Histology

Histology is the study of tiny parts inside living things, especially how tissues are built. It is a part of biology that looks at very small structures using a microscope. While gross anatomy studies bigger parts of the body that we can see without tools, histology zooms in to see the smaller details.

In the past, scientists divided the study of tiny body parts into different areas: organology for studying organs, histology for studying tissues, and cytology for studying cells. Today, all these areas are usually included under histology.

In medicine, histopathology is a part of histology that looks at diseased tissue under a microscope to help doctors understand health problems. In paleontology, scientists use paleohistology to study the tiny structures inside fossil remains of ancient animals and plants.

Biological tissues

Animal tissue classification

Main article: Anatomy § Animal tissues

Animals have four main types of tissues: muscle tissue, nervous tissue, connective tissue, and epithelial tissue. These four types include all the different tissues in an animal's body. For example, blood is a type of connective tissue because its cells float in a liquid called plasma.

• Epithelium
  • Simple epithelium.
    • Simple squamous epithelium.
    • Simple cuboidal epithelium.
    • Simple columnar epithelium.
  • Pseudostratified columnar epithelium
  • Stratified epithelium
    • Stratified squamous epithelium
    • Stratified cuboidal epithelium
    • Stratified columnar epithelium
    • Transitional epithelium
  • Multicellular glands
• Muscle tissue
  • Smooth muscle
  • Skeletal muscle
  • Cardiac muscle
• Connective tissue
  • General connective tissue
    • Loose connective tissue
    • Dense connective tissue
  • Special connective tissue
    • Cartilage
    • Bone
    • Hemopoietic
    • Blood
    • Lymph
• Nervous tissue
  • Central nervous system
  • Peripheral nervous system
  • Special receptors

Plant tissue classification

Main article: Plant Anatomy

Plants also have different types of tissues, studied in plant anatomy. The four main types are:

• Dermal tissue
• Vascular tissue
• Ground tissue
• Meristematic tissue

Medical histology

Histopathology is a part of histology that studies diseased tissues under a microscope. It helps doctors diagnose diseases like cancer by looking at tiny pieces of tissue. Special doctors called pathologists examine these samples to help treat patients.

People who prepare these tissue samples for viewing are called histotechnicians, histology technicians, and other similar job titles. They work in labs and help doctors by getting the tissues ready for examination.

Sample preparation

Most samples for studying tiny parts of living things need special steps before looking at them with a microscope. These steps depend on the sample and how we plan to look at it.

Fixation

Main article: Fixation (histology)

Chemicals are used to keep the shape of tiny parts of living things just as they are. This also makes the tissues firmer so they can be cut into very thin slices for looking at under a microscope. One common chemical used for this is called formalin, which helps keep the tissues in place by linking together parts of proteins.

Selection and trimming

When not all of a sample is needed, scientists pick the parts they want to study. They might keep the rest in case they need it later. They also cut the samples to show the important parts and to make them the right size for further steps.

Embedding

Tissues are put into a harder material to support them and to help cut thin slices. First, water is removed from the tissues and replaced with a material that hardens, like wax.

Paraffin wax

Mainly, wax called paraffin is used for this. The tissue goes through steps to remove water, then alcohol, and finally the wax to harden it.

Other materials

Sometimes other materials like epoxy or acrylic are used, especially for very thin slices needed for special microscopes.

Sectioning

Main article: Microtome

Thin slices of tissue are cut using special tools. For regular microscopes, slices are about 5 to 15 very small units thick. For very powerful microscopes, slices can be even thinner.

Staining

Main article: Staining

Tissues often need special colors to show details better. One common way is to use hematoxylin and eosin, which color certain parts of cells different shades to help see them clearly.

Light microscopy

Hematoxylin and eosin is often used to show the general look of tissues. Hematoxylin makes cell centers blue, while eosin colors other parts pink.

Historadiography

Slides can sometimes be looked at using X-rays, or special methods can show where certain materials have gone in the body.

Immunohistochemistry

Main article: immunohistochemistry

Special molecules can be used to find specific parts in cells, like proteins. This helps scientists learn more about what cells are doing.

Electron microscopy

Main article: Electron microscope § Sample preparation

For very powerful microscopes, metals are used to add contrast and show details in the tissues.

Specialized techniques

Cryosectioning

Main article: Frozen section procedure

Tissues can be frozen quickly, cut, and looked at to study certain parts, like in surgeries to check for unwanted cells.

Ultramicrotomy

Main article: Ultramicrotomy

Very thin slices are made for the most powerful microscopes, using special tools and materials.

Artifacts

Sometimes, the steps to prepare tissues can change how they look, hiding real details or adding false ones. For example, some chemicals can leave colors or shrink parts of the tissue.

History

In the 1600s, an Italian scientist named Marcello Malpighi used microscopes to look at tiny parts of living things. He is often called the founder of histology, which is the study of tiny parts of tissues. Malpighi looked at parts of bats, frogs, and other animals and discovered important details about how oxygen gets into the blood.

In the 1800s, histology became a special area of study. A French scientist named Xavier Bichat introduced the idea of tissues in the body in 1801. The word "histology" was first used in a book by Karl Mayer in 1819. During this time, many new ways to prepare and look at tissues were invented by different scientists. In 1906, two histologists, Camillo Golgi and Santiago Ramon y Cajal, won the Nobel Prize for their work on the brain.

Future directions

In vivo histology

Scientists are working on new ways to study body tissues while people are still alive. They want to use special imaging tools, like MRI, so doctors can learn about healthy and sick tissues without needing to take samples from the body. This could help doctors understand and treat illnesses better.