Cell Membrane Structure & Function PDF

Cell is the smallest structural and functional unit of the organism able to live by itself, formed from other cells by cell division - proliferation. The cell composed of nucleus and cytoplasm, contains organelles limited by the plasma membrane. The organelles can be classified to membranous and no...

Cell is the smallest structural and functional unit of the organism able to live by itself, formed from other cells by cell division - proliferation. The cell composed of nucleus and cytoplasm, contains organelles limited by the plasma membrane. The organelles can be classified to membranous and non-membranous organelles. Membranous organelles: 1. RER and SER – Rough and Smooth endoplasmic reticulum. RER – protein synthesis by polyribosomes which are destined to secretion out of the cytosol. SER - phospholipid and steroid hormones synthesis, detoxification of toxins, Calcium release (abundant in muscle cells, sarcoplasmic reticulum) 2. Golgi apparatus – modify and pack proteins synthesized in the RER. 3. Lysosomes – site of intracellular digestion. 4. Mitochondria - aerobic respiration and production of ATP 5. Peroxisomes – contain enzymes involved in lipid metabolism. 6. Nucleus Non-membranous organelles: 1. Free floating Ribosomes – protein synthesize for the cell use 2. Centrosome – composed of pair of centrioles, organizing center for microtubule. 3. Cytoskeleton - Microtubules, Actin and Intermediate filaments All eukaryotic cells are enveloped by a limiting membrane composed of: Lipids - Phospholipids and Cholesterol, glycolipids Proteins Chains of oligosaccharides (small number of simple sugars, monosaccharides) linked to phospholipid and protein molecules. Characteristic of biological membrane : Asymmetric Fluid Mosaic Membranes range from 7.5 to 10 nm in thickness, visible only in Electron Microscope; Trilaminar organization – 2 hydrophilic heads, sandwiched layer of hydrophobic tails.. Membrane phospholipids consist of two non-polar hydrophobic tails, linked to a charged polar hydrophilic head group – stability to the membrane – Amphiphilic charateristic. Cholesterol is also present (1:1 ratio with phospholipids), insert among the phospholipid fatty acid and restricting their movement. It contains proteins called integrin that are linked to both cytoplasmic cytoskeletal filaments and extracellular matrix components. Through these linkages, there is a constant exchange of influences in both directions, between the ECM and the cytoplasm. Proteins of membrane, synthesized by RER, can be divided into: Integral transmembrane proteins – transfer of chemical substances by channels, transporters or pumps. 1. Transporters(Carriers) \ Channels – diffusion by gap junctions of small ions, molecules, water 2. Pumps – active transport of specific ions across the membrane using energy e.g. Na+\K+ pump 3. Transport using vesicles – endocytosis and exocytosis 4. ABC Transporters – using ATP for molecules transfer, connection to MDR: MDR (Multi drug resistance). Chemotherapy resistance occurs when cancers that have been responding to a therapy suddenly begin to grow. In other words, the cancer cells are resisting the effects of the chemotherapy. You may hear statements like the "cancer chemotherapy failed." When this occurs, the drugs will need to be changed – protein that transports the drug across the cell wall stopped work, thus cancer cell shows resistance to the drug. Peripheral proteins – exhibit looser association with one of the two membranes surfaces (inner our outer surfaces). The distribution of membrane proteins is different in the two surfaces of the cell membranes – asymmetric characteristic. The external surface of the cell shows a carbohydrate rich region called the glycocalyx, a layer which made of carbohydrate chains linked to membrane's proteins and lipids. The glycocalyx has a role in cell recognition and attachment to other cells and to extracellular molecules.  The membrane characterized by fluid mosaic appearance because of the combination of the fluid nature of the lipid bilayer, and the mosaic appearance of the membrane proteins. Cell membrane functions: 1. Selective barrier which regulate the passage of materials into and out of the cell in order to keep constant ion concentration – regulates the intracellular environment. 2. Carry out number of specific recognition and regulatory functions 3. Interactions of the cell with its environment including adhesion, cell to cell signaling. Endocytosis: Materials may get across the plasma membrane in a general process called endocytosis, which involves folding and fusion of a membrane to form vesicles which encloses the material transported. Phagocytosis – "cell eating" – WBC such macrophages and neutrophils engulf and remove matters, such as bacteria, protozoa, dead cells. The membrane of these cells surrounds the bacterium, fuse with it, and enclosing it in a phagosome. Pinocytosis – "cell drinking" – small particles brought into the cell, forming an invagination and suspended within vesicles which break down by lysosomes. Receptor mediated endocytosis – "absorptive pinocytosis" - cells absorb metabolites, hormones, proteins – by inward budding of plasma membrane vesicles containing proteins with receptor sites. In Exocytosis, a vesicle fuses with the plasma membrane, resulting in the release of its contents into the extracellular space without compromising the integrity of the plasma membrane (opposite to endocytosis). Cell Surface Specialization: The cell surfaces exhibit apical, lateral and basal domains. Apical domain and its modifications includes: Microvilli (1um high and 0.08 um wide) - fingerlike cytoplasmic projections that have a wide vary of appearances. The number and shape of a cell's microvilli correlate with the cell's absorptive capacity: Cells with tall, closely packed microvilli are likely to function in transportation and absorption of metabolites while cells with small irregular shaped microvilli will be less active in transportation and absorption. Within the microvilli are clusters of actin filaments that are cross linked to each other and to the surrounding plasma membrane by several other proteins. Stereocilia/ Stereovilli - extremely long, immotile microvilli that facilitate absorption. They are limited only to the epididymis, and sensory hair cells of the inner ear. Cilia and flagella are motile processes, covered by cell membrane, with highly organized microtubule core. Cilia's main function is to sweep fluid along the surface of cell sheet. Both cilia and flagella have the same structure, composed of 9 peripheral microtubular surrounding two central microtubules (9+2 pattern – axoneme) Lateral Domain: Seals to prevent the flow of materials between the cells – Tight (=Occluding) junctions, made by oclodine proteins) Sites of adhesion (adhesive or anchoring junctions, made by cadherin proteins)  Channels for communication between adjacent cells (gap junctions – made by connexin proteins). Such junctions are present in a definite order from the apical to the basal ends of the cells. Tight junctions à adhering junctions à desmosome à gap junctions à hemidesmosome. The tight junction (zonula occludens) and adherent junction (zonula adherens) are typically close together and each forms a continuous band around the cell's apical end. As said, occluding junctions prevent passive flow of material between the cells but they are not very strong; therefore, an adhering junctions found immediately below them and serve to stabilize and strengthen these circular bands, help hold the layer of cells together. Desmosome form very strong attachment points, bound to intermediate filaments, which supplement the role of the zonulae adherens and play a major role to maintain the unity of an epithelium. Gap Junctions (connexons attach in the adjacent cell membranes), have little strength but serve as intercellular channels for flow of molecules. Basal Domain: 1. Basement membrane (term used in light microscopy) - mediates attachment of epithelial cells to underlying connective tissue. Serves to support, nourishment and bind the cell to neighboring structures. 2. Cell to ECM junctions: Focal adhesions – anchor actin filaments of the cytoskeleton into the basement membrane, by doing so, creating a dynamic link between the actin cytoskeleton and extracellular matrix proteins. Hemidesmosomes – anchor the intermediate filaments of the cytoskeleton into the basement membrane. 3. Basal cell membrane in folding: Increase the surface area of the basal cell domain, allowing for more transport proteins and channels to be present. Abundant in cells that participate in active transport of molecules, thus mitochondria are typically concentrated at this basal site to provide the energy requirements for active transport.  1. Mitochondria Elongated structure 0.5-1 µm in diameter and lengths up to 20µm. Double membrane-bound organelle found in most eukaryotic cells. Mitochondria have been described as "the powerhouse of the cell" – ATP production. In addition to supplying cellular energy, mitochondria are involved in other tasks: signaling cellular differentiation cell death, maintaining control of the cell cycle and cell growth Calcium storage, oxidation of fatty acids. Endosymbiotic theory - The fact that mitochondria have certain bacterial characteristics has led to the hypothesis that mitochondria originated from an ancestral aerobic prokaryote. The mitochondrial matrix contains a small circular chromosome of DNA, ribosomes (limited proteosynthesis), mRNA and transfer RNA. Protein synthesis occurs in mitochondria. The mitochondria genome codes only a fraction of mitochondrial proteins, these proteins have a small amino acid sequence that is a signal for their uptake across the mitochondrial membrane and they transported by translocases. Chemiosmotic process (theory) relates to the generation of ATP by the movement of hydrogen ions across a membrane during cellular respiration. Hydrogen ions (protons) will diffuse from an area of high proton concentration to an area of lower proton concentration, and an electrochemical concentration gradient of protons across a membrane can be harnessed to make ATP. The Mitochondria move through the cytoplasm along microtubules. The number of mitochondria in cell is related to the cell's energy needs. Both mitochondrial membranes contain a large number of proteins and have reduced fluidity. The outer membrane containing many transmembrane proteins called porins that form channels through which small molecules readily pass to enter the intermembrane space from the cytoplasm. The inner membrane is folded to form a series of long infoldings called cristae which greatly increase the membrane's surfaces area. The number of cristae also corresponds to the energy needs of the cell. Integral proteins in the impermeable lipid bi-layer make the inner membrane selectively permeable to the small molecules required for ATP synthesis. Matrix enzymes include those that oxidize pyruvate and fatty acids to form acetyl CoA and those of the citric acid cycle that oxidize acetyl CoA releasing Co2 as waste product, and small energy rich molecules which provide electrons for the electron transport chain (respiratory chain).  2. Rough and Smooth Endoplasmic Reticulum RER is prominent in cells specialized for protein secretion. It consists of saclike cisternae limited by membranes that are continuous with the outer membrane of the nuclear envelope. The surface of RER is abundant by protein manufacturing ribosomes, giving it a rough appearance. Although there is no continuous membrane between the endoplasmic reticulum and the Golgi apparatus, membrane-bound vesicles shuttle proteins between these two compartments. RER manufactures of lysosomal enzymes, secreted proteins, integral membrane proteins. SER (smooth endoplasmic reticulum) lack bound polyribosomes, and in most cells is less abundant that RER. The roles of the SER are: Synthesis of the various phospholipid molecules that constitute all cellular membranes (and direct to the membrane by vesicles or by phospholipid transfer proteins) SER occupies a large portion of the cytoplasm in cells that synthesize steroid hormones (cells of the adrenal cortex), and contains some of the enzymes required for steroid synthesis. Detoxification of drugs and toxins – abundant in liver cells, where it contains enzymes responsible for the oxidation, conjugation and methylation processes that degrade certain hormones and neutralize noxious substances. Another function of the SER is to store and release calcium in a controlled manner, which is part of the rapid response of cells to various external stimuli. This function is well developed in muscle cells, where the SER participates in the contraction process and assumes a specialized form called the sarcoplasmic reticulum. 3. Golgi Appartus: The Golgi apparatus packages proteins into membrane-bound vesicles inside the cell, before the vesicles are sent to their destination. It is of particular importance in processing proteins for secretion, containing a set of glycosylation enzymes that attach various sugar monomers to proteins as the proteins move through the apparatus. Structure: two main networks- the cis Golgi network (CGN) and the trans Golgi network (TGN). The CGN is a collection of fused, flattened membrane-enclosed disks known as cisternae. This collection of cisternae is broken down into cis, middle, and trans compartments. The TGN is the final cisternal structure, from which proteins are packaged into vesicles destined to lysosomes, secretory vesicles, or the cell surface. The Golgi apparatus is a major collection and dispatch station of protein products received from the endoplasmic reticulum (ER). Proteins synthesized in the ER are packaged into vesicles, which then fuse with the Golgi apparatus. These cargo proteins are modified and destined for secretion via exocytosis or for use in the cell. In this respect, the Golgi can be thought of as similar to a post office: it packages and labels items which it then sends to different parts of the cell or to the extracellular space. The Golgi apparatus is also involved in lipid transport and lysosome formation. 4. Lysosome: Lysosomes are cellular organelles that contain acid hydrolase enzymes that break down waste materials and cellular debris. They can be described as the stomach of the cell. Lysosomes digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. The membrane around a lysosome allows the digestive enzymes to work at the pH they require. At pH 4.8, the interior of the lysosomes is acidic compared to the slightly basic cytosol (pH 7.2). The lysosomal membrane protects the cytosol, and therefore the rest of the cell, from the degradative enzymes within the lysosome. The cell is additionally protected from any lysosomal acid hydrolases that drain into the cytosol, as these enzymes are pH-sensitive and do not function well or at all in the alkaline environment of the cytosol. This ensures that cytosolic molecules and organelles are not destroyed in case there is leakage of the hydrolytic enzymes from the lysosome. 5. Peroxisomes: are spherical membrane limited organelles approx. 0.5µm in diameter. They contain enzymes involved in lipid metabolism. They oxidize specific organic substrates by removing hydrogen atoms that are transferred to molecular oxygen. It abundant in the liver and kidney cells because of the ability of catalase enzyme (which found in all peroxisomes) to detoxification of ethanol. 6. Melanosomes – cell organelles containing pigment melanin derived from amino acid tyrosine, protection against UV radiation. The Nucleus contains a masterplan for all cell structures and activities encoded in the DNA of the chromosomes. It also contains the molecular machinery to replicate its DNA and to synthesize and process all types of RNA. Macromolecular transfer between the nuclear and cytoplasmic compartments is regulated, and the proteins needed for the nucleus to function imported from the cytoplasm. The nucleus has oval shape and usually found in the center of the cell. Its main components are the nuclear envelope, chromatin consisting of DNA and associated proteins, and a specialized region of chromatin called the nucleolus. Nuclear Envelope composed of two membrane layers around the nucleus, separated by a narrow (30-50nm) perinuclear space. The Endoplasmic reticulum is a direct continuation to the outer portion of the envelope, whereas the inner portion called nuclear lamina, fibrous proteins, mainly intermediate filament proteins called lamins, which helps to stabilize the nuclear envelope. At sites where the inner and outer membranes of the nuclear envelope fuse, the resulting lipid-free spaces contain nuclear pore complexes – nucleoporins – which regulates most bidirectional transport between the nucleus and the cytoplasm. Chromatin: Chromatin is the chromosomal material in a largely uncoiled state. Two types of chromatin can be distinguished under the EM or LM: Heterochromatin - tightly packed form of DNA, it has been associated with several functions, from gene regulation to the protection of chromosome integrity. Some of these roles can be attributed to the dense packing of DNA. Euchromatin is the lightly packed form of chromatin (DNA, RNA and protein) that is rich in gene concentration, and is often (but not always) under active transcription. Euchromatin comprises the most active portion of the genome within the cell nucleus. 92% of the human genome is euchromatic. The remainder is called heterochromatin. Chromatin is composed mainly of coiled strands of DNA bound to basic proteins called histones and to various non-histone proteins. The basic structural unit of chromatin and histones is the nucleosome which has a core of eight small histones (two copies of each histones - H2A H2B H3 H4), around which is wrapped DNA with about 150 base pairs. Each nucleosome also has a larger linker histone (H1) that binds both wrapped DNA and the surface of the core. DNA bound to nucleosomes is then folded further in the next order of chromatin organization (30nm fiber). Higher orders of chromatin coiling into microscopically visible stained structures, the chromosomes, which are especially important during the condensation of chromatin for mitosis and meiosis. 8 Histones with DNA wrapped = Nucleosome à Chromatin à Chromosome Important* - The chromatin pattern of a nucleus is a guide to the cell's activity. Generally cells with lightly stained nuclei are more active in protein synthesis then those with condensed, dark nuclei. In light stained nuclei with much euchromatin (=chromatin loose and not dense, ready for transcription for later protein synthesis, and thus light stained) and few heterochromatin, more DNA surface is available for transcription of RNA. Sex chromatin: The male cell has one X chromosome and one Y chromosome, whereas the woman has two X chromosomes. The X and Y chromosomes contain genes which determine whether an individual will develop as a female or a male. In humans most cells of the body, the somatic cells, contain 22 pairs of autosomes in addition to one pair of sex chromosome. Each of these 23 pairs of chromosomes contains one chromosome originally derived from the mother and one derived from the father. The members of each chromosomal pair are called homologous chromosomes, because although from different parents, they contain forms (alleles) of the same genes. Somatic cells are considered diploid (2N chromosome – 46) whereas sperm and mature oocytes considered haploid (N chromosome – 23) – each pair of chromosomes separated during meiosis. The number and characteristics of chromosomes encountered in an individual are known as the karyotype. Nucleolus present in the nuclei of cells active in protein synthesis. The intense basophilia of nucleoli is due not to heterochromatin, but to the presence of densely concentrated rRNA which is transcribed, processed, and complexed into ribosomal subunits in that nuclear region. The molecules of rRNA synthesized and modified in the nucleolus very quickly, associate with the many ribosomal proteins which are imported from the cytoplasm through the nuclear pore complexes. The newly organized small and large ribosomal subunits are then exported back to the cytoplasm through those same nuclear pores. There are few non-membranous cell organelles: Free Floating Ribosomes Cytoskeleton Centrosome 1. Free Floating Ribosomes: Ribosomes are about 20-30nm in size, composed of small and large subunits which compose of proteins and rRNA. Because of numerous phosphate groups of the rRNA, Ribosomes are basophilic, thus, sites in the cytoplasm rich in ribosomes stain by hematoxylin (or any basic dye). Ribosomes which producing proteins are gathered by mRNA into polyribosomes, and synthesized proteins used by the cell itself (hemoglobin, actin and myosin, mitochondrial enzymes). rRNA - Ribosomal RNA – responsible for functionality of the ribosome – translation of mRNA into proteins. The small subunit serves for connecting the components at one place – mRNA, tRNA and elongation factors. The large subunit helps to form peptide bonds. Small and Large subunits bind with each other via mRNA. The proteosynthesis is gene expression: Cellular organisms use messenger RNA (mRNA) to convey genetic information from DNA (using the letters G, U, A, and C to denote the nitrogenous bases guanine, uracil, adenine, and cytosine) that directs synthesis of specific proteins. Translation – Proteins are formed by 21 different amino acids. Sequence of three nucleotides – codon – determines one amino acid. mRNA which brings the codons from the DNA is decoded on ribosomes. 2. Cytoskeleton: The cytoskeleton can be referred to as a complex network of interlinking filaments and tubules that extend throughout the cytoplasm, from the nucleus to the plasma membrane. In eukaryotes, the cytoskeletal matrix is a dynamic structure composed of three main proteins, which are capable of rapid growth or disassembly dependent on the cell's requirements. There is a multitude of functions that the cytoskeleton can perform. v it gives the cell shape and mechanical resistance to deformation, so that through association with extracellular connective tissue and other cells it stabilizes entire tissues. v The cytoskeleton can also actively contract, thereby deforming the cell and the cell's environment and allowing cells to migrate. v involved in many cell signaling pathways, in the uptake of extracellular material (endocytosis), v segregates chromosomes during cellular division, v cytokinesis (the division of a mother cell into two daughter cells), v provides a scaffold to organize the contents of the cell in space and for intracellular transport (for example, the movement of vesicles and organelles within the cell) 3 types of cytoskeleton: Microfilaments, Microtubules, Intermediate filaments. 3. Centrosome - is an organelle that serves as the main microtubule organizing center (MTOC) of the animal cell as well as a regulator of cell-cycle progression. v Centrosomes are composed of two centrioles, each centriole composed of 9 triplets of microtubules. Centrosomes are associated with the nuclear membrane during prophase of the cell cycle. In mitosis the nuclear membrane breaks down and the centrosome nucleated microtubules can interact with the chromosomes to build the mitotic spindle. The centrosome replicates during the S phase of the cell cycle. During the prophase in the process of cell division called mitosis, the centrosomes migrate to opposite poles of the cell. v The centrosome is copied only once per cell cycle so that each daughter cell inherits one centrosome. v Basal Body - formed from a centriole and several additional protein structures. Centrioles, from which basal bodies are derived, act as anchoring sites for proteins that in turn anchor microtubules, and are known as the microtubule organizing center (MTOC). These microtubules provide structure and facilitate movement of vesicles and organelles. The cytoplasmic cytoskeleton is a complex network of microtubules, microfilaments (actin filaments) and intermediate filaments. These protein structures determine the shape of cells, play an important role in the movements of organelles and cytoplasmic vesicles, and also allow the movement of entire cells. Microtubules: They have an outer diameter of 24nm with a dense wall 5nm thick, and a hollow lumen. The protein subunit of a microtubule is a heterodimer composed of α and β tubulin molecules. The tubulin heterodimers polymerize to form microtubules. Polymerization of tubulins to form microtubules is directed by microtubule organizing centers (MTOC), which include centrosomes and the basal bodies of cilia. Microtubules are polarized structures and growth, via tubulin polymerization, occurs more rapidly at one end of existing microtubules. Microtubules show dynamic instability with tubulin polymerization and depolymerization dependent on concentrations of chemical substances as well as specific microtubule- associated proteins (MAPs). Cytoplasmic microtubules are stiff structures that play a significant role in the formation and maintenance of cell shape. Complex microtubule networks also participate in the intracellular transport of organelles and vesicles (chromosome movements by mitotic spindle, vesicle movements among different cell compartments etc..) Transport along microtubules is under the control of special MAPs (microtubule associated proteins) called motor proteins, which use ATP to move molecules and vesicles. Kinesins carry organelles away from the microtubule organizing centers, toward the plus end of microtubules. Cytoplasmic Dyneins carry vesicles in the opposite direction. Microtubules provide the basis for several complex cytoplasmic components including centrioles, basal bodies, cilia and flagella. Centrosome composed of pair of centrioles. Before cell division, during the S phase, each centrosome duplicates itself so that now each centrosome has two pairs of centrioles. During mitosis the centrosome divide into halves, which move to opposite poles of the cell and become organizing centers for the microtubules of the mitotic spindle. Cilia and flagella are motile processes, covered by cell membrane, with highly organized microtubule core. Cilia's main function is to sweep fluid along the surface of cell sheet. Flagella's function in sperm cell used for motility. Both cilia and flagella have the same structure, composed of 9 peripheral microtubular surrounding two central microtubules (9+2 pattern – axoneme) At the base of each cilia or flagella is a basal body, which controls the assembly of the axoneme. Microfilaments Microfilaments or actin filaments are the thinnest filaments of the cytoskeleton – 5-7nm, a structure found in the cytoplasm of eukaryotic cells. These linear polymers of actin subunits are flexible and relatively strong. Microfilaments are highly versatile, functioning in cytokinesis and changes in cell shape. Actin is usually found in cells as polymerized filaments of F actin mingled with free globular G actin subunits. Within cells, actin microfilaments (F-actin) can be organized in several forms. 1. In skeletal muscle, they assume a stable array integrated with thick 16nm myosin filament. 2. In most cells, microfilaments form a thin sheath or network just beneath the plasmalemma (cell membrane). These filaments are involved in all cell shape changes such as those during endocytosis, exocytosis and cell locomotion (‫)ניידות‬. 3. Microfilaments are intimately associated with several cytoplasmic organelles, vesicles, and granules and play a role in moving or shifting cytoplasmic components (cytoplasmic streaming) 4. Cytokinesis A large number of actin binding proteins with different activities have been demonstrated in various cells, and include: Actin motor proteins such as myosin (II, I, V) , which carry other molecules or vesicles along microfilaments Actin-capping proteins such as tropomyosin which bind the free end. Actin filament severing proteins such as gelsolin which break microfilaments into short pieces Actin bundling proteins such as fimbrin, villin and actinin, which crosslink microfilaments, Actin branching proteins such as formin, which produce branch points along a microfilament. Intermediate filaments Intermediate in size between the other two cytoskeletal components – diameter averaging 10-12nm. Intermediate filament proteins have been organized chemically and genetically into four major groups:  Keratins (cytokeratins) – diverse family of more than 20 proteins found in all epithelial cells and in the hard structures produced by epidermal cells. Keratins strengthen the tissue and provide protection against abrasion and water loss. Vimentin – most common intermediate filament protein in mesenchymal cells derived from the middle layer of the early embryo. Desmin (vimentin like) found in almost all muscle cells, Glial fibrillary acidic protein (GFAP) found in astrocytes, supporting cells of the CNS tissues. Neurofilaments Lamins – The nuclear lamina is a dense (~30 to 100 nm thick) fibrillar network inside the nucleus of most cells. It is composed of intermediate filaments and membrane associated proteins. Besides providing mechanical support, the nuclear lamina regulates important cellular events such as DNA replication and cell division. Additionally, it participates in chromatin organization and it anchors the nuclear pore complexes Cell junctions consist of multiprotein complexes that provide contact between neighboring cells or between a cell and the extracellular matrix. Intercellular Adhesion Several membrane associates structures contribute to adhesion and communication between cells and found numerous and prominent in epithelia – make them extremely cohesive. Those intercellular adhesions are especially marked in epithelial tissues that are subjected to pressure (e.g. – skin). Intercellular adhesion functions: Seals to prevent the flow of materials between the cells – Tight (=Occluding) junctions, made by oclodine proteins) Sites of adhesion (adhesive or anchoring junctions, made by cadherin proteins) Channels for communication between adjacent cells (gap junctions – made by connexin proteins). Such junctions are present in a definite order from the apical to the basal ends of the cells. Tight junctions à adhering junctions à desmosome à gap junctions à hemidesmosome. Cell junctions consist of multiprotein complexes that provide contact between neighboring cells or between a cell and the extracellular matrix.  Anchoring Junctions connect adjacent epithelial cells and serve as a tight bond between them. They do not affect passage of materials between cells and made by Cadherin protein. There are three types of anchoring junctions: 1. Desmosomes : serve as a point of attachment between cells, anchored to the intermediate filament inside the cell.. 2. Hemidesmosomes: In contact area between epithelial cells and the basal lamina, hemidesmosomes, 'half desmosome', bind the cell to the basal lamina. While in desmosomes the attachment contains cadherins, in hemidesmosomes it contains integrin, transmembrane proteins that are receptor sites for the extracellular macromolecules. 3. Adherent junction: provides firm adhesion of one cell to its neighbor, connects to the microfilament (actin filament) by catenin mediator protein. Tight junction: major component of zonula occludens are each cell's transmembrane proteins called claudins, which make tight contact across the intercellular space, creating a seal. They seal off body cavities and acts like a barrier. Gap junction: Channels for communication between adjacent cells, made by connexins proteins. Each connexons complex has hydrophilic pore about 1.5nm in diameter. Gap junction permit the rapid exchange between cells of molecules withsmall, under 1.5nm, diameters. The tight junction (zonula occludens) and adherent junction (zonula adherens) are typically close together and each forms a continuous band around the cell's apical end. As said, occluding junctions prevent passive flow of material between the cells but they are not very strong; therefore, an adhering junctions found immediately below them and serve to stabilize and strengthen these circular bands, help hold the layer of cells together. Desmosome form very strong attachment points, bound to intermediate filaments, which supplement the role of the zonulae adherens and play a major role to maintain the unity of an epithelium. Gap Junctions (connexons attach in the adjacent cell membranes), have little strength but serve as intercellular channels for flow of molecules. Cell division, mitosis, is a process in which a parent cell divides, and each of the daughter cells receives a chromosomal set identical to that of the parent cell. The period between mitoses is called interphase, during which the DNA is replicated and the nucleus appears as it is most commonly seen in histological preparations. P (Professor) M (Met) A (Ana) T (Talpen) Prophase: replicated chromatin condenses into rod-shaped bodies, the chromosomes, each consisting of duplicate sister chromatids. Outside the nucleus, the centrosomes with their centrioles separate and migrate to opposite poles of the cell. Simultaneously, the microtubules of the mitotic spindle appear between the two centrosomes and the nucleolus disappears as transcriptional activity there stops. Late in prophase, the nuclear envelope breaks down. Metaphase: The condensed chromosomes attach to microtubules of the mitotic spindle at the kinetochore. The kinetochore is the protein structure on chromatids where the spindle fibers attach during cell division to organize the chromosome in the Equatorial plate (center of the cell) and later pull sister chromatids apart toward opposite poles of the mitotic spindle. The kinetochore forms in eukaryotes, assembles on the centromere and links the chromosome to microtubule polymers from the mitotic spindle during mitosis and meiosis. Anaphase: The sister chromatids separate from each other and are slowly pulled at their kinetochores toward opposite spindle poles by kinesin motors moving along the microtubules. Telophase: The two sets of chromosomes are at the spindle poles and begin reverting to their non-condensed state. Microtubules of the spindle depolymerize and the nuclear envelope begins to reassemble around each set of daughter chromosomes. At the cytokinesis part of the Telophase, a contractile ring containing actin filaments develops in the peripheral cytoplasm. This ring constricts and divides the cytoplasm and its organelles to two daughter cells, each with one nucleus. Meiosis – Cell division that occurs only in the formation of sperm and egg cells. https://www.youtube.com/watch?v=16enC385R0w The cells produced are haploid (n, 23) with just one chromosome from each pair present in the rest of the body's somatic cell. Union of haploid egg and sperm cells at fertilization forms a new diploid cell – the zygote – which can develop into a new individual. During a greatly elongated prophase of the first meiotic division, Prophase 1, chromatin condenses as usual, but early in condensation, homologous chromosomes begin to come together physically in synapsis to form tetrads, and mixing of the genetic material both from maternal and paternal genes occurs. Prophase I is normally extended for 3 weeks during male gametogenesis in humans, whereas oocytes arrest in this meiotic phase from the time of their formation in the fetal ovary through the woman's reproductive maturity (puberty). When synapsis and crossing over are completed, the chromosomes continue to regular metaphase, anaphase and telophase, till the division of the cell into two cells. The anaphase I separation involves the homologous chromosomes that came together during synapsis. Each of the separated chromosomes still contains two chromatids held together at the centromere. Each of the two new cells now divides again, much more rapidly and without a new phase of DNA replication. In this division the chromatids now separate at the centromere and are pulled to the opposite poles as individual chromosomes. In summary, mitosis is a cell division that produces two diploid cells. Meiosis consists of two processes of cell division and produces four haploid cells. During meiotic crossing over, new combinations of gene alleles are produces and every haploid cell is genetically unique. Lacking synapsis and the opportunity for DNA recombination, mitosis yields two cells that are the same genetically. The Cell Cycle DNA replication occurs during interphase. The cyclic alternation between mitosis and interphase, known as the cell cycle, occurs in all tissues with cell turnover. The cell cycle has four distinct phases: G1 (gap between mitosis and DNA replication), S (synthesis of DNA), and G2 (gap between DNA duplication and the next mitosis). During G1 there is active synthesis of RNA and proteins which control the cell cycle, and the cell volume which reduced to one half by previous mitosis, grows to its previous size. During S phase the DNA and histones synthesizes, as well as duplication of centrosome. In G2 phase, proteins required for mitosis accumulate. As postmitotic cells begin to specialize and differentiate, cell cycle activities may be temporarily or permanently suspended and the cells are referred to as being in the G0 phase. Regulation of Cell Cycle: Cycling in post-mitotic cells (bypassing the G0 state) is triggered by protein signals from the extracellular environment called mitogens or growth factors. Nutrients and proteins required for DNA replication accumulate and when all is ready (at the restriction point) DNA synthesis begins. Entry or progression through each phase of the cycle is controlled by specific sets of proteins, the cyclins and cyclin-dependent kinases (CDKs). The progression through the cell cycle is also regulated by various signals which halt cycling under adverse conditions such as DNA damage (which leads to repair of the DNA). If the problem encountered at any checkpoint cannot be corrected while cycling is halted, tumor suppressor genes or proteins, such as P53, are activated and the cell's activity is redirected toward apoptosis. The gene encoding P53 is often mutated in cancer cells, thus reducing the cell's ability to detect and repair damaged DNA. Stem cell and tissue renewal In rapidly growing adult tissues and perhaps in other tissues there are slowly dividing populations of stem cells. Stem cells divide asymmetrically, producing one cell that remains as a stem cell and another which becomes committed to a differentiate pathway. Such cells have been termed progenitor cells, each of which cells eventually stops diving and becomes fully differentiated. Apoptosis Cell suicide or programmed cell death called Apoptosis. It is an highly regulated cellular activity produces small membrane enclosed apoptotic bodies, which quickly undergo phagocytosis by neighboring cells or macrophages specialized for debris ‫ פסולת‬removal. An example for apoptosis: Inside the thymus, T lymphocytes with the potential to react against self-antigens receive signals that activate the apoptotic program and they die before leaving the thymus. Apoptosis is an important means of eliminating cells whose survival is blocked by lack of nutrients, by damage caused by free radicals or radiation, or by the action of tumor suppressor proteins. The process of Apoptosis: Loss of mitochondrial function – release of Cytochrome C into the cytoplasm where it activates proteolytic enzymes called Caspases à Fragmentation of DNA: Endonucleases are activated which cleave DNA between nucleosomes into small fragments à Shrinkage of nuclear and cell volumes à Formation and phagocytic removal of the apoptotic bodies. Preparation of tissues for study Histology is the study of the tissues of the body and how these tissues are arranged to constitute organs. Tissues are made of two interacting components: cells and ECM. The ECM consists of many kinds of molecules, most of which are highly organized and form complex structures, such as collagen fibrils and basement membranes. The main functions of the ECM are furnishing mechanical support for the cells, transport nutrients to the cells, carry away catabolites and secretory products. Cells are producing the ECM, and both reacts together to stimuli and inhibitors. Each of the fundamental tissues is formed by several types of cells and typically by specific associations of cells and ECM. These characteristic facilitate the recognition of the many subtypes of tissues. Most organs are formed by an orderly combination of several tissues, except the CNS which formed almost solely by nervous tissue. To study tissues, we are making use of light microscope – under the light microscope tissues are examined via a light beam, that transmitted through the issue. Tissues and organs are usually too thick for light to pass through them, therefore they must by sectioned to obtain thin sections. In order to preserve the same structure and molecular composition as the tissue has in the body, we are making preparation of the tissue being studying. Taking of human samples – Samples can be obtained from living organisms – Biopsy – or after death – Necropsy. It must be obtained by quick and safe technique, painless for the patient. It can be obtained by few ways: 1. Excision ‫ –כריתה‬Cutting Out – samples are cut out by sharp device (Scalpel) – Skin. 2. Needle biopsy (puncture) is used for taking samples from solid organs (lymph node, liver, kidney, bone marrow, brain, thyroid, breast, skeletal muscle etc), Fine Needle biopsy – for superficial lumps just under the skin. 3. Curettage (‫ – )גרידה‬endometrial biopsy – mucosa of the uterus 4. Endoscopic using an endoscope, taking of samples by means of special tube , the tube gets in to hollow organs and cavities in the body (stomach, intestine, bronchus, urinary bladder, heart) 5. Exfoliative cytology – cells are examined in smears –uterus All samples must be labelled – on the vessel (name, year of birth, birth certificate number), and complete dispatch note of material for histological examination (patient identification, probable diagnosis, previous therapy, data for insurance company, name and address of physician). Important Values: Sample size < 1cm3 (in em