HISTOLOGY, the science or study of the tissues, is that branch of biology which deals with the appearance and nature of the structural elements of living organisms, whether animal or vegetable. The term was introduced by A. F. J. K. Mayer in 1819; when used alone, it is understood to refer to animal tissues, otherwise the term "vegetable histology" is usually employed. As such study is largely with the microscope, histology has come in the English-speaking countries to be almost synony mous with "microscopic anatomy." In Germany, Holland and other countries, however, emphasis is still laid on the nature of the tissue under consideration (including origin, development and characteristics), so that textbooks are commonly entitled : "Text book of Histology and Microscopic Anatomy." Thus in the microscopic study of an organ such as the lung, the form, position, etc., of alveoli, vestibules, capillaries and so on is strictly speak ing a matter of microscopic anatomy, whereas consideration of the component tissue—epithelium, endothelium, collagen, muscle fibres and so on—is a matter of histology. As the function of the various tissues may properly be considered under the wider use of the term histology, its study in some medical schools is more closely linked with physiology than with anatomy.
History.—As the history of the subject will be taken up under other headings, it will not be discussed here (see HOOKER, SIR J. D.; GREW; SWAMMERDAM; LEEUWENHOEK; MALPIGHI; BICHAT; SCHWANN; VIRCHOW).
Fresh Material.—Preparations may be scraped or teased (with two mounted needles) either dry on a slide, or in an indifferent fluid, such as physiological salt solution. Though not in much demand today, this method is useful, with the addition of caustic potash, for, the demonstration of fungi. Maceration with ap propriate fluids will in rare cases give the best demonstration of the structures being studied, as in the scroll-like structure of heart muscles or the vascular supply of an organ. Very thin sections of fresh tissues, formerly done with . a hand razor or crude instrument, can be prepared with modern instruments on frozen tissue with almost the same accuracy and perfection as with hardened embedded material. With the greater speed avail able, this method is therefore considered by some to be preferable to the use of fixed tissues in pathology, both because quicker diagnosis of the diseased tissue is possible and also because less distortion has occurred. In skilful hands, such sections can be cut, stained and ready for examination within a very few minutes, though ir, some pathological laboratories preliminary partial fixation in heated formalin for a few hours is considered prefer able. In some surgical clinics this method is employed while the operation is still in progress as a routine in doubtful cases of malignant disease; in others it is felt that tissues, doubtful to the surgeon on gross examination, deserve the more thorough ex amination that fixation permits.
Fixed Tissues.—For proper examination with the modern micro scope (magnifying up to 1,500 diameters) tissues must be cut so thin that after appropriate staining they can be accurately examined by transmitted light and without the confusion of sev eral superposed layers of cells. The vast majority of histological materials are prepared for microscopic examination by preliminary "fixation," hardening, sectioning, staining and mounting, processes which altogether require from 2 to io days, depending on which of the innumerable methods is used. The purpose of the fixing fluid (formalin, Zenker's, Orth's, Bouin's, Miiller's, etc., etc.) is to penetrate and kill the tissues quickly, so that the various tissue elements, and especially the cell nuclei, are preserved as lifelike and undegenerated as possible. If, for instance, the natural pro cess of "mitosis" (important in tumour diagnosis) may be com pleted in twenty minutes, the need for quick fixation becomes at once apparent.
As hardening fluids, formalin and alcohol are preferred and both can be subsequently used indefinitely as preservatives, though, especially with the latter, distortion by shrinkage occurs.
To get sufficiently even and thin (e.g., one thousandth of a millimeter) sections with the microtome (introduced by Purkinje and His), the tissue must by gradual transitions be imbedded in paraffin wax or celloidin, the former having the advantage that "ribbons" of serial sections can be more easily obtained. The paraffin sections, fastened with egg albumin to glass slides and rid of their impregnated paraffin, may be stained and counter stained in the greatest variety of ways, depending on the micro chemical result to be obtained. (See EHRLICH.) Then after further transitions through alcoholic and oily fluids, the preparations are permanently mounted beneath a coverslip in a reagent such as balsam or damar, which harden with a refraction index similar to that of the microscope lenses, and thus minimize distortion. The celloidin technique, used especially in the examination of the nervous system, is similar and simpler yet more time-consuming.
Smears or Spreads.—The cellular contents of the blood and various body fluids may be spread in thin films, and like sections of body tissues, be examined either fresh, stained or unstained, or after fixation with a great variety of stains, designed to bring out diverse properties or pathological changes. (See hematological treatises such as Ferrata's "Emopatie" or any textbook of clinical microscopy or pathology.) Vital Stains.—In recent years, methods of intra-, supra- and post-vital staining have developed a considerable technique for the study of living cells and tissues. Here dyes are used which enter but do not kill the cell. They usually select certain parts of the cell (nuclei, granules, vacuoles, mitochondria, etc.) and the cell only becomes diffusely stained after it has died. Such dyes may be injected in the living organism and the cells which have selected the stain studied, or may be applied to cells withdrawn from the body but still alive. Such procedures are comparatively free from the objection that death of the tissue may introduce artefacts in the form of the cell or tissue and also have the advantage of permitting the study of the differentiated components under vary ing experimental living conditions. It must be recognized, how ever, that even such stains are toxic to a greater or less degree, so that the cells being studied are to a corresponding degree abnormal.
Examination of Living Tissues Under Normal Condi tions.—The classic studies of Cohnheim of the phenomena of inflammation in the living mesentery or tongue of the frog at once come to mind; but greatly improved methods have recently become available. The Clarks' method of studying the living tadpole's tail permits microscopic examinations, indefinitely repeated, over weeks, whereby normal growth or reaction to various tests or injuries can be studied in a way hitherto impossi ble. Repeated examination of the same living cell or vessel ad libitum without extraneous damage is bound to furnish informa tion of fundamental value. Very recently Sandison has applied the same principle to a window inserted in a rabbit's ear, so that mammalian tissue is now also subject to examination under these favorable conditions, even with the oil immersion lens.
The elementary tissues of the body may be more or less arbi trarily subdivided into : 1, epithelial (of various types and includ ing such modifications as hair, nails, tooth enamel, glands, etc.) ; 2, nervous tissue; 3, connective tissue (including fibrous and elastic tissue, fat tendon, bone, cartilage, dentine) ; 4, muscular tissue (voluntary, involuntary and cardiac). The vascular system (including lymphoid tissue) is sometimes considered an elementary tissue as a matter of convenience, though, more strictly speak ing, it is a combination of elements of the other tissues mentioned. The blood is also sometimes spoken of as a tissue, though, more strictly speaking, its cellular elements are mobile, functioning cast-offs from various fixed sources.
These elementary tissues are derived from the three primary blastodermic layers of the embryo (with trifling exceptions) as follows: 1 from ectoderm and entoderm; 2 from ectoderm; and 3 and 4 from the mesoderm.
Epithelial tissue serves chiefly as a protective covering and for secretion and excretion. According to shape, epithelial cells are called: flat or squamous (covering skin, adjacent mucous mem branes, alveoli of the lungs, some kidney tubules, etc.) ; or col umnar (lining the respiratory and digestive tract, the ducts of glands, uterus and oviducts, forming the secreting cells of glands) ; pyramidal, etc. They may occur in single layers or stratified and various modified forms are found, such as "ciliated" (uterus, bronchi, etc.), "goblet" (upper respiratory tract, stomach, colon), "transitional" (parts of urinary system) and greatly modified for special uses (as in the retina, ear, taste buds) . The single-layered flat cells lining the vascular system and the large enclosed body spaces (pleura, etc.) are called endothelial cells (or endothelial and mesothelial respectively) .
Connective tissue is found in such forms as the embryonal jelly of the umbilical cord, mucous, reticular, loose fibro-elastic or areolar, dense fibrous, elastic tissue, cartilage and bone. (For details, see appropriate sections.) It contains a variety of cells such as fibroblasts, lymphocytes, plasma cells, macrophages (clasmatocytes), giant cells, eosinophiles, mast cells, etc.
Muscle tissue (i.e., a form of protoplasm in which the property of contractility predominates) is of three main types : I. smooth or involuntary; 2. striated or voluntary; 3. cardiac. Some striped muscle, such as oesophagus and diaphragm, is only partly volun tary. Smooth muscle, consisting of elongated pointed cells with central oval nuclei, mitochondria, granules, etc., is found in the following systems ; alimentary, respiratory, genitourinary, vascu lar, and in glandular ducts, the capsules of the spleen and lymph nodes, the skin and some eye muscles. Voluntary muscle, dis tinguished by definite longitudinal and cross striae, a complex sys tem of lines and cross discs, and a peripheral nucleus, is of obvious distribution through the body. Cardiac muscle (structurally in termediate between the other two) forms a close meshwork with complete loss of the original cell boundaries, and centrally placed nuclei. Myofibrils pass beyond the "intercalated discs," structures the nature of which is still in doubt.
Nervous tissue, receiving, originating and conducting nerve impulses, is made up of the neuron (or nerve cell in its broadest sense) and the supporting neuroglia, previously mentioned. While details may be found under the heading NERVOUS SYSTEM, a few examples of the many kinds of nerve cells may be given. The giant pyramidal cell of Betz in the cerebral cortex, the lesser ganglion cells of wide distribution (unipolar, bipolar and multi polar), the characteristic Purkinje cells of the cerebellum, the various types of Golgi cells, the ependymal cells and long and short rayed astrocytes of the glial tissue (to say nothing of these cells in various stages of immaturity found in tumour tissue), will indicate the great variation of structure to be found, though unfortunately there is still a tremendous lack of correlation with function. While tigroid bodies, axone, dendrites, medullated sheath, neurolemma and terminal arborizations are common properties of most nerve cells, their infinite variety adds still further to the complexity of this subject. (See EPITHELIUM ; ENDOTHELIUM; NERVOUS SYS TEM ; CONNECTIVE TISSUE; CYTOLOGY.) Organs are formed from combinations of the various units of these tissues, sometimes of two units, usually of several. The separate subjects are to be found further described under appro priate headings.