What is Histology ?

Histology is the scientific study of the fine detail of biological cells and tissues using microscopes to look at specimens of tissues that have been carefully prepared using special processes called histological techniques.

It is a discipline that is essential for the understanding and advancement of biology, medicine, veterinary medicine and many sub-disciplines within those scientific subjects.

The word histology comes from the combination of the Greek words , meaning 'tissue', and which is sometimes translated as '-logia' and is a suffix generally used to denote the study of a subject or the branch of knowledge of a discipline.

Short Definition of Histology

Test or exam questions about histology and/or cells sometimes ask for a definition of histology. Here are some short definitions of the term histology:

• Histology is the scientific study of biological tissues.
• Histology is the microscopic study of the structure of biological tissues using special staining techniques combined with light and electron microscopy.
• Histology is the study of the microscopic structures of cells and tissues of plants and animals. It is often carried out by examining a thin slice (called a 'section') of tissue under a light microscope or an electron microscope. In order to distinguish different biological structures more easily and accurately histological stains are often used to add colours to or enhance the colours of certain types of biological structures differently from other types of structures that may be located next to and/or in contact with each other. Histology is an essential tool of biology, medicine and veterinary medicine.

Exam Tip: In a test or exam look at the number of marks allocated to answers for each part of the question and give short (concise) answers where only one mark is available but include more detailed information where several marks are indicated.

Uses of Histology

• Education - Histology slides are often used in teaching laboratories to help students learn about the microstructures of human (and animal) biological tissues.
• Diagnosis for treatment - Biological tissue samples taken from a patient (that is, a specific person or animal's body) may be studied in detail to enable medical or veterinary experts to learn more about the patient's condition and hence perhaps understand its causes and make recommendations for treatment or management of the condition.
  Note: Although the study of the microstructure of diseased cells and tissues (e.g. to help inform decisions about treatment in clinical medicine) is an aspect or use of histology because it uses histological techniques, study of diseased tissues is more accurately called histopathology.
• Forensic investigations - Forensic histology, immunohistochemistry and cytology involving microscopic study of biological tissues using various stains can help clarify the cause of sudden unexpected deaths and other issues in forensic science.
• Autopsy - Biological tissues from a deceased person or animal can be studied using histological techniques enabling experts (e.g. pathologists re. unexplained death of a person) to learn about the circumstances and possibly cause of death.
• Archaeology - Study of biological cells and tissues recovered from archaeological sites can provide information about history, even ancient history. The state of preservation of the biological material is critical and sometimes sufficient e.g. for bone histology and dental histology.

Who does what ?

Histologists are people who have and use the special skills necessary to process samples of biological tissues in histology laboratories. Tissue obained from the patient (or sometimes from a suspect in the case of forensic science labs) is processed using a series of techniques (listed below) to prepare appropriate very tiny samples of tissue then mount them on slides and stain the tiny sample of tissue on each slide using chemicals called histology stains that have been carefully selected in order to help the people who will look at the slides to distinguish between the different types of biological material within the tiny sample on the slide.

The skills needed to interpret information on histology slides are different from the skills needed to prepare the slides. Histologists use considerable practical skills and precision to produce high-quality samples for examination under a microscope. The resulting microscope slides are often studied by pathologists, i.e. physicians whose specialist area is in studying human biological microstructures such as tissues and cells and interpreting their observations so that they or other people can then make decisions based on them. Those decisions might concern how to treat a medical problem, how someone received an injury, or died, and so on.

The microscopic study of diseased biological tissue is called histopathology (a sub-discipline within pathology).

Medical specialists who study and interpret diseased tissues in microscopic detail are called histopathologists.

Key Stages in Preparing Histology Slides

Note: Students of first-level courses in Anatomy & Physiology (e.g. ITEC) and Human Biology (e.g. A-Level) may not be required to demonstrate practical skills in histological techniques but it is useful to have general awareness of the steps involved in preparing histology slides. Not only does such appreciation contribute to a general insight into laboratory biology and techniques used in medical research, but familiarity with the terminology is useful when reading around the subject or communicating with professionals working in various fields within health sciences, e.g. during work experience or if researching histology jobs.

The five main stages in the preparation of histology slides are:

1. Fixing
2. Processing
3. Embedding
4. Sectioning
5. Staining

These are described individually on How to Prepare Histology Slides.

Common Histology Stains

There are many different histology stains, which are generally selected according to the type of tissue to be observed. Some histology stains are only used to study very specific types of biological tissue.

Hematoxylin and eosin (H&E stain) is the most commonly used histology stain for light microscopy.
Hematoxylin stains the nuclei within cells blue (recall that the cell structure of every biological includes a nucleus). Eosin stains the cytoplasm of cells pink. It is because H&E is such a common histology stain that so many histology slides seem to be dominated by colours ranging from blue to pink - though of course there are many exceptions due to the other stains used. H&E is a permanent histology stain, as opposed to a temporary stain.

More about Histology

Specific aspects of histology have their own names, such as histopathology, histochemistry, historadiography and immunohistochemistry (beyond the scope of requirements for most introductory-level courses such as A-Level and ITEC Exams).

See also Histopathology, How to Prepare Histology Slides, Histology Stains and the Structure of a Cell.