Cytology and Histology Past Paper PDF 2023/24

Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández LESSON 5: VITAL MANIFESTATIONS OF THE CELL B. CYTOSKELETON The cytoplasm of animal cells contains a cytoskeleton which is a 3D meshwork of protein filaments that are responsible for the maintenance of cellular morpholog...

Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández LESSON 5: VITAL MANIFESTATIONS OF THE CELL B. CYTOSKELETON The cytoplasm of animal cells contains a cytoskeleton which is a 3D meshwork of protein filaments that are responsible for the maintenance of cellular morphology, as well as, cellular motion including the intracellular transport (organelles and vesicles within cytoplasm), certain regions of the cell or the entire cell. The cytoskeleton has three components: actin filaments (microfilaments), intermediate filaments and microtubules. I. ACTIN FILAMENTS OR MICROFILAMENTS Actin is an abundant protein in all cells, particularly muscle cells. Actin filaments (diameter 6–8 nm) are thinner, more flexible and shorter than microtubules. They consist of two forms of actin: free G-actin (globular actin) and polymerised F-actin (filamentous actin). Actin filaments form a dense network that interconnects individual organelles. Functions: 1) The filaments extend into peripheral cell processes and provide structural support for the cytoplasm. 2) They act together with myosin (tropomyosin) filaments to bring about cell contraction and associated motility (Lesson 10). 3) Cross-linking of actin filaments into parallel bundles by the actin-bundling protein fimbrin gives rise to the structural core of specialised cell surface projections known as microvilli and stereocilia (Lesson 2). 4) Aggregates of actin filaments contribute to the contractile ring that divides the cell during the final phase of mitosis. 5) Mediate processes associated with endo- and exocytosis, facilitate intramembranous movement of transport proteins and expedite cellular movement. 6) By combining with filamin and -actinin to form a flexible mesh, actin also contributes to the gel state of the cytoplasm. II. INTERMEDIA FILAMENTS Intermediate filaments are polypeptide chains that provide structural support for the cell. They are considered to be the least soluble components of the cytosol. Intermediate fibres are typically arranged in parallel, passing along lines of pressure and tension within the cytoplasm. Their diameter (8–10 nm) lies between that of actin filaments and microtubules (hence ‘intermediate’ filament). They having been classified according to their constituent protein such as desmin filaments, keratin filaments (cytokeratins or tonofilaments), neurofilaments, vimentin, nuclear lamin and glial filaments (Glial fibrillary acidic protein-GFAP). 1 Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández In contrast to actin filaments and microtubules, intermediate filaments do not disassemble and reform. Instead, they constitute substantial, permanent structural elements of the cell. Intermediate filaments are abundant in those cells under heavy mechanical stress, such as neuronal axons, muscle cells (at the Z line), and epithelial cells (at the surface). Functions: 1) Provide structural support for the cell. Their main function is to withstand mechanical stress, mainly stretching. 2) Form a deformable 3D structural framework for the cell. 3) Anchor the nucleus in place because they form a net that spreads from the nuclear envelope to the plasma membrane. 4) Provide an adaptable connection between the cell membrane and the cytoskeleton, as well as, with other cells (desmosomes, hemidesmosomes and zonula adherens) or extracellular matrix. 5) Furnish a structural framework for the maintenance of the nuclear envelope forming the nuclear lamina as well as its reorganization subsequent to mitosis. III. MICROTUBULES Microtubules are elongated, narrow protein cylinders with a consistent diameter of 25 nm and composed of the globular polypeptide tubulin, which in turn consists of two polypeptide subunits: α- and β-tubulin. The tubulin dimers (formed from free cytoplasmic α- and β-tubulin molecules) polymerise end-to-end, the α-subunit of one molecule binding to the β-component of another. The resulting linear chains are termed protofilaments. Thirteen protofilaments combine to form the wall of the microtubule. Microtubules are impermanent structures that can be rapidly assembled and dismantled. Arising from the microtubule-organising centre (MTOC) (containing the centrioles), microtubules are formed by end-to-end attachment of free tubulin molecules. These molecules originate from the disassembly of microtubules elsewhere in the cell. Microtubules also grow outward from basal bodies, the organising centre for cilia and flagella. Microtubules are not contractile, rather they serve as attachment sites for contractile proteins. Functions: 1) contributing to the spatial organization of organelles. 2) intracellular vesicular transport (e.g. secretory vesicles, endosomes, lysosomes). 3) attachment of chromosomes to the mitotic spindle and movement of chromosomes during mitosis and meiosis. 4) movement of cilia and flagella. 5) elongation (axon extension) and motility of cells (e.g. fibroblast). 6) maintenance of cell shape. 2 Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández NOTE: Microtubule-associated transport proteins direct intracellular movement of organelles and cytoplasmic inclusions towards their intended destination. Entities destined for the cell surface (e.g. secretory vesicles) are bound to microtubules by kinesin. Dynein carries intracellular structures towards the centre of the cell and to the nucleus. Axonemal dyneins are responsible for the movement of cilia and flagella. C. CENTRIOLE I. DEFINITION The centriole is one of the specialised formations of microtubules and is involved in cell division and the formation of cilia and flagella. Ultrastructurally, centrioles show a cylindrical morphology in longitudinal section and measure about 0.25 µm in diameter by 0.75 µm in length. As centrioles occur in pairs in interphase cells, with both centrioles arranged perpendicularly, they are also called "diplosome". The diplosome is also located in the vicinity of the Golgi complex, next to the nucleus, near the geometric centre of the cell, hence also called the 'cell centre' and 'centrosome'. II. STRUCTURE The wall of the centriole is made up of nine microtubule triplets (9x3). The microtubules of the triplets are denoted by the letters A, B and C. Microtubule A is the innermost and its wall is complete. Microtubule B occupies a central position and does not complete its wall in the area of contact with A. Microtubule C is the outermost microtubule and does not complete its wall in the area of contact with B. Each triplet is linked to the previous and next triplet by a filament of the protein nexin, which links microtubule A to microtubule C. The microtubules A of each triplet extend towards the centre of the centriole to form the radial spokes (in the most proximal portion of the centriole, which is closest to the nucleus). There is also an electrodense material surrounding the triplets, which provides a framework or support for the centriole and is called the pericentriolar matrix (called also the microtubule-organising centre). This matrix appears to be made up of ribonucleoproteins. III. ORIGIN Centrioles originate from another centriole, a phenomenon called duplication, with the daughter centriole arranged perpendicular to the parent centriole. This occurs in all animal cells that undergo mitosis. It appears that the parent centriole, but especially the pericentriolar matrix, acts as a microtubule-organising centre. Centrosomes are comprised of two centrioles (mother and daughter) connected via an interconnecting fiber. The mother centriole has additional distal and sub distal appendages. The centrioles are surrounded by a matrix of proteins, the pericentriolar material (PCM). During the cell cycle, each centriole (the original mother and daughter centriole) duplicates once, growing a new daughter centriole from their sides. The 3 Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández original mother centriole duplicates at a faster rate than the original daughter centriole. The original daughter centriole acquires additional appendages and thus becomes a new mother centriole. Mitosis separates the two centrosomes (duplicated centrioles) resulting in two cells each with a differentially aged mother centriole. Differences between these cells regarding cell fate and regulation are beginning to emerge. IV. FUNCTIONS 1) Mitosis as the mitotic spindle is formed. 2) Act as centres directing microtubule formation (microtubule-organising centres). 3) Originate the cilium and support it. Figure 2. Detail of a centriole (cross section). Nexin filaments can be seen joining the triplets. MET. D. CILIA Cilia are motile projections found in various animal cells. The ciliary apparatus consists of the axoneme, the basal plate, the basal body and the striated rootlets. I. AXONEMA This is the name given to the free structure of the cilia and can measure from several µm to 1 or 2 mm. It is surrounded by a plasma membrane. It consists of nine peripheral microtubule doublets and two single central microtubules. The basic structure is 9x2+2. The microtubules are designated by the letters A (for innermost and complete) and B (for outermost and incomplete). From microtubule A depart extensions, the so-called dynein arms, which in all microtubules are oriented in the same direction, clockwise when looking at the axoneme from base to tip. Dynein is an ATP-ase that is activated by magnesium and calcium. The outer arm resembles a hook, while the inner arm is attached to the microtubule B of another doublet by a nexin filament coming out of it. The arms repeat periodically every 24 nm. Joining each microtubule A to the so-called central pair sheath is a radius. The central pair sheath is a protein structure that surrounds the two central microtubules. 4 Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández The central pair microtubules are complete and also emit projections that connect to the microtubules and constitute the central pair sheath. II. BASAL PLATE On the cell surface is the basal plate, which marks the transition from the structure of the axoneme (9x2+2) to that of the basal body or centriole (9x3+0). The basal plate has the nine peripheral doublets, but the two central microtubules (9x2+0) and the rays are missing, replaced by a central electrodense area called the axosome. III. BASAL BODY AND STRIATED ROOTLETS The basal body is found in the cytoplasm. It is identical in structure to the centriole and gives rise to and supports the cilia. From the distal end of the basal body emerge the striated rootlets, consisting of parallel microfilaments with ATP-ase activity. All the rootlets converge at one point, usually on one side of the nucleus. It has been suggested that they are responsible for the coordinated movement of the cilia. Figure 3. Numerous transversely cut axonemes (9x2+2) and their basal bodies (9x3+0) on the cell surface. MET. IV. CILIARY MOVEMENT Ciliary movement is explained in a similar way to muscle contraction, based on two fundamental facts: 1) the need for ATP for movement, energy which is supplied by the numerous mitochondria in the vicinity of the basal body and, 2) the presence of two contractile proteins, tubulin and dynein, and one flexible protein, nexin. The most common ciliary movement consists of two phases. In the first phase, the cilia projects forward by flexing its basal portion, describing a 90° angle; this is the effective stroke. In the second phase, the cilia recovers its primitive position; this is the slow recovery stroke. 5 Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández The ciliary movement is produced by the displacement of the microtubule doublets one on top of the other, breaking the dynein bridges and generating new ones. This ciliary movement is metachronous in the plane parallel to the direction of movement and isochronous in the plane perpendicular to the direction of movement. E. CELL DIFFERENTIATION I. CONCEPT Cell differentiation is the set of transformations that some of the cellular components undergo, thus enabling the cell to carry out a specific function. The phenomena of cell differentiation take place in interphase, which is why there is an antagonism between division and differentiation and why highly differentiated cells lose some or all of their ability to divide. II. CELL CLASSIFICATION ACCORDING TO DEGREE OF DIFFERENTIATION 1. Stable cell populations, which are highly differentiated cells that have lost their ability to divide, e.g. neurons. 2. Expanding cell populations, which are highly differentiated cells that can regain their ability to divide at a certain point in time, e.g. liver cells, which can divide in the event of liver damage. 3. Labile cell populations, poorly differentiated cells that divide very easily, e.g. blood cells. F. CELL DEATH Cell death is defined as the irreversible cessation of all vital manifestations of the cell. The types of cell death such as necrosis, apoptosis, pyroptosis and necroptosis, will be studied in depth in the discipline of General Anatomical Pathology. The term necrosis has evolved to mean death by swelling of the cell (oncosis) with eventual rupture of cell membranes because an irreversible cell injury. Necrotic cell death typically involves groups or zones of cells and elicits an inflammatory reaction because of the release of cell contents into the extracellular cell matrix. Morphologically, necrosis is characterised by condensation and fragmentation of the chromatin and destruction of the nuclear envelope, dilation of the rough endoplasmic reticulum, disruption of lysosomes, formation of amorphous densities in mitochondria, and destruction of the plasma membrane with the release of the cellular contents to the exterior. 6 Cytology and Histology (Academic course 2023/24) Verónica Mª Molina Hernández Apoptosis is a form of programmed cell death that is important in embryologic development, homeostasis, and involution of organs or tissues deprived of hormonal stimulation or growth factors. It is also a regulated form of cell death that is directed by signaling pathways in response to certain types of injury. In apoptosis, nuclear chromatin is condensed and fragmented, but the nuclear envelope is maintained. The cytoplasmic volume decreases due to water loss and protein condensation, but most of the organoids remain intact. The second stage of the process is characterised by the blebbing of the plasma membrane, which eventually fragments and the appearance of so-called apoptotic bodies that contain nuclear fragments, organelles, and condensed cytosol enveloped by these membrane fragments. The plasma membrane that surrounds apoptotic bodies prevents the inflammation occurring with necrotic cell death but does express factors to attract phagocytes and stimulate heterophagy. Pyroptosis (Pro-inflammatory cell death) resembles apoptosis in that it is receptor-mediated, but the differences are that it involves plasma membrane rupture, leakage of cytoplasmic contents, cell vacuolisation and inflammatory reaction similar to necrosis. In fact, pyroptosis is generally a primary response to infectious organisms. Pyroptosis is induced in cells of the innate immune system, such as monocytes, marcrophages, and dendritic cells in the presence of pathogen-associated molecular patterns (PAMPs) expressed on microbial pathogens or by cell-derived DAMPs. Necroptosis (Pro-inflammatory Programmed Necrosis) is a receptor-induced cell death that occurs as a defence mechanism against neurodegenerative processes and viral infections when the cell cannot enter apoptosis. Thus, necroptosis is a cell defense pathway that is activated under conditions in which apoptosis is inhibited. The similarities of necroptosis with apoptosis are receptor-mediated and that it occurs in individual cells, but morphologically it is more similar to necrosis (cell swelling, cytoplasmic rupture, outgrowth of cytoplasmic contents, induces inflammatory reaction). 7