HISTOLOGY: HISTORY, WHAT IT STUDIES AND STUDY METHODS - BIOLOGY - 2025

The histology (from the Greek: histos = frame; loggia = science) is the branch of anatomy that describes and explains the microscopic structure of the plant and animal tissues, from the cell to the levels of organs and organ systems level.

The objective of anatomy is the systematic understanding of the principles underlying the external shape and internal architecture of multicellular organisms. Gross anatomy, or gross anatomy, considers structural features that can be inspected with the naked eye.

Source: User: Uwe Gille In turn, histology or microscopic anatomy, considers structural characteristics that can only be inspected using a microscope, being a fundamental device to understand gross anatomy. Its integration with cellular and molecular biology allows understanding the organization and function of cells.

History

Marcello Malpighi (1628–1694) was the forerunner of histology. He used the microscope to study plants and animals.

Marie-François-Xavier Bichat (1771–1802), considered the father of modern histology, coined the term “tissue”. Despite not using a microscope, in 1800, by dissecting cadavers and chemical tests, he identified 21 human tissues. In 1819, Carl Mayer (1787–1865) coined the term "histology."

In 1826, Joseph J. Lister (1786–1869) designed a revolutionary optical microscope, correcting for chromatic and spherical aberrations. Thanks to this, during the rest of the century, modern histology could develop. In 1827, Thomas Hodgkin (1798–1866) and Lister proved that red blood cells lack a nucleus.

In 1847, Rudolf Virchow (1821–1902) postulated that diseases have their origin in disturbances of cells. For this and other contributions, he is considered the founder of histopathology.

By the beginning of the 20th century, histology had matured. This was also made possible by:

- The development of chemical agents to fix tissues and of the microtome to section them throughout the 19th century.

- The embedding and preservation of tissues in blocks of Canadian balsam in 1832 and paraffin in 1869.

- Photomicrography in 1844.

What do you study?

The development of comparative histology has been possible thanks to descriptive studies of animal and plant tissues. Comparative histology includes histopathology, cytopathology, histochemistry, functional histology, and plant pathology. It also applies to the study of the evolution and systematics of living beings, as for example occurs with paleohistology.

Histopathology studies and diagnoses human and animal diseases. To do this, he uses tissue samples (biopsies) that are fixed, sectioned and examined by a professional known as a pathologist.

Cytopathology also studies and diagnoses human and animal diseases. The difference is that it does so at the level of microscopic fragments of free tissues and cells.

Histochemistry combines biochemical and histological techniques to analyze the chemistry of tissues. It is based on the use of chromogenic markers that serve to reveal positive cellular processes for certain substances.

Functional histology investigates the dynamic aspects of the organization of tissues. One of its most notable promoters was Santiago Ramón y Cajal (1852–1934), whose research on neurons laid the foundations for twentieth-century neuroscience.

Phytopathology studies plant diseases caused by viruses, bacteria, protozoa, parasitic plants, fungi and nematodes.

Human histology

Epithelial tissue

The basic types of human and animal tissues are: epithelial, muscular, nervous, and connective.

Epithelial tissue is made up of layers of cells that line (epithelium) the body surface, surround (endothelium) the body cavities or form glands and their ducts.

Epithelial tissue is classified into simple (a single layer of cells), stratified (several layers of cells), pseudostratified (a layer of cells attached to a basement membrane), squamous (flattened cells), cuboid (rounded surface cells), and columnar. (cells taller than they are wide).

The airways are lined by pseudostratified columnar epithelium. The body surface is covered by stratified squamous epithelium rich in keratin. The moist cavities, such as the mouth, vagina, and rectum, are lined by stratified squamous epithelium lacking keratin.

The glands are made up of secretory epithelium. They synthesize, store and release various types of substances, including: proteins (pancreas), lipids (adrenal and sebaceous glands), complexes of carbohydrates and proteins (salivary glands) and all of the above substances (mammary glands).

Muscle tissue

Muscle tissue is made up of elongated cells or fibers, with contractile properties. Based on its structure and function, three types of muscle are recognized: skeletal, cardiac, and smooth.

Skeletal muscle contains highly elongated, striated, multinucleated bundles of cells. Each muscle fiber is made up of smaller units called myofibrils.

These in turn consist of filaments composed of actin and myosin that form a regular alternating pattern. It is attached to bones. Its contraction is fast, vigorous and voluntary.

The heart muscle is also made up of elongated, striated cells. Its fibers are similar to those of skeletal muscle. However, they are uninucleated and show branching attached to those of other cells, being called intercalary discs. It is located in the heart, the aorta and the pulmonary trunk. Its contraction is vigorous, rhythmic and involuntary.

Smooth muscle is composed of medium-long, uninucleated spindle cells. It is not striated because actin and myosin do not form a regular alternating pattern.

It is layered in hollow visceral organs and blood vessels. It is also associated with hair follicles. Its contraction is prolonged, slow and involuntary.

Nervous tissue

Nervous tissue is made up of a network of many billions of nerve cells (neurons), all assisted by cells for support, nutrition and defense (glial cells). Each neuron has hundreds of long interconnections with other neurons.

Nervous tissue is distributed throughout the body, forming a system that controls behavior patterns as well as body functions (eg, blood pressure, respiration, hormone levels).

Anatomically, it is divided into:

- CNS, central nervous system, consisting of a large aggregation of neurons (brain, spinal cord).

- PNS, peripheral nervous system, consisting of nerves (cranial, spinal, peripheral) and small aggregations of neurons (ganglia). The PNS conducts sensory and motor nerve impulses to and from the CNS.

Connective tissue

Connective tissue is made up of cells associated with extracellular matrix. Serves for the union or support of other tissues. It includes bone, cartilage, tendons, fibrous tissue, adipose tissue and bone marrow, all of them with a solid extracellular matrix. It also includes blood, with a liquid extracellular matrix (plasma).

Plant histology

Fundamental tissue

The basic types of plant tissues are:

- Fundamental (or basic), subdivided into parenchyma, collenchyma and sclerenchyma.

- Vascular, subdivided into xylem and phloem.

• Dermal, subdivided into epidermis and peridermis.

The parenchyma is made up of cells, alive when mature, of irregular shape and a thin primary wall, storing sugars and starch, which can participate in photosynthesis and retain the ability to differentiate into other types of cells. It makes up most of the biomass of plants, including the interior of the stem, leaves and fruits.

Collenchyma is made up of cells, alive at maturity, irregular in shape and thick primary wall, rich in pectin. It provides structural support without losing the elasticity necessary for the elongation of the plants. It is located under the epidermis of the stems and in the petioles.

The sclerenchyma is made up of cells, with secondary walls, internal to the primary, thick and rich in lignin. These secondary walls, which persist after cell death, provide strength to the parts of the plant that need it and are no longer elongating. The sclerenchyma consists of fibers and sclereids.

Vascular tissue

Vascular tissue is typical of vascular plants, that is, pteridophytes (eg, ferns), gymnosperms (eg, pines and firs) and angiosperms (flowering plants).

The xylem distributes water with mineral solutes taken from the soil. The conduction of this fluid is carried out by tracheids (all vascular plants) and conducting vessels (mainly angiosperms). The tracheids and the elements that make up the conducting vessels are dead cells.

The phloem distributes sap, made up of water, sugars produced by photosynthesis and nutrients previously stored in other cells.

The conduction of this fluid is carried out by sieve cells (pteridophytes, gymnosperms) or by sieve tube elements (angiosperms). The sieve cells and the sieve tube elements are live cells.

Dermal tissue

Dermal tissue surrounds the entire body of plants. Above the ground, the dermal tissue protects the plant from water loss. Under the ground, it allows to take water and mineral salts. The epidermis is the only dermal tissue in plants, unless there is lateral thickening. In this case, the epidermis is replaced by peridermis.

Study methods

In general, a histological study requires:

1- Obtaining the sample

2- Fixation

3- Staining

4- Inlay

5- Sectioning

6- Microscopic observation.

Obtaining the sample consists of acquiring a part of the human or animal body (biopsy) or plant, of sufficient size (usually very small) and representative of the tissue of interest.

Fixation includes physical (eg, flash freezing) and chemical (eg, formalin) procedures that stabilize the sample so that it remains unaltered during and after the following steps.

Cells are colorless and must therefore be stained, allowing the structures of interest to be highlighted. Staining is performed using chromogenic (eg, hematoxylin, eosin, Giemsa), histochemical, or immunohistochemical reagents.

Embedding consists of infiltrating the tissue with a transparent or translucent liquid (for example, paraffin, acrylic resin) that will later harden by cooling or polymerization, forming a solid block.

Sectioning consists of slicing, using a microtome, the previous solid block. The sections obtained, typically 5–8 µm thick, are called histological sections.

Microscopic observation is carried out using optical, electronic, confocal, polarizing, or atomic force microscopes, among others. At this stage, digital images of the cuts are generated.

References

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