Cytology and Histology PDF 2023/24

Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández LESSON 1: CONCEPT OF CYTOLOGY AND HISTOLOGY I. CONCEPT OF HISTOLOGY Histology emerged as a branch of Anatomy, and it was a consequence of technological development and morphological research. Histology is based on key i...

Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández LESSON 1: CONCEPT OF CYTOLOGY AND HISTOLOGY I. CONCEPT OF HISTOLOGY Histology emerged as a branch of Anatomy, and it was a consequence of technological development and morphological research. Histology is based on key instruments which are the microscope and the microtome. Histology derives from the Greek words histos (tissue) and logos (treatise or study), so etymologically, histology means the study of tissues. This science, which in its beginnings was concerned only with forms at the microscopic level, gradually expanded with the appearance of the electron microscope, which allowed the description of new structures and the clarification of others at the cellular level. Tissues are similar structures in all species, but the tissue organisation for the formation of organs is specific to each animal species. Therefore, the analogies, differences and peculiarities of animal structures must be known by the veterinary histologists. The cell is the morphological and functional unit of living beings. The grouping of cells of the same nature, differentiated in a certain way, regularly arranged and performing a certain function, constitutes a higher-order unit called tissue. Tissues are similar structures in all species but, fundamentally in the formation of organs, the architecture is specific to each animal species. Organ is therefore defined as the combination of two or more tissues that constitute a larger unit. Systems are assemblies of organs, made up of the same types of tissues, which can perform independent acts. On the other hand, the combination of several organs, which may be of very different tissues and which act in coordination in the performance of a function, is called apparatus. The concept of Histology is based on all of the above principles, since histological science deals with the structural (carried out with the optical microscope) and ultrastructural (carried out with the electron microscope) study of the cells, tissues and organs of living beings and their modifications in relation to their function and their physico-chemical and immunological characteristics. II. HISTORICAL BACKGROUND The development of Histology is closely related to advances in microscopy. At the end of the 16th century, the first simple microscope was made by the Dutchmen Hans and Zacharias Janssen (1590), who were eyeglass dealers and inserted lenses into a cylinder, although they did not give it any biological application. It was Francesco Stelluti (1625) who, in his laboratory in Rome, gave it its first biological application when he observed and made a drawing of a bee which he had magnified five times. 1 Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández In Italy, a series of important histologists would emerge, such as Marcello Malpighi, the true founder of Microscopic Anatomy and Embryology. Malpighi (16281694) studied the structure of animal and plant tissues and found that both were made up of independent units delimited by thin membranes which he called "saccules". He described that within these saccules there was a more or less lumpy liquid, which he called "utricles". Malpighi is thus credited with the discovery of the cell membrane. Later, Antoni Van Leeuwenhoek (1674) continued to perfect the microscope, developing his own model with which he achieved up to 200 magnifications, and is known for being the first to apply stains to tissues in order to observe them under the microscope. However, many later researchers rejected the microscope, including François Xavier Bichat (1771-1802), who is considered to be the true creator of Histology. The ideas of this anatomist caused a profound upheaval in the medical-philosophical world. Bichat realised the mistakes that had been made by some of the researchers of the 18th century, and as a result, he rejected the microscope outright as an instrument of observation, so he used methods of dissection, desiccation, boiling and many other techniques based on physics and chemistry to investigate the structure of living beings. He carried out so many studies that Bichat is considered the father of Histology, defining tissue as the morphological unit of all living beings and thus creating the tissue theory.3 In 19th century, Schleiden and Schwann, postulated the cell theory. Schleiden (1804-1881) promulgated the theory that the nucleus played an important role in the development of the plant cell. Schwann (1810-1882), on the other hand, based on this theory, recalled that he had observed something similar in the dorsal chord of the notochord of animals, and deduced that the origin of the animal cell need not be different from that of the plant cell. All these facts led Schwann to promulgate the cell theory: "every organism consists of cells, which are the unit of form and function". This theory denied cell division. However, Virchow (1821-1902) postulated his aphorism omnis cellula e cellula (every cell comes from another maternal cell by very different mechanisms), establishing that "the cell is the morphological, functional and originating unit of living beings", in such a way that Virchow completes the cellular theory although the paternity of the cellular theory is still given to Schleiden and Schwann. The cellular theory is applied to all tissues with the exception of the nervous system, as Camillo Golgi (18431926) defended the theory of continuity, "all neurons continue each other by dendritic prolongations". All these theories demonstrated the great interest that existed in applying cellular theory to nerve tissue. This concern was taken up by Santiago Ramón y Cajal (1852-1934), who postulated and proved the individuality and independence of nerve cells. Golgi and Cajal, who shared the Nobel Prize in Medicine in 1906 for their studies on the nervous system, met only in Stockholm, to receive the award. Golgi gave his Nobel lecture first, in which he tied to his belief in “reticular” neural networks, which was entirely contradicted by Cajal’s Nobel lecture. Cajal, a strenuous supporter of the 2 Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández contiguity theory (and not the continuity theory) in which individual cells representing the basic units of the nervous system, fought for his ideas until his death. III. THE CELL: GENERAL CHARACTERISTICS The cell is considered the morphological, functional and originating unit of the living being. It can also be defined as the smallest portion of protoplasm with functional and structural independence, which makes up the living matter of the animal and plant kingdoms. In the animal world we can consider two types of cells: prokaryotic and eukaryotic. Prokaryotes are cells that have a cell membrane or envelope that isolates the protoplasm from the extracellular environment and do not have separate genetic material (DNA) from the protoplasmic components of the cell. An example of prokaryotic cells are bacteria. In contrast, eukaryotic cells are cells enveloped by a complex cell membrane and their genetic material is separated from the rest of the cell organoids by a nuclear envelope. Thus, in the eukaryotic cell, two essential parts are distinguished: the cytoplasm, where all the cell organoids are located, and the nucleus, where the genetic material of the cell is located. The cell as a physical unit, with a defined structure, has the characteristics of a physical body such as shape, size, volume and color. The shape of the cell depends on whether it is associated with other cells or not. In general, an isolated cell the shape tends to be spherical. On the other hand, when under the influence of pressure from neighbouring cells, it tends to be polyhedral (with 8, 12 or 14 sides; e.g., squamous epithelium) being the tetradecahedron (14 sides) the ideal shape for cells constituting a cell tissue; flattened (e.g., squamous epithelium, endothelium and the upper layers of the epidermis); cuboidal (e.g., in thyroid gland follicles); columnar (e.g., the cells lining the intestine); discoidal (e.g., red blood cells or erythrocytes); spherical (e.g., eggs of many animals); spindle shaped (e.g., smooth-muscle fibers); elongated (e.g., nerve cells or neurons); or branched (e.g., chromatophores or pigment cells of skin). However, the shape depends not only on contiguity with other cells, but also on the surface tension of their cytoplasmic membranes, the state of their protoplasm, whether it is in the sol or gel state, and especially on the degree of differentiation or specialisation in which it is found. Differentiation can determine a specific shape on its own, as in the case of muscle cells, which are usually cylindrical or spindle-shaped, the optimum shape for contracting and dilating. Another example is nerve cells, which adopt stellate shapes in order to receive and transmit nerve information. Size is another variable characteristic of the cell. Most cells in tissues range between 12 and 24 micrometres or microns (µm) in diameter, but there are cells below this size, between 3-12 µm, which are traditionally called dwarf cells, e.g. some lymphocytes, germ cells of epithelia or erythrocytes. In contrast, cells between 30 and 300 µm can be found in the animal organism and are referred to as giant cells, such as megakaryocytes and osteoclasts. 3 Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández The cell volume is constant for each cell type and independent of the size of the animal. This concept is called Driesch's Law. The cells have an approximate volume between a minimum of 300 and a maximum of 5000 µm3. Cells are normally colorless, except for those that contain pigments in their cytoplasm. Since cells are colorless, when they are to be observed, they must be stained by dyes, either while they are still alive or after death by suitable physical or chemical agents. In the first case, the cells phagocytose the dye and can thus be observed, whereas in the case of dead cells, they must be fixed with fixatives and then stained with dyes. IV. THE CYTOPLASM It is the part of the protoplasm that surrounds the nucleus and shows different constitution depending on the cellular zone. Thus, there is a thin inner strip, around the entire cytoplasmic membrane, which is called ectoplasm, which is free of organoids and has a sol state with a thin and shakable consistency, while towards the interior of the cell there is the endoplasm, in which all the organoids are located, and which is in a gel state. The cytoplasm consists of the hyaloplasm or cytosol (cytoplasmic matrix) which is the fundamental matrix of the cell and in where most cellular biochemical reactions occur; the cytoskeleton, which consists of a network of microfilaments, intermediate filaments and microtubules; organoids and metabolic inclusions. V. METABOLIC INCLUSIONS A number of inert substances called metabolic inclusions are found in the cytoplasm of cells. These substances perform a vital function in cells, but they are reversible and appear or disappear as a result of cell metabolism. Within this group of substances, the following stand out: inclusions of glycogen, lipids, proteins, pigments and a large group of condensation vesicles of the Golgi complex and the vacuoles. Among them, we highlight: 1. Glycogen inclusions Cells contain large amounts of carbohydrates, which are involved in both the internal metabolism of the cell and the general organism. Carbohydrates can be stored as a polymer termed as glycogen. Glycogen is the long-term storage unit of glucose within the cell, typically found in hepatocytes, Purkinje cells, chondrocytes, and muscle fibers. Under light microscopy, glycogen can be visualized in tissue using a specific histologic stain called the periodic acid-Schiff (PAS) stain that gives a magenta color to the carbohydrates including glycogen as a polysaccharide and mucopolysaccharides. The electron microscope clearly detects the various forms of glycogen in the cells of the animal organism: - Alpha particles: these are arranged as rosettes of electron-dense granules (beta particles), consisting of a central granule surrounded by peripheral granules. These 4 Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández rosettes are typically 500-2,000 Å in diameter, and the dense granules are about 300 Å in diameter (Figure 1a). - Beta particles: these are isolated electron-dense granules (single spherical particles) about 300 Å (=30 nm) in diameter. - Gamma particles: glycogen occurs as large amorphous deposits of medium electron density and small dense granules can be observed within them. 2. Lipid inclusions They are the most abundant inclusions in the cell, although it should be noted that these inclusions are structural fat. These inclusions are difficult to observe due to the fact that the usual processing techniques use lipid solvents (i.e., xilol), and therefore, when these inclusions are observed with the light microscope and sometimes with the electron microscope, only the place they used to occupy is visible (Figure 1b, asterisk). 3. Protein inclusions These inclusions are difficult to observe because they must form clusters or hyaline granules to be seen. They are usually associated with carbohydrates and lipids, mostly forming the structural proteins of the cell (Figure 1b, arrow). By the electron microscopy they are electron-dense. a b * * Figure 1. a) Glycogen inclusions (alpha particles) (electrodense); b) Lipid inclusions (adielectronic) (*) and protein inclusions (electrodense) (arrow). 5