal resources of two cells, egg and spcim, to foim an independent starting point for a new series of mitoses.
If mitosis can be defined as making two cells out of one, then sexual reproduction may be described as making one cell, that is, a fertilized egg, out of two cells (Fig. 94).
As has been shown, mitosis is an elaborate process by means of which the same number of chromosomes is maintained throughout all the successive generations of cells that make up an individual. When cells from two such individuals unite, some provision must be made for the reduction in the number of their chromosomes, else the cells of the new series of mitoses in the new individual will have double the typical number of chromosomes for the species. This situation is met by a preparatory process of maturation, or “maturing,” whereby the chromosomal material in each cell is cut in half. This process of the disposal of half of the germ-cell chromosomes is termed meiosis.
Meiosis includes two mitotic divisions, of a special type, following one another in rapid succession. Let us take as an example a species in which the general body cells contain four chromosomes. During ordinary mitoses of these cells the number of chromosomes is temporarily increased to eight; but, with the distribution of half to each daughter cell, the number immediately returns to four. In the same species the two meiotic divisions, which are always accompanied by only one period of chromosome-splitting, also have only eight chromosomes to distribute. The two successive cell divisions during meiosis, however, result in the formation of four cells and, with the equality of distribution typical of ordinary mitoses, each of these four granddaughter cells receives its share of the eight chromosomes, that is, two, the reduced number.
The union of the two germ cells, each equipped with the reduced number of chromosomes, results in the formation of a fertilized egg with the full complement of chromosomes, half of which were contributed by the mature egg and half by the sperm (Fig. 95). In basic plan, therefore, there is for each chromosome derived from the mature egg a comparable one from the sperm. Thus the chromosomes may be said to exist in pairs, each pair consisting of one member contributed by the male parent and one by the female.
The Determination of Sex
In man there are 23 pairs of mated chromosomes (autosomes) in each cell of the body and in addition in the male a mismated pair of sex chromosomes (allosomes), sometimes known as x and y, making a total of 48 all together (Fig. 96). In the female besides the 23 pairs of autosomes there is a pair of allosomes, both of which are of the x type, likewise making a total of 48.
As a result of this inequality, when the sex cells undergo meiosis and reduce to one-half their equipment of chromosomes, the mature eggs are all alike (23 + x), while the mature sperm cells are of two sorts (23 + x) and (23 + y). The sex of the future individual is consequently dependent upon which kind of sperm unites with the egg, as follows:
There is one outstanding exception to the rule that all individuals of any particular species have the same number of chromosomes in every one of their structural units. Among many species there is found to be one more chromosome in each cell of the female than of the male, although curiously in birds as well as in butterflies and moths (Lepidoptera) the reverse is true, the male showing one more chromosome than the female in each component cell. This sexual difference in chromosomal count occurs because the allosomes, instead of being represented in one sex by a mismated pair, are present in that sex as an odd chromosome. In this case they are labelled x and o, the o indicating absence of an allosome. Determination of the sex of the resulting individual, however, comes out exactly as in the case of the mismated allosomes, that is:
Still other combinations of allosomes have been observed, but although the number of allosomes and autosomes varies in different species of animals and plants, all cases agree in producing either one kind of mature eggs, and two kinds of mature sperm, or the reverse, so that the determination of sex in a new individual is referable to a definite combination of the chromosomes, making the chances fifty to fifty that either sex results, which agrees approximately with observed findings.
A World of Billions
The total number of cellular units taking part in the structure of the human body is beyond all imagination. Dr. Keen notes that the hair of a man’s beard grows one millimeter in twenty-four hours. The constituent cells in the make-up of a millimeter of hair are, by count and computation, roughly 10,000, so that seven or eight new cells per minute are formed for every hair. Multiplying this number by the total estimated hairs of the head, one arrives at figures that even a mathematician has difficulty in comprehending.
The example given concerns but one of the many kinds of organic units which take part in the formation and repair of the human frame. When it is remembered that each one of these cells arises from a preceding cell by the elaborate machinery of mitosis, the laziest person may feel well assured that he has accomplished something at the close of every day.