What is histology?

Histology is the discipline that studies biological tissues including their microscopic structure, development and function. It is a multidisciplinary science that encompasses, among others, microscopic anatomy, cytology and biochemistry, and is considered essential for biology, medicine, physiology and many other branches of science.

Definition and a little history

The word histology comes from the Greek word “histos” (ἱστός), meaning “tissue”, and “logos” (λογία), meaning study or knowledge. The definition, however, is subject to many nuances, although it is generally understood as the study of the microscopic structure of biological cells and tissues, which is why it is sometimes considered practically synonymous with microscopic anatomy, although histology studies tissues in a meaningful way. much broader than the purely structural, as it includes the study of tissue development and the structural relationship to its function.

The study and development of histology throughout history has gone hand in hand with the development of microscopes. One of the first histologists to achieve notoriety was Marcello Malpighide of Italian origin, who in the 17th century made numerous observations of the tissues of bats, frogs and other animals under a microscope.

malpighi also studied the human lung, described the alveoli and hair-like structures that connected veins and arteries in the lungs; he called these hairlike structures capillaries (from the Latin capillaris, capillus, meaning “hair”), one of the basic types of blood vessels.

However, histology would not appear as a scientific discipline in its own right until the 19th century. In fact, the first reference to the concept of “tissue” dates back to 1801 (Bichat) and the first reference to histology as the specific study of tissues 1819 (Karl Mayer).

Throughout this 19th century there were several milestones in the history of histology, also due to the development of the microscope, but above all thanks to two things. One, for the development of microtomes, machines capable of making very thin tissue cuts; the first microtome was made in 1848. And another, in staining techniques developed by Camilo Golgi, known for giving his name to the Golgi apparatus, the cellular organelle in which proteins mature and lipids in the plasma membrane are synthesized.

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It was these staining techniques that allowed Santiago Ramón y Cajal to carry out his studies on nervous tissue and to develop the doctrine of neuron theory according to which nervous tissue is formed by a network of neurons as discrete units without direct contact with each other, but which communicate through chemical signals (neurotransmitters) that are dumped into clefts or synaptic spaces.

For these contributions, Golgi and Ramón y Cajal shared the Nobel Prize in Medicine in 1906.

In the second half of the 20th century, various histopathologies began to become common as a diagnostic technique, including forensic tests. Since the late 1980s, immunohistochemistry, a very specific branch of histology, has become fundamental in the diagnosis of cancer.

Application examples

Listing all applications of the knowledge provided by histology is impossible, it would be an endless list, here are just a few applications and frequent uses of histology:

Histopathology: Histopathology studies the microstructure of diseased cells and tissues, which can be used as a diagnostic method and as a possible means to recommend a particular treatment, as well as to better understand the pathological mechanisms of the disease. Autopsies and forensic investigations: the study of the biological tissues of the deceased can serve to clarify deaths and other problems related to forensic medicine. Archaeology: Histology applied to archaeological remains is a great source of information to reconstruct history and learn about our ancestors Educational: Histological sections are part of the teaching material in many scientific disciplines to help understand the microstructure of the human, animal or plant body and how each tissue is structurally and functionally related to other tissues.
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General types of fabrics

If histology studies biological tissues, it is not uncommon to classify them into several types. In mammals, more precisely in humans, there are only four basic types of tissue:

Muscle tissue Nervous tissue Connective tissue or connective tissue Epithelial tissue

With these four tissue types and their subtypes, every organ, system and fluid in the body. For example, blood is considered a type of connective tissue since blood cells are suspended in a matrix (plasma), while blood vessels are made up of a combination of muscle tissue and epithelial tissue.

In plants, fungi and other types of living beings, the tissues are different from those of animals. In plants, for example, the four basic tissue types are epidermal tissue, vascular tissue (phloem and xylem), collenchyma, and meristematic tissue.

histological sections

Histological studies are commonly performed by microscopic examination, either light microscopy or electron microscopy. The sample observed is what is known as a histological section, and consists of an ultrathin section of tissue, usually obtained using a microtome. In histological studies, cell culture in artificial environments is also common.

The staining of histological sections has been and continues to be a central aspect of histology. Its objective is that different types of cells and microscopic structures react differently to various dye substances, which allows to increase contrast and be able to observe them under the optical microscope.

Preparing a histological section usually includes the following steps:

Fixation: this step aims to stop the degradation of the tissue, maintaining its structure, including the structure of the cell and its organelles. Fixation can be chemical, for example with formaldehyde, which preserves structures but damages DNA, RNA and proteins. Among the fixation techniques without chemical products, cryofixation stands out. Dehydration, cleaning and infiltration: dehydration seeks to remove water and other liquids from the material so that it remains in a solid state and can be cut. In addition to dehydration, some material that forms a support matrix is ​​infiltrated to aid in cutting, usually through epoxy-type resins. Subsequently, the samples are soaked in different materials, sometimes the same as the infiltration substances, or frozen for later dissection. Dissection or cut: to visualize the histological sample, it must be cut with a very thin thickness that allows the passage of light from the optical microscope. The cut is made with a device called a microtome, capable of cutting tissue samples with thicknesses of 2-10 µm, if it is for light microscopy, or 60-100 nm if it is for electron microscopy. staining: Biological tissues have very little contrast and a staining process is required to visualize them. If the staining mechanism is known at the molecular level, then it is called histochemistry. One of the most commonly used staining techniques is hematoxylin and eosin staining, although there are many others.
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With the optical microscope, high resolutions can be achieved, down to 0.2 µm with the scanning electron microscope down to 1nm or less.

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