Histology Basic Tissues

[Tiffany Crutch]

Histologyis the study of tissues. All of the various tissues of the human body can be categorized into four basic tissue types (see philosphical/historical note). All organs are built of these four tissues, which have consistent characteristics and arrangements from organ to organ. Thus an appreciation of the major features of these four basic tissue types can greatly simplify your understanding of the cellular composition of the many organ systems.

Epithelial Tissue covers body surfaces (epi, on + thelium, surface). Epithelial tissue consists of cells attached to one another to form an uninterrupted layer of cells that separates the underlying tissues from the outside world. The body's epithelium not only covers the body's obvious surfaces (such as the epidermis of the skin and the linings of respiratory, urinary, and digestive tracts) but also extends into all of the complex invaginations which form lungs, kidneys, sweat glands, digestive glands, liver, etc. Epithelial tissue provides the essential functions of protection, containment, absorption, and secretion. It keeps bad stuff out, keeps bodily fluids in, and moves materials in and out. Embryonically, most epithelial tissues are derived either from ectoderm (e.g., epidermis) or endoderm (e.g., epithelium of trachea and lung). [More] [Examples]

Connective Tissue supports other tissues. Connective tissue consists of several cell types and extracellular products which, together, provide essential functions of mechanical reinforcement, immune surveillance, transport/diffusion of nutrients and wastes, and energy storage (fat). Embryonically, connective tissues derive from mesoderm or mesenchyme. [More] [Examples]

Nervous Tissue is responsible for rapid long-distance signalling, coordination, and "thinking." Nervous tissue consists of highly specialized nerve cells and support cells which are derived from embryonic neuroectoderm and neural crest. [More]

A note on pathology nomenclature: The names of neoplasms reflect the fundamental nature of their source tissues. Thus a carcinoma is a cancer of epithelial origin, while a sarcoma is a cancer of mesenchymal (connective tissue or muscle) origin.

Parenchyma is typically the focus of attention. Because organ-specific function usually centers on parenchymal cells, histological (and physiological) accounts often emphasize parenchyma. Unfortunately, stroma is commonly ignored as just boring background tissue.

Pay attention to the stroma. No organ can function without the mechanical and nutritional support provided by the stroma. If an organ is inflamed, the signs of inflammation appear first in the stroma. (For an example of inflammation from liver, see WebPath.)

Historical note: Ignoring inconspicuous tissue features, such as stroma, can have consequences. Stromal capillaries are seldom evident in tissue specimens. Nothing calls them to one's attention, so they are often ignored and forgotten. Unfortunately, just such inattention may have delayed for decades the realization that tumors depend on ingrowth of capillaries for their unchecked expansion, so that interfering with tumor vascularization might powerfully inhibit tumor growth.

Philosophical note: The concept of "four basic tissue types" provides a simple (and powerful) framework for organizing and learning a great wealth of detail. This concept is more than just a convenient intellectual construct. There is a real boundary layer, the basal lamina (visible microscopically with appropriate stain), which separates tissues of different types.

Nevertheless, nature does remind us every so often that her organization is not bound by our simplifying concepts. Although most tissues do correspond closely to one of the four basic tissue types, the usual caveat ("all rules have exceptions") does apply.

Exceptions include germ cells, which don't fit the scheme at all, and several specialized varieties of connective tissue which masquerade as epithelium, such as synovial membranes of joint capsules and stria vascularis of the inner ear.

Basic knowledge of tissue preparation, including staining, is important to know when interpreting pathology reports on either in-patient or out-patient biopsies. It is not always the case that the interpreting pathologist has thoroughly analyzed the tissue sample by including appropriate histologic staining, and this deficiency can retard accurate diagnosis.

Four basic types of human tissue can be stained and viewed using various histological techniques. Epithelium, connective tissue, muscle tissue, and nervous tissue have commonalities but look very distinct structurally after staining. Each stain exists to highlight an important feature or component within a tissue type. For example, one of the most common stains, Hematoxylin, is a basic dye that stains proteins a blue color, while Eosin stains proteins a pink color. These two stains are commonly used together to define intracellular organelles and proteins. Because of the variety of the proteins that exist, some stains were created to highlight a particular protein, which this review will discuss in the following sections. The benefit of using a special stain is that it can highlight the specific protein very well. However, because of its specificity, the other structures will not be seen. For this reason, multiple slides will often be created from a given specimen so that multiple stains can be performed to gather the full range of needed information.

Almost all tissue stains are performed on tissue that has been removed from the body. However, in rare instances, very specialize stains called vital stains can work on tissue remaining in the body. These stains are used for the identification of specific types of tissue and identification of abnormal tissue, so a subsequent biopsy can be more accurate in obtaining abnormal tissue.

Before specific staining can occur, tissue samples must undergo preparation through the following stages: Fixation, processing, embedding, sectioning, and sometimes antigen retrieval. In modern histology laboratories, most of these steps are automated.

Fixation: Fixation uses chemicals to preserve the structure of the tissue in its natural form and protects it from degradation by irreversibly cross-linking proteins. Although several specialized fixatives are available, Neutral Buffered Formalin is a common choice for this step. The fixation step is vital to the rest of the histologic staining procedure because by retaining the chemical composition of the tissue, the sample is hardened and makes the sectioning phase easier. Paraffin-formalin is another effective fixative. Its benefit is that it is the fixative of choice for immunostaining; however, it requires preparation at the time of the fixation. Bouin is a fixative used for examining embryo and brain tissue because of its superior preservation of delicate nuclei and glycogen. Its downside is that it does not preserve kidney tissues well and also distorts mitochondrial structure.[1]

Dehydration: The addition of ethanol accomplishes the dehydration of a sample. It removed water from the sample and further hardens the tissue for eventual light microscopy. After ethanol is applied, and following the completion of tissue dehydration, xylene is used to remove the ethanol.[1]

Embedding: Embedding is the process of putting the sample into a paraffin wax or a plastic resin to enhance the process of extracting cellular structures. This step is to be performed with caution if the goal is to perform immunostaining because the paraffin wax will inhibit the penetration of antibodies, and lead to a false result.[1]

Sectioning: Sectioning involves mounting the specimen on a microtome and cutting it into sections. The preferred thickness is 4-5 micrometers so that it can be stained and put on a microscope slide for examination.[1]

Antigen Retrieval: This step is to retrieve antigens that could have been covered in the fixation and embedding stages. If the cross-linking of proteins conceals the antigen sites, there may not be as robust of an immunohistochemical response. Antigen retrieval is achieved through heating and proteolytic methods to break down the cross-links and reveal the epitopes and antigens that were previously covered.[1] Although this step carries the risk of denaturing both the fixative and the antigens themselves, a successful antigen retrieval method can lead to a much more effective immunostaining intensity.

As the name implies, it is two stains done in subsequent steps. The hematoxylin is a basic dye that stains acidic structures. The resulting color is a purple/blue hue, and structures that are targeted with this dye are named Basophilic. Basophilic structures include DNA in cell nuclei, RNA in ribosomes, and the rough endoplasmic reticulum.[1]

Eosin is a counterstain done after hematoxylin and is an acidic dye that targets basic structures. The resulting color is a pink/red hue, and structures that attract eosin are called eosinophilic.[1] The cytoplasm is an example of an eosinophilic structure.

The gram stain is a sequential staining technique invented for differentiating bacterial species. Its major utility lies in determining the causative organism of bacterial infection by staining the cell wall.[2] While not all bacteria have a cell wall and thus cannot be stained with this method, it is still a very useful and commonly performed stain. A bacterial sample can be heat-fixed and undergo gram stain with these four steps: Primary staining with crystal violet, secondary staining with grams iodine, decolorized with alcohol or acetone, and counterstained with safranin. Gram-positive bacteria are those that contain a thick layer of peptidoglycan, making them retain the violet stain and appear purple. Alternatively, the gram-negative bacteria have a thin layer of peptidoglycan and more lipids in the cell wall, so the decolorizing step washes out the violet more, and the sample appears pink.[2]

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