Body Tissue Histology
Elementary Tissue Histology
The various parts of the body are composed of a small number of " elementary tissues." Each of these elementary tissues has a definite structure, but the details of that structure may vary within certain limits in different parts of the same mass or in different situations within the body. Such variations can usually be referred to differences in the functional activity assigned to the tissue, which is not always exactly the same throughout the body. For example, epithelium is an elementary tissue consisting of cells which are nearly always rich in cytoplasm and are separated from each other by a very small amount of homogeneous intercellular substance. Wherever epithelium is found it has these general peculiarities of structure. But the functions demanded of epithelium are of widely diverse character in different situations, and its structure shows a corresponding diversity in its details. The fact that it is made up almost exclusively of cells leads to the natural inference that the usefulness of epithelium depends upon cellular activities. Inasmuch as these may be of very different character, we should expect the tissue to vary chiefly in the structure and arrangement of its component cells according to the particular activity which was needed and the manner in which it was utilized. Such, as a matter of fact, is the case. These considerations will be made clearer if we follow a little more closely the example offered by epithelium.
In some situations epithelium serves to protect the underlying tissues from injury. But the usual injurious influences which threaten the tissues differ in different parts of the body, and must, therefore, be averted by different means. Upon the surface of the skin they are chiefly of a mechanical or chemical nature, and to resist them the cells of the epithelium forming the epidermis undergo a modification in structure, resulting in the formation of a superficial horny layer which is highly resistant to abrasion and chemical change. Upon the inner surfaces of the respiratory passages the conditions are different. Here the tissues require protection from particles of dust that may be inhaled. For this purpose the epithelial cells lining those passages are provided with minute, hair-like processes, "cilia," which execute lashing movements toward the outlets of the passages and occasion the transportation of substances coming into contact with them toward the outer world. In the digestive tract the conditions are again different. The tissues underlying the epithelial lining need protection from the chemical action of the fluids in the stomach and intestine, as well as from friction with their solid contents. The cells of the epithelium meet these needs by a secretion of mucus, which is discharged upon the inner surfaces of the digestive organs, where it serves as a protective layer and as a lubricant.
In other situations epithelium has an excretory function, which is less clearly of value in protecting its immediate surroundings, but is essential for the protection of the whole organism from substances which would exert an injurious effect if they were permitted to accumulate in the circulating fluids of the body. These substances are absorbed from those fluids by epithelial cells, from which they are discharged from the body either unchanged or after transformation into other chemical compounds. Here the most obvious products of cellular activity are of no use in the economy, and are eliminated from it; but it is not improbable that the cells which separate them or their antecedents from the circulating fluids may also discharge useful substances into those fluids (" internal secretion ").
We must not assume that the most obvious function exercised by a tissue is the only service it does to the organism.
The epithelium which carries on this eliminative function is nearly always associated with other elementary tissues to form an organ, called a "gland," in which the epithelium is the functionally active tissue, the other tissues being subservient to it. The glands of the body differ considerably in both structure and function, but in all of them it is epithelium which elaborates the materials essential to the formation of their normal secretions. Mention has already been made of those glands which furnish secretions charged with waste materials to be eliminated from the body. Such glands are called excretory glands, and are exemplified by the kidney. Other glands, distinguished as secretory in a restricted sense, furnish secretions which are of service to the organism. Examples of such glands are those which discharge their secretions into the alimentary tract where, by virtue of the ferments they contain, they prepare the food for absorption. Another example of a secretory gland is furnished by the sebaceous glands of the skin, which produce an oily substance serving to keep the epidermis upon which it is discharged soft and pliable.
In the secretory glands the cells of the functional epithelium elaborate within their bodies the substances necessary to give the glandular secretion its peculiar and useful characters. These sub stances accumulate within the cells, where they are stored until required, when they are discharged into the secretion. While in the stored condition within the cells these substances may have a different chemical constitution from that which they acquire when they are discharged from the cells. A simple example of this chemical transformation is furnished by the liver, in the epithelial cells of which carbohydrates are stored as glycogen, to be liberated as a closely related chemical substance, glucose. In like manner the ferments stored in the epithelial cells of the digestive glands are not fully formed while in that situation, but exist in states known as " zymogens," from which the potent ferment appears to be readily formed when the cells are called upon to furnish it.
It is apparent, then, that the elementary tissue, epithelium, can not have the same microscopical structure in all the situations in which it is found ; but, notwithstanding these variations, wherever epithelium occurs it presents certain general structural peculiarities which are constant and which distinguish it from the other elementary tissues. Similarly, each of the other elementary tissues presents variations in the details of its structure in different situations, but always retains certain general structural characteristics distinguishing it from all the other elementary tissues. It is the first task of the student of histology to learn to recognize and identify these elementary tissues wherever they occur and however they may vary from the type which is first presented to him for study.
In the following chapters an attempt is made to give the student an idea of the essential structure of the elementary tissues, so that he may recognize them in specimens which he examines with the microscope. For this purpose they have been arranged in the order of their structural simplicity.
When examining a specimen under the microscope with a view to recognizing the elementary tissues it contains, the student should habitually ask himself the following questions : (1) What are the general characters of the cells entering into the structure of the tissue (2) What kind of intercellular substances separates those cells (3) How arc the cells arranged with reference to each other and the intercellular substances Correct answers to these three questions will enable him to quickly determine the nature of the tissue he is observing, even if it should vary considerably in structural details from examples of the same tissue with which he has already become familiar. display_block('basic_tissue'); ?>