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Histology is defined as the scientific study of the microscopic structure (microanatomy) of cells and tissues. The term "histology" comes from the Greek words "histos," meaning tissue or columns, and "logia," which means study. The word "histology" first appeared in a 1819 book written by German anatomist and physiologist Karl Meyer, tracing its roots back to 17th-century microscopic studies of biological structures performed by Italian physician Marcello Malpighi.
How Histology Works
Courses in histology focus on the preparation of histology slides, relying on previous mastery of anatomy and physiology. Light and electron microscopy techniques are usually taught separately.
The five steps of preparing slides for histology are:
Cells and tissues must be fixed to prevent decay and degradation. Processing is required to prevent excessive alteration of tissues when they are embedded. Embedding involves placing a sample within a supporting material (e.g., paraffin or plastic) so small samples can be cut into thin sections, suitable for microscopy. Sectioning is performed using special blades called microtomes or ultramicrotomes. Sections are placed on microscope slides and stained. A variety of staining protocols are available, chosen to enhance the visibility of specific types of structures.
The most common stain is a combination of hematoxylin and eosin (H&E stain). Hematoxylin stains cellular nuclei blue, while eosin stains cytoplasm pink. Images of H&E slides tend to be in shades of pink and blue. Toluidine blue stains the nucleus and cytoplasm blue, but mast cells purple. Wright's stain colors red blood cells blue/purple, while turning white blood cells and platelets other colors.
Hematoxylin and eosin produce a permanent stain, so slides made using this combination may be kept for later examination. Some other histology stains are temporary, so photomicrography is necessary in order to preserve data. Most of the trichrome stains are differential stains, where a single mixture produces multiple colors. For example, Malloy's trichrome stain colors cytoplasm pale red, the nucleus and muscle red, red blood cells and keratin orange, cartilage blue, and bone deep blue.
Types of Tissues
The two broad categories of tissues are plant tissue and animal tissue.
Plant histology usually is called "plant anatomy" to avoid confusion. The main types of plant tissues are:
- Vascular tissue
- Dermal tissue
- Meristematic tissue
- Ground tissue
In humans and other animals, all tissue may be classified as belonging to one of four groups:
Subcategories of these main types include epithelium, endothelium, mesothelium, mesenchyme, germ cells, and stem cells.
Histology may also be used to study structures in microorganisms, fungi, and algae.
Careers in Histology
A person who prepares tissues for sectioning, cuts them, stains them, and images them is called a histologist. Histologists work in labs and have highly refined skills, used to determine the best way to cut a sample, how to stain sections to make important structures visible, and how to image slides using microscopy. Laboratory personnel in a histology lab include biomedical scientists, medical technicians, histology technicians (HT), and histology technologists (HTL).
The slides and images produced by histologists are examined by medical doctors called pathologists. Pathologists specialize in identifying abnormal cells and tissues. A pathologist can identify many conditions and diseases, including cancer and parasitic infection, so other doctors, veterinarians, and botanists can devise treatment plans or determine whether an abnormality led to death.
Histopathologists are specialists who study diseased tissue. A career in histopathology typically requires a medical degree or doctorate. Many scientists in this discipline have dual degrees.
Uses of Histology
Histology is important in science education, applied science, and medicine.
- Histology is taught to biologists, medical students, and veterinary students because it helps them understand and recognize different types of tissues. In turn, histology bridges the gap between anatomy and physiology by showing what happens to tissues at the cellular level.
- Archaeologists use histology to study biological material recovered from archaeological sites. Bones and teeth are most likely to provide data. Paleontologists may recover useful material from organisms preserved in amber or frozen in permafrost.
- Histology is used to diagnose diseases in humans, animals, and plants and to analyze the effects of treatment.
- Histology is used during autopsies and forensic investigations to help understand unexplained deaths. In some cases, a cause of death may be evident from microscopic tissue examination. In other cases, the microanatomy may reveal clues about the environment after death.