Normal histology

Preface

The general plan and scope of this outline of histology are the result of experience in teaching this subject to students of medicine under conditions which require economy of time. In order to accomplish the greatest amount of instruction under these circumstances it seemed necessary to present, early in the course, certain generalizations which might be kept constantly in mind and assist the memory in retaining facts by showing their logical correlation. The broad ideas of almost universal applicability which were chosen for this purpose were : first, the conception of the cell as tlie active constituent of tissues ; second, the general rule that the elementary tissues are composed of cells and intercellular substances, and that their differences depend upon the proportions or characters of those constituents ; third, that the structural details of the different tissues are intimately correlated to their usefulness — i. e., that function and structure are mutually dependent upon each other, being but two aspects of a single device or arrangement; and, fourth, that the ability of tissues to perform active functions is, in the main, roughly proportional to the number or size of the cells entering into their structure.

The author believes that his experience has shown that the early introduction of these fundamental ideas has been a distinct aid to the student ; and it is his belief that such would also be the case no matter to what limits the course of instruction in this subject might be extended.

In the elaboration of these ideas the author has been in the habit of arbitrarily dividing the activities of the cells of the body into four theoretical groups : the reproductive activities, leading to the production of new cells ; the formative activities, through wliich the structural modifications resulting in the formation of different varieties of tissue were brought about; the riidritive activities^ maintnining the integrity of the tissues already formed ; and the functional

PREFACE.

activifics, through which the tissues serve the whole orgauism of which they are constituent parts.

Assuming that histology should invariably be taught largely by practical work and experience in the laboratory, and recognizing the fact that it must have a place in the beginning of a medical curriculum when the student’s technical facility is least developed, the writer believes that the best way to teach the subject is by means of demonstrations at which the students have ample opportunities to become familiar with the structural details of the best attainable specimens so prepared as to reveal the presence and arrangement of the tissue-elements. In addition to such a course of demonstrations he should receive practical instruction in histological technique, during which he may become versed in the methods employed in the preparation of the specimens he uses in the demonstration course. This work may be so arranged as to permit his preparing a collection of specimens for himself.

With such related courses of instruction in mind, the writer has added a section on Histological Technique to the present volume. In tliis part of the book he has endeavored to make clear those various methods of preparing tissues for microscopical study which he has found to yield excellent results, not only in his own hands, but also when employed by those who have had no previous experience in such work. The more complicated methods, requiring such experience, have been omitted.

The student occasionally desires to consult the original sources of information which has been published. To encourage this practice, the author refers him to The Journal of the Royal Microscopical Society, Archiv fur mikroskopische Anatomie, and the Zeitschrift fur wissenschaftliche Mikroskopie, in which abstracts of the current literature will be found. Admirable text-books upon technique are Lee’s 3ficrotomists’ Vade 3Ieciim, of which a more recent revised edition is Lee and Mayer’s Grundzuge der mikroskopischen Technik, and Mallory and Wright’s Pathological Technique.

338 East Twenty-sixth St., New York, May, 1900.

E. K. D.

HISTOLOGIC A I. TECHNIQUE.

CHAPTER XXVI.

PRACTICAL SUGGESTIONS FOR THE CARE AND USE OF THE MICROSCOPE— MICROSCOPICAL TECHNIQUE 265

NORMAL HISTOLOGY.

Introduction

Dueing life all parts of the human body are the seat of constant activity. This is a fact too readily overlooked by the student who gains his knowledge of the structures of the body by a study of the tissues after death. To make that study of use to him in his medical thinking he should constantly bear in mind that he is viewing the mechanism of the body while it is at rest, and, furthermore, that the metliods employed in the study of the minute structure of the parts not only arrest the normal activities of those parts, but expose them to mutilation. He must, therefore, constantly supplement the knowledge of structure he gains by his histological studies by recalling to mind and applying that which he has acquired by a study of physiology, habitually associating his ideas of structure and functional activity, until he can hardly think of what a structure is without at once recalling what it does. This he cannot do till he has mastered at least the general outlines of systematic anatomy and of physiology. Those two fundamental subjects are brought together by an intelligent study of the minute structure of the body, histology, which, for this reason, has also and appropriately been called physiological anatomy.

But the student of medicine must go beyoud this. To the conception of the body during health, which he has formed by this thoughtful method, he must tlien add a conception of the influence exerted, both on the structure and activities of the body, by abnormal conditions wliicli disturb or thwart the usual workino; of that complex mecluiuism. The more closely he can make those conceptions agree with observed facts, the more perfect will l)ecome his ability to interpret the pliysical signs and symptoms of disease, 2 17

INTROl) UCTION.

and the clearer will grow his insight into the causes and tendencies of the processes of which they are an expression.

In all his studies he must seek not merely to train his powers of observation ; he must endeavor to cultivate his ability to interpret what he sees ; to deduce the processes and causes that have wrought the results he perceives, and to compare those deductions with the conceptions of living things he has already formed, so that his ideas may remain in perfect accord with one another as his grasp of the subject enlarges. By so doing he may hope to create a lifelike mental picture of the body both in health and during disease.

The activities of the body involve changes in the substances of which it is composed. Some of these changes are always destructive in character — that is, they result in chemical rearrangements which convert more complex combinations of less stable nature into simpler combinations of greater stability. Such chemical changes, whether they take place within the body or in external nature, among organic or inorganic substances, are always accompanied by a liberation of energy hitherto locked up or stored in latent or potential form in the compounds of higher complexity. It is this liberated or kinetic energy which is utilized by the bodily mechanism for the performance of internal or external work. When directed in various ways and operating through different structures, this energy occasions visible movement, appears as heat, etc., or passes again into the latent form in the elaboration of more complex chemical substances from those of simpler constitution.

These associated transformations of matter and energy involve a continual loss to the bodily economy. The stock of energy is diminished during the execution of external work and by the dissipation of heat. The store of useful chemical substances is reduced by their progressive conversion into compounds that are insusceptible of further utilization, and which, in many cases, may act injuriously upon the structures of the body. Under normal conditions such substances are eliminated from the body.

It is evident, then, that the body is constantly suffering a loss of both energy and matter. This loss must be made good if tlie activities of the body are to be maintained, and this is accomplished, during h.ealth, through the absorption of fresh material, containing latent energy, from the food taken into the body.

The activities of the body are not tlu' same in all its parts. They are all alike in one })articular — namely, that each part must main-

INTRODUCTION.

tain its own nutrition, incorporating the food-materials that are accessible to it and using them in such a way as to keep its structure in a normal condition. But, aside from this duty whicli is common to all, each part lias a duty to perform for the good of the whole organism ; and, as we shall see, this duty often appears to be paramount, the activities which it necessitates being carried on even if they involve a sacrifice in the nutrition or structure of the individual part.

Each part of the body has some particular kinds of work assigned to it, which constitute its functions, and which it performs for the benefit of the whole body. The development and life-history of each part has direct reference to those functions, through which it co-operates with all the other parts in maintaining the integrity and normal activities of the whole body, all the parts being interdependent upon each other and subservient to the general needs.

The foregoing considerations prepare us for the fact that the structure of the various parts of the body differs in its details. The study of those finer details can only be pursued with the aid of the microseope, for the microscopical constituents of the tissues are the elements which confer upon them their particular properties and powers. This study is called histology.

Investigation has shown that there is one form of tissue-element which is always present in all parts of the body. This is the cell. It does not always possess the same form or internal structure, but in all its variations the same general plan of construction is adhered to. These cells are the essentially active constituents of the tissues. It is within them that the transformations of matter and energy are chief! V carried on, and it is due to their activities that the tissues forming the body are elaborated and enabled to perform their several functions. These marvellous powers possessed by the cell have created our conception of life, and, in spite of eager study, remain inscrutable. We do not know why a living cell differs from a dead cell, but we do know that the mysterious vital powers are only derived from pre-existent living cells and are not antagonistic to the chemical and physical laws governing unorganized matter.

All the cells of the body are descendants of a single cell, the egg, from which they arise by successive divisions, and throughout the existence of the body they retain some of the characters of the original cell. But as the body develops the cells of the different parts display divergent tendencies, which finally result in the for-

INTRODUCTION.

mation of a considerable variety of tissues, grouped in various ways to form organs or systems of very different kinds of utility to the whole organism. This divergent development is known as differentiation and results in a specialization of the different parts of the body. Its study constitutes embryology, but it will make the comprehension of histology easier if some of the simpler and broader facts derived from a study of development are first briefly summarized.

A new individual arises through the detachment of a single cell, the ovum (Fig. 1), from the parent organism. This cell divides

Fig. 1.

- ...

■f

e a iisy, '

Section of human ovum and its immediate surroundings within the ovary. (Nagel.) a, zona pellucida ; b, cytoplasm of the ovum ; c, granules and globules of stored food materials within the cytoplasm, collectively known as the metaplasm or deutoplasm; d, germinal vesicle or nucleus of the ovum containing, in this case, two germinal spots or nucleoli ; e, zone of epithelial cells immediately surrounding the ovum ; /, cells of the discus proligerus ; g, perivitelline spaces separating the zona pellucida from the cytoplasm of the ovum.

into two cells, which, even at this stage of development, differ slightly from each other. These daughter-cells in turn divide in two, and this process of division is continued, each cell giving rise to two new cells, until a considerable aggregate of cells has resulted (Fig. 2). Then the cells assume a definite arrangement into layers. Some become disposed in a superficial layer enclosing the rest of the cells and a body of fluid. Tliis layer is called the primitive ectoderm. The remaining cells accumulate in an irregular laminar mass beneath the jirimitive ectoderm at the site of the future embryo. This mass of cells is tlie primitive entoderm. Thus, at

INTRODUCTION.

this stage of development, there is a cellular sac, containing fluid, with a reinforcement of its wall at the region occupied by the primitive entoderm (Fig. 3).

Fig. 2.

Segmented egg of Petromyzon Planeri : Surface view of the collection of cells.

are invisible. (Kupifer.)

The nuclei

Subsequent to these events a third layer of cells becomes interposed between the primitive ectoderm and entoderm. Most of its

Fig. 3.

Ovum of rabbit: a, primitive ectoderm in section; 6, primitive ectoderm, surface view; c, primitive entoderm ; d, dividing cell of the ectoderm, (van Beneden.)

cells are derived from those of the primitive ectoderm, but the

INTRODUCTION.

primitive entoderm may also participate in its formation. This third layer is called the mesoderm. Soon after its formation, the mesoderm divides at the sides of the embryo into two layers — a parietal, which joins the under surface of the ectoderm, and a visceral, attached to the upper surface of the entoderm. Tlie space between these two layers is occupied by fluid, and is destined to form the future body-cavities. In the axis of the embryo the three earlier layers remain in continuity, forming a cellular mass around the site of the future spinal column (Fig. 4).

Fig. 4.

a

Embryo of Necterus in cross-section. (Platt.) ed., ectoderm ; mesoderm ; ento-

derm ; a, neural groove ; ch, site of future spinal column.

From these three embryonic layers of cells the body of the foetus is developed. The entoderm, with the visceral or lower layer of the mesoderm, turns downward and inward to meet its fellow of the opposite side and form the alimentary tract. The ectoderm and parietal or upper layer of the mesoderm also turn downward and inward, outside of the alimentary tube, and join those of the other side to form the walls of the body.

Meanwhile, the upper surface of the ectoderm over the axis of the embryo becomes furrowed. The edges of this furrow grow upward, deepening the groove between them, and finally arch over it and coalesce, forming a canal around which the central nervous system is developed (Fig. 5). Traces of this canal persist through life as the central canal of the spinal cord and the ventricles of the brain.

The embryonic layers have a deeper significance than the mere furnishing of the architectural materials from which the body is built up. They are evidences of a distinct differentiation in the development of the cells of which they are composed. The ectoderm gives rise to the functional part of the nervous system and to the epithelial structures of the skin and its appendages. The cells of the mesoderm elaborate the muscular tissues and that great group

INTRODUCTION.

known as the connective tissues, and the entoderm contains the cells that build np the linings of the digestive tract, inchiding its glands, and of the respiratory organs. It appears, then, that this division of the cells of the embryo into three layers marks a distinct ditference in the destinies of the cells composing those layers. This distinction persists through life, the tissues arising from a given layer showing, in general, a closer relationship to each other than the tissues arising from different layers. But this relationship is not always revealed by a similarity in structure, for the latter is determined by the functions the tissues are destined to perform, and tissues of like function acquire a similarity in structure. Thus, for example, the neuroglia in the central neryous system resembles

Fig. 5.

Cross-section of fish embryo. (Ziegler.) a, neural canal, cells enclosing it not represented; 5, chorda dorsalis, site of future spinal column ; ao, aorta ; Bf, external layer of mesoderm ; c, c, body-cavity ; d, alimentary canal, not yet completely closed * *, passes through the external layer of the mesoderm to its inner surface ; e, deutoplasm, or yolk of egg.

some of the connective tissues, although one develops from the ectoderm and the other from the mesoderm ; and the ganglion cells of the central nervmus system differ greatly in structure from the epithelium of the skin, nails, etc., and the cells of the neuroglia, notwithstanding the fact that they all spring from the cells of the ectoderm. The explanation is to be sought in the similarity of the usefulness of neuroglia and connective tissue and the difference in the functions of ganglion cells and those of the other tissues emanating from the ectoderm.

During-the early stages of development the cells of the germinal layers are very similar in character, although, as we have seen, their potential qualities are quite diverse. As growth proceeds, they begin to vary in size, shape, and internal structure in the dif-

INTRODUCTION.

ferent parts of the foetus. Their relative positions become modified. The primitive organs are defined and the tissues of which they are composed become elaborated.

The elaboration of the tissues is wrought by the cells, which display what is called their formative powers in the production of materials of various sorts which lie between them, and are called the intercellular substances. The amount and kind of intercellular substance vary, each form of tissue having its own peculiarities in this respect, dependent upon the role it is to play in the general economy. Some of the tissues perform functions which require the active processes that can be carried on only in cells, and in these the intercellular substances are either small in amount and apparently structureless, as in epithelium, or their place is taken by a tissue of separate origin, while the cells, relieved of the necessity for exercising their formative powers in this direction, become highly specialized to meet the functional demands imposed upon them. This development is met with in the muscular and nervous tissues.

Other tissues of the body are of use mainly because of their physical properties, such as rigidity, elasticity, tensile' strength, pliability, etc. These tissues, collectively called the connective tissues, are essentially passive. They require little or no cellular activity for the performance of their functions, and it is in the elaboration of these tissues that the cells exercise their most marked formative powers during the development of the body, causing the deposition of intercellular substances which possess the requivsite physical characters— rigidity and elasticity in the case of bone, pliability and tensile strength in the case of ligamentous structures, etc. As these substances are perfected, the cells decrease in activity, until they merely preside over the integrity of the intercellular substances they have already produced.

It may be well to point out here a distinction that divides the tissues of active cellular function into two groups. The first group, including the various modifications of epithelium, displays its activity in the elaboration of material products, taking the form of either new cells which are continually being produced, or of certain chemical substances which appear as a secretion. The second group, comprising the muscular and nervous tissues, exercises its functional activities in the storage of latent energy in such substances of unstable chemical nature and in such a manner that it

INTRODUCTION.

can be lil^erated wlien required and directed toward the accomplisliinent of some detinite purpose. The functions of both groups require an active intracellular metabolism, resulting in the formation of particular chemical substances. In this they are alike. But in the first group the production of those substances is, in itself, the functional purpose of the process, while in the second group those substances are merely a means for holding energy in the latent condition. If we may so express ourselves, the first group utilizes energy for the elaboration of material, the second group elaborates material for the utilization of energy.

In the adult, under normal conditions, each kind of cell, if it reproduce at all, gives rise to cells only of its own kind. But when the conditions are morbid, a sort of reversion may take place, the progeny of a given cell then showing less evidence of specialization than the parent cell. Such reverted cells, or their descendants, may never develop into more specialized cells, or they may regain the original degree of specialization possessed by the first cell, or, finally, they may become specialized along some divergent line of development, giving rise to a tissue that is nearly or remotely akin to that from which they started, according to the degree of reversion which has taken place. The reversion appears never to extend further back than the degree of specialization that is marked by the formation of the three embryonic layers in the history of development ; for example, epithelium which springs from either the entoderm or ectoderm does not revert to a primitive condition from which it can develop into bone or some other form of connective tissue normally derived from the mesoderm. Examples of reversion will be met with in the chapters on Inflammation, Tumors, and Metaplasia.

PART I.

NOEMAL HISTOLOGY.

Chapter I. The Cell

As has been stated in the introductory chapter, the cells of the body are not all alike. Most of them have undergone modifications fitting them for the performance of some definite function, and the majority of them are in consequence not appropriate objects for a study of the general characters of a cell. The extent to which this modification has afiPected the visible structure of the cell is, however, very different in the different tissues, and in some of them the cells retain so much of their original embryonic appearance as to clo.sely resemble the unspecialized cell.

This is true of the cells of some varieties of epithelium. But, though in appearance they give little evidence of specialization, in their functional activities they display very marked modifications of the powers of the 'primitive cell. Some of those powers, perhaps the 'nutritive, perhaps the secretory, have become exaggerated, while others, e. g., the locomotory, or reproductive, have fallen into abeyance, or suffered almost total extinction.

On the other hand, it is obvious that such cells as constitute the whole body of unicellular animals must retain all the powers essential to a living cell in relatively equal states of development. No one of them can be extinguished or thrown out of its proper balance with respect to the others if the cell is to remain normal. And yet, even among the unicellular organisms, certain parts of the cell may be very evidently specialized for the performance of particular functions. For example, the cilia of infusoria have the power of executing much more rapid movements than the other

NORMAL HISTOLOGY.

parts of the same cell. And it is probable that all protozoa, e. unicellidar animals, possess similar, though less oljvions and internal, heterogeneity of constitution.

Tlie less the degree of specialization or differentiation in the structure of an organism, the less highly developed is the functional activity of which it is capable, and the less perfect its ability to cope with possible unfavorable environment. The value to the whole organism of a diversity in its parts is, therefore, unquestionable, and the higher we go in the animal kingdom, the greater we find the development of this diversity, coupled with a more and more perfectly adjusted co-operative interdependence of the different parts of the body.

In the protozoa the single cell does all the work of the whole organism. In the multicellular animals, the metazoa, this work is distributed among the component cells of the body, each of which has developed an efficiency for performing its special work that would be incompatible with a wider range of duties.

It is quite impossible to find in nature any example of a cell devoid of all individual peculiarities attributable to differentiation or specialization. We must, therefore, study several varieties of

Fig. 6,

Amoeba pellucida. (Frenzel.) c(, ectoplasm ; b, endoplasm ; c, nucleus ; d, nucleolus ; e, large contractile vacuole ; /, incorporated foreign body ; g, g, pseudopodia.

cell in order to gain an ideal conception of such a cell. This accomplished, we may consider those cells which occur in nature as special modifications of that type.

Perhaps the simplest cell leading an independent existence is the protozoon, amceba (Fig. 6). This animal is widely distributed in

THE CELL.

moist earth, upon the surfaces of acpiatic plants, and in the soil at the margins of ponds and sluggish streams.

The body of the amoeba consists of a gelatinoid substance which has received the name protoplasm, or, more definitely, cytoplasm. Within this cytoplasm and sharply defined from it is a round or oval, vesicular body, called the nucleus, which in turn contains one or more particularly conspicuous granules, the nucleoli.

The most superficial layer of the cytoplasm appears perfectly clear, colorless, and homogeneous. It envelops the rest of the cytoplasm, which has a granular appearance. The clear peripheral portion is distinguished as the hyaloplasm,’’ or ectoplasm the

granular internal portion as spongioplasm,” or endoplasm.”

The terms hyaloplasm and spongioplasra are also used in a different and more restricted sense, as will presently apj^ear.

When viewed under the microscope, the granules of the cytoplasm are seen to possess a constant, slight, vibratile motion, the Brownian movement, to which is added now and then a flowing movement from one part of the cell to another. At intervals there is a protrusion of the ectoplasm at some point, extending for some distance from the body of the cell, a pseudopodium. This may soon be retracted again, merging with the rest of the ectoplasm, or some of the endoplasm may flow into the central portion of the pseudopodium, converting it into a broad extension of the cell-body. This may subsequently be withdrawn, or the whole mass of cytoplasm, with the nucleus, may flow into the pseudopodium, gradually increasing its size, until the whole cell occupies the original site of the pseudopodium. In this way the animal executes a slow, creeping locomotion.

These pseudopodial movements and the locomotion occasionally incident to them appear to be wholly spontaneous, i. e. dependent upon internal conditions of which we have no knowledge. They may, however, be influenced by external circumstances. Certain substances evidently attract the amoeba, others are either matters of indifference to it or repel it. If a pseiidopodium comes in contact with some particle in the surrounding medium, it may retreat from it, appear indifferent to it, or be attracted and proceed to incorporate it. This is accomplished by the cytoplasm flowing around the foreign body and coalescing on its further side so as to enclose it. It is then conveyed to the body of the cell, either by cytoplasmic currents, by the withdrawal of the pseudopodium containing it, or

NORMA L IIISTOL OGY.

by the streaming of the cell-body into that protrusion. The fate of the particle thus incorporated dcj)ends upon its nature. If it be serviceable as food, it is gradually digested and absorbed, or sucli parts of it as are digestible are so utilized, and the remainder, no longer of use to the amoeba, is extruded from its body.

These phenomena reveal powers of perception and selection on the ])art of this cell which are very closely akin to the intelligence of more complex organisms. They also demonstrate its power of assimilating material from without, to serve as nourishment and the source of the energy which it expends in executing its movements and in carrying on the chemical processes pertaining to its internal economy.

At intervals, there appears within the endoplasm a small, clear, spherical spot. This gradually increases in size and constitutes a little drop of fluid, sharply defined from the surrounding cytoplasm. Alter it has attained a certain size, it suddenly disappears, the cytoplasm around it' coalescing and leaving no trace of its existence. Such a clear space, filled with fluid, within the body of a cell is called a vacuole, and those which are suddenly obliterated, contractile vacuoles. Their purpose is not clearly understood, but probably has to do with a primitive circulatory or respiratory function, since contractile vacuoles are not observed in the cells of higher organisms where those functions are carried on by more elaborate mechanisms.

Eventually the amoeba reproduces its kind by dividing into two similar cells, each of which grows into a likeness to the parent individual.

Let us now compare the amoeba with some other varieties of cell, in order to learn what they all have in common.

The amoeba has an outer, soft, transparent layer of cytoplasm, the ectoplasm. This is not present in all cells. In many the granular cytoplasm has no envelope, but appears to be quite naked. In other varieties it is enclosed in a distinct membrane.

In the great majority of cells the active streaming of the cytoplasm and the pseudopodial protrusions described in the amoeba are wanting, but the Brownian movement of the granules is more constantly present. The cells have fixed positions and their food is brought to them, usually in solution, so that the more active movements so essential to the welfare of the amoeba would be superfluous.

THE CELL.

For a similar reason, as already intimated, they can dispense with the contractile vacuole.