Introduction to histology
Introduction to histology

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Introduction to histology

1.4 Identifying cells and structures

Most cells and cellular elements are virtually transparent, so it is difficult to distinguish individual cells and cellular structures without staining the cells in some way before viewing them. There are two main ways of staining cells. The traditional method involves the use of dyes that selectively bind to different structures within the cell. More recently, techniques using antibodies have been developed which can stain individual molecules.

1.4.1 Histochemistry

Histologists have developed a wide variety of different staining techniques to identify different elements within tissues. This methodology is referred to as histochemistry. The most commonly used stain for medical diagnosis is haematoxylin and eosin (commonly abbreviated H&E). Haematoxylin is an alkaline dye which stains acidic structures a blue/purple colour. Eosin is an acidic dye which stains basic structures a deep pink colour. Cellular and extracellular components that are neutral take up neither stain and appear relatively clear.

SAQ 1

What colour will cell nuclei be stained by H&E? Why?

Answer

Deep purple/blue. The nucleus of a cell contains DNA (an acid) so it binds the basic dye.

Figure 2 shows 'sections' that illustrate the principles of histology and histopathology - the great majority of these sections have been stained with H&E (Figure 2). Some structures, such as the extracellular matrix, do not stain well with H&E, so we have included a few sections with different stains that allow the identification of particular tissue elements. These additional stains include Giemsa, Masson's trichrome (Figure 3), and Van Gieson. The characteristics and use of different stains is explained more fully in the course Introduction to microscopy [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] .

Figure 2 A section of the adrenal cortex, stained with haematoxylin and eosin. Cell nuclei are stained deep blue and the cytoplasm is pink. The collagenous capsule of the organ (longer arrow) is also pink. Red blood cells (erythrocytes) seen within small blood vessels (short arrow) are stained red. Scale bar (bottom right) = 50 μm.
Figure 3 Two sections across the wall of the aorta from a child; (a) is stained conventionally with haematoxylin and eosin; (b) is stained with elastic Van Gieson which identifies the elastin fibres (black) in the media of the artery and the collagen (red) in the adventitia.

1.4.2 Immunohistochemistry

In the last thirty years staining methods using antibodies have been increasingly developed. When applied to staining for the light microscope, these techniques are collectively referred to as immunocytochemistry (ICC) or immunohistochemistry (IHC) (Figure 4). Antibodies specifically bind and detect individual proteins, so they can be selected to identify the presence or location of a single protein, that is diagnostically discriminating. Such characteristic proteins of a particular cell type or disease are called markers. Introduction to microscopy examines markers in greater depth and the ways in which antibodies can be used to stain sections.

Figure 4 Neurons in the cortex of the brain are identified by immunohistochemistry using an antibody directed against neurofilaments (protein filaments in the axons of the neurons). (Courtesy of Dr Eva Del Valle Suarez)
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