histology is the discipline that studies biological tissuesincluding your microscopic structure, development and function. It is a multidisciplinary science that encompasses, among others, microscopic anatomy, cytology, and biochemistry, and is considered essential to biology, medicine, physiology, and many more branches of science.
Definition and a bit of 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 structurea of biological cells and tissues, which is why it is sometimes considered practically synonymous with microscopic anatomy, although histology studies tissues in a much broader sense than the purely structural, since it includes the study of tissue development and the structural relationship with 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 Malpighiof Italian origin, who in the 17th century made numerous observations of the tissues of bats, frogs and other animals under the microscope.
malpighi too studied the human lung, described the alveoli and hair-like structures that connected veins and arteries in the lungs; he called these hair-like 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 itself until the nineteenth century. In fact, the first reference to the concept of "fabric" dates back to 1801 (Bichat) and the first reference to histology as the specific study of the tissues of 1819 (Karl Mayer).
Throughout this nineteenth 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, to the development of microtomes, machines capable of making very thin tissue cuts; the first microtome was made in 1848. And another, at staining techniques developed by Camilo Golgi, known for giving his name to the Golgi apparatus, the cellular organelle in which proteins mature and lipids of the plasma membrane are synthesized.
It was these staining techniques that allowed Santiago Ramón y Cajal to carry out his studies on nervous tissue and develop the neuron doctrinetheory according to which the nervous tissue is formed by a network of neurons as discrete units without direct contact with each other, but rather communicate through chemical signals (neurotransmitters) that are poured into the 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 histopathology began to become common as a diagnostic technique, including forensic tests. Since the late 1980s, the immunohistochemistrya very specific branch of histology, has become fundamental in the cancer diagnosis.
Application Examples
Listing all the applications of the knowledge provided by histology is impossible, it would be an endless list, here are just some 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 certain 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.
- Archeology: histology applied to archaeological remains is a great source of information for reconstructing history and learning about our ancestors
- educational: histological sections are part of the teaching materials 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.
General types of fabrics
If histology studies biological tissues, it is not uncommon for it to classify them into various types. In mammals, more precisely in humans, there are only four basic types of tissue:
- Muscle tissue
- Nervous tissue
- Connective or connective tissue
- Epithelial tissue
With these four types of tissue and their subtypes, all organs, systems and fluids of the body. For example, blood is considered a type of connective tissuesince 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 things, the tissues are different from those of animals. In plants, for example, the four basic types of tissue are epidermal tissue, vascular tissue (phloem and xylem), collenchyma, and meristematic tissue.
histological sections
Histological studies are commonly performed by microscopic examination, both light microscopy and electron microscopy. The observed sample is what is known as histological cut, and consists of a ultra-fine cut of tissue, usually obtained using a microtome. In histological studies, it is also usual to cell culture in artificial environments.
The staining of histological sections It has been and continues to be a central aspect of histology. Its goal is that different types of cells and microscopic structures react differently to various staining substances, which allows increase contrast and be able to observe them under an optical microscope.
The preparation of a histological section usually includes the following steps:
- Fixation: this step aims to stop tissue degradation while maintaining its structure, including the structure of the cell and its organelles. The fixation can be chemical, for example with formaldehyde, which preserves the structures but damages DNA, RNA and proteins. Among the chemical-free fixation techniques, 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 help cutting, generally by means of epoxy-type resins. Subsequently, the samples are embedded in different materials, sometimes the same as the infiltration substances, or frozen to subsequently proceed to dissection.
- Dissection or cut: to visualize the histological sample, it must be cut with a very thin thickness that allows the light of the optical microscope to pass through. 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 optical microscopy, or 60-100 nm if it is for electron microscopy.
- staining: Biological tissues have very little contrast and a staining process is necessary for them to be visualized. If the staining mechanism is known at the molecular level, then it is called histochemistry. One of the most widely used staining techniques is hematoxylin and eosin staining, although there are many others.
With the optical microscope, high resolutions can be achieved, up to 0.2 µmwith the scanning electron microscope up to 1nm or less.