Histology: recognise the organisation of tissues
Looking at a microscope slide provides information about the organisation of a tissue. When comparing slides for the same type of tissue, there are two main issues: the cut plane (how the slide was prepared) and stain preparation. Examine the general organisation first. Then examine the individual cells. Instead of just being able to identify which cell/tissue is a particular colour, link what you see to the specific purpose that tissue serves.
Look for a covering or lining
Epithelium consists of tightly packed cells, with minimal intercellular substance between them. Cells may be flat, cuboidal or columnar, and arranged in one or several layers. Epithelia can occur in glandular structures (to secrete substances), on the lining of internal organs, on the skin, etc., and serve to protect, absorb nutrients and/or secrete substances.
A surface lining has an exposed side and an attachment to underlying tissue. The shape and arrangement of the cells provide information about how that lining is organised. Describe those features before trying to name a specimen. This approach makes a comparison between two teaching slides more useful than simply matching each one with a familiar colour or outline.
Pay attention to what surrounds the cells
Connective tissue contains cells that are surrounded by an extracellular matrix. A significant difference exists with regards to the amount as well as type of components within each matrix. Bone, cartilage, fat and blood belong to this tissue family. Identify if fibres, open spaces or a firm supporting substance make up most of the space. It may be easier to describe the matrix rather than count the number of observable cells prior to establishing the general structure of the tissue.
The matrix is an important part of connective tissue, not merely empty space between cells. Its composition helps explain why different connective tissues have different properties. A field dominated by fibres presents a different pattern from one containing a firm supporting material. Describe both the cells and their surroundings so that the account represents the tissue as a whole.
Relate specialised cells to their work
Muscle cells contract to produce force. Skeletal, cardiac, and smooth muscles have different forms based on their functions. The orientation in which you cut the sample will affect what structures you can see. Nerve cells and other support cells help coordinate actions by forming nervous tissue. Be careful when viewing individual nerve cells; they often cannot be viewed for their entire length. Thin sections show only a small portion of an overall 3D structure.
A long cell can look quite different when cut across rather than along its length. The same issue affects the appearance of bundles and layers. This is one reason to establish orientation before interpreting a tissue pattern. A thin section also shows only part of a cell, so the absence of a feature in one profile needs careful interpretation.
Move from the whole field to the details
Use lower magnification to assess the boundary, layering and repetition of microscopic structure. At higher magnification, examine the cells, nuclei and matrix within a selected area. Document all that can be seen before you attempt to decide what you are seeing. Stains help reveal structures; however, relying solely on colour is unreliable. Compare your findings with the information provided by the specimen label, the orientation of the slide and how the sample was prepared. An educational image cannot establish a diagnosis without appropriate laboratory and clinical assessment.
Moving between magnifications should answer a question. A lower-power view establishes the arrangement; a closer view helps examine selected details within it. Return to the wider view when those details become hard to place. A labelled learning slide provides a useful setting for this practice, while diagnostic interpretation also requires specimen preparation information and the relevant clinical assessment.
A useful viewing sequence
- Overall pattern
- Tissue arrangement
- Cell features
- Function
Check your understanding
A short quiz, with an explanation after each answer.
Which feature is typical of epithelium?
For a teaching slide, write three observations before naming the tissue. Include its arrangement, characteristics of the cells and the material separating them. Finally, add one statement relating what you observed to its function. This gives you a description that someone else can check.