Lesson 2: Cellular Wrapping 2023/24 PDF

Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández LESSON 2: CELLULAR WRAPPING I. INTRODUCTION Cells are bounded on the outside by the cell membrane, also called the plasma membrane, which separates the cytoplasm from the outside environment. This membrane has plasticit...

Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández LESSON 2: CELLULAR WRAPPING I. INTRODUCTION Cells are bounded on the outside by the cell membrane, also called the plasma membrane, which separates the cytoplasm from the outside environment. This membrane has plasticity properties, which allow cells to have specific and variable shapes. It is also permeable, which facilitates the exchange of substances between the inside and outside of the cell, linking the two compartments. It also allows the cell to perform complex functions such as immunological and cell recognition. II. STRUCTURE The cell membrane is organised in a complex structure made up of three components: 1. externally, the glycocalyx or microenvironment; 2. the cytoplasmic membrane, the most important element from a functional and morphological point of view of this triad; and 3. an inner layer or ectoplasm, which in turn constitutes the outermost zone of the cytoplasm (Figures 1-3). Membrane unit corresponds to the trilaminar structure described for the cell envelope, which is similar for all cellular component membranes, with slight differences in its thickness and in the proportion of its components. 2 1 3 Figure 1. Diagram of a cell. Figure 2. Detail of the cell membrane. Figure 3. Detail of figure 2 showing the trilaminar membrane. From these three structures of the cell membrane, the cytoplasmic membrane is the most complex in terms of both architecture and chemical composition. It can be defined as a flexible and dynamic structure that separates the intracellular from the extracellular environment. Its functions are to preserve cellular integrity, control the traffic of substances between the inside and outside, participate in cell recognition and adhesion, and transfer extracellular signals to the cell interior. 1 Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández The cell membrane is not observable with the optical microscope. With the electron microscope, it is generally observed as an electron-dense line about 7.5-8 nm thick. At high magnifications, this electron-dense line has a trilaminar structure: two electron-dense sheets (external and internal) about 2-3 nm thick, separated by an adielectronic sheet 3-4 nm thick (Figures 1-3). This structure is due to the arrangement of its components: lipids and proteins. Lipids are amphipathic phospholipids that have a hydrophilic head and a hydrophobic tail and are arranged in two layers so that the hydrophobic tails of each layer face each other, constituting the adielectronic sheet, and the hydrophilic heads are located peripherally, constituting the external and internal electron-dense sheets. Other lipids that are involved in the composition of the membrane are glycolipids and cholesterol. Proteins, according to their location in the lipid bilayer, are classified as: • Integral or transmembrane: they are tightly bound to lipids and constitute transport channels through the lipid bilayer. • Peripheral: associated with the inner or outer layer of the membrane and weakly attached to lipids. Among the functions of these proteins are: - Be receptors that intervene in cell recognition and adhesion processes - Transport of substances to the outside or inside of the cell - Be enzymes that catalyze membrane reactions - Be receptors that connect the membrane with the cytoskeleton, with another adjacent cell or with the extracellular matrix - They are part of the glycocalyx III. GLYCOCALYX It is a thin sheet located on the outer sheet of the cell membrane and of variable thickness (5-10 nm). It is particularly developed on the surface of cells with great absorption activity, as occurs in the intestine, where it reaches a thickness of about 50 nm. With the optical microscope, it is PAS positive due to its composition rich in glycoproteins, and with the electron microscope, it has a filamentous appearance. Its functions include enzymatic activity and participation in protection, recognition and adhesion processes. IV. EXCHANGES WITH THE EXTERNAL ENVIRONMENT The exchange of substances between the outside and inside of the cell is carried out through the cell membrane, which acts as a semipermeable barrier, allowing the passage of some substances and restricting the passage of others. Thus, small non-polar molecules, such as oxygen, nitrogen, benzene, water, urea, glycerol, CO 2, etc., diffuse 2 Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández easily, while polar molecules and ions (sugars, nucleotides, vitamins, etc.) pass through transport proteins passively, or with energy expenditure through ATP degradation. V. HISTORICAL MODELS Dawson and Danielli (1935) described the cytoplasmic membrane as consisting of a lipoprotein layer in which the proteins were arranged in two layers, one on the outer side and the other on the inner side, and that the centre was occupied by a lipid bilayer with its hydrophilic poles facing outwards (the proteins) and the hydrophobic poles occupying its centre (Figures 4 and 5). This biochemical model of the cytoplasmic membrane is joined by the model of Robertson, who in ultrastructural studies described the membrane as two electrodense protein layers on its outer and inner sides and a central one of low density of lipid nature. This model of trilaminar structure is referred to as the "membrane unit". Figure 4. Biochemistry of membrane unit structure. Figure 5. Scheme of an electron microscopy image of the membrane with two electrodense layers (black) and one adielectronic layer (white). Thus, as shown in figure 5, the cytoplasmic membrane has a trilamellar structure: two electrodense layers and a central adielectronic layer. This membrane model is extrapolated to all other cell membranes, although they vary in composition and also in thickness. However, this scheme of the membrane unit is rigid and does not explain the passage of many substances through the membrane. 6 Figure 6. Scheme of the fluid mosaic membrane model. Figure 7. Electron microscopy image showing the detail of two membranes. Considering that the lipid bilayer is the base of the membrane and acts as a relatively impermeable barrier to the passage of most water-soluble molecules, most of 3 7 Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández the specific functions would be regulated by proteins. The type of proteins that had been described in previous models corresponded to fibrous or structural proteins, which allow little permeability and plasticity. Therefore, in 1972, Seymour Singer and Garth Nicolson defined the so-called "fluid mosaic" membrane model. This structure is made up of fibrous or structural proteins and globular or functional proteins. The fibrous proteins would form most of the outer layers, between which the globular proteins would be inserted. In addition, these proteins would be distributed within the lipid layer (Figures 6 and 7). VI. PLASMA MEMBRANE SPECIALISATIONS The membranes of numerous cell populations, especially epithelial cells, show modifications by means of which they relate to the external environment and, above all, to neighbouring cells. The relationship between cells is not only mechanical or adhesive, but also involves ion exchange, as in the case of cardiac cells. These specialisations, according to their location on the perimeter of the cell and taking the epithelial cell as a model, are as follows: A) Apical border: microvilli, cilia and stereocilia. Microvilli are thin filiform cell extensions 1 to 2 µm high and 0.10 µm thick. Their stability is due to the presence of actin filaments arranged parallel to the major axis of the microvilli. They can be found in the digestive epithelium and the cells of the proximal convoluted tubules of the kidney, among others. Their main function is the exchange of substances between tissues and the extracellular environment. With the optical microscope, in those epithelia that have many, it is seen as a more refractive superficial layer. As it presents a highly developed glycocalyx, it is stained with the PAS technique. In the intestine it is called striated veneer and in the kidney, brush border. Cilia are filiform cell extensions between 10-15 µm in length and 0.25 µm in diameter. They are usually densely packed on the free apical surfaces of numerous cells. The epithelia of the respiratory tract are the best example. Being able to move, their main function is to displace fluids as well as any other substance trapped between them. Flagella are a special type of cilium that appears on sperm. Stereocilia, despite their name, are modified microvilli. For example, they can be found in the cells of the epididymis and in the sensory cells of the inner ear. They lack movement and their functions are secretory and sensory. B) Lateral border: B.1. Cell-cell junction modes: o Zonula occludens or tight junction (Figure 8). o Zonula adherens or adherent junction (Figure 8). o Macula adherens or desmosome (Figure 8). 4 Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández o Interdigitation (Figure 8). The zonula occludens or tight junction is a form of cell-cell junction. Tight junctions were first observed by transmission electron microscopy (TEM) as apical membrane contacts extending over 200–500 nm with electron-dense areas forming a continuous belt-like attachment by fusing the outer membrane leaflets of two neighboring cells into a ≈3 nm thick structure. By forming a tight seal between neighbouring epithelial cells, they restrict the movement of water or molecules into the intercellular space. Tight junctions are visualized as a zone in which adjacent plasma membranes are closely apposed, that circumscribes the cell as a belt together with adherent junctions. At high magnification, tight junctions are characterized as focal attachments of adjacent cell membranes that exclude the intercellular space (Figures 8, 9 and 10). The zonula adherens or adherent junctions is usually located below the occluding zone (Figure 8). This membrane specialisation is constituted by the membranes of neighbouring cells, which are arranged in parallel. The cytoplasmic faces of the membranes of the neighbouring cells have a reinforcement of electrodense material which is called an electrodense band or sheet (Figure 10). Functionally, it is one of the most labile of all binding modes. The macula adherens or desmosome can even be seen as punctate structures under the light microscope because of their high refractivity. The membranes of the two neighbouring cells are arranged parallel to each other, separated by a space of approximately 250 Å. In the intermembrane space there is an electrodense material formed by the overlapping of the glycoprotein meshes of the glycocalyx of the two neighbouring cells. And, on the inner side of the plasma membranes, electron microscopy methods show an amorphous electrodense plate to which numerous microfilaments are attached, arranged in a hairpin fashion with the curvature on the dense cytoplasmic plate and the ends directed towards the endoplasm (Figure 8). These microfilaments correspond to intermediate filaments, and are primarily tonofilaments, which define the epithelial cell population, although actin microfilaments are also present (Figure 11). Interdigitations are lamelliform processes of the cell membrane that invaginate into the neighbouring cell, increasing the contact surface (Figure 8). * Figure 8. Tight junction (arrow), adherent junction (arrow head), desmosome (*) and interdigitation (+). Figure 9. Tight juntions showing close contacts between plasma membranes of adjacent cells. 5 Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández 11 10 Figure 10. Tight junction and adherent junction. Figure 11. Desmosome. Figure 12. Comunicating junction in heart (N). B.2. Substance exchange: o Gap junction or communicating zone The gap junction or communicating zones are plate-shaped and vary in size depending on the cell type. The intercellular space is about 20 Å. The membranes of neighbouring cells are not fused and there is no reinforcement in the cytoplasm. Hexagonal structures are observed extending from one membrane to the other and are approximately 90 Å apart. These structures have a narrow pore-like channel in their centre that connects the two neighbouring cells and through which ions and molecules of low molecular weight pass (Figure 12). Their function is not binding. C) Basal border: o Hemidesmosome Hemidesmosomes are specialisations of the membrane at its basal edge that anchor the cell to the basement membrane. Structurally, proteins called integrins act as anchors between the cell and the underlying basement membrane. 6 12