Histology PDF

# Histology Histology is the study of the microscopic structure of normal tissues. It allows us to understand how tissues are built to carry out their functions - Tissues are discrete, organized collections of cells having similar morphological characteristics. (epithelial, muscular, nervous, conn...

# Histology Histology is the study of the microscopic structure of normal tissues. It allows us to understand how tissues are built to carry out their functions - Tissues are discrete, organized collections of cells having similar morphological characteristics. (epithelial, muscular, nervous, connective) - Size of a cell: 10-30 micrometres; to observe them we use an instrument that improves the power of our sight: microscope. We need two sets of lenses to study the structure of a cell ## Light-transmission microscopy - It uses visible light that has to go through a sample. - Objective lenses enlarge and project the illuminated image of the object towards the eyepiece. Objectives used in histology include X4 for observing a area of the tissue at low magnification; X10 for medium magnification of a smaller field; and X40 for high magnification of more detailed areas. - The two eyepieces or oculars magnify this image another X10 and project it to the viewer, yielding a total magnification of X40, X100, or X400 - The sample must be transparent; a fluid like blood is transparent and we can put it on top of the coverglass. A piece of tissue is not transparent and we have to prepare it by creating a section of it. - Consequence of creating sections of a tissue: From a 3-dimensional image we obtain a 2-dimensional image which can have different forms depending on where we cut the tissue. ## Preparation of a tissue: 1. **Fixation:** Small pieces of tissue are placed in special solutions of chemicals that cross-link proteins and inactivate degradative enzymes, which preserve cell and tissue structure. 2. **Dehydration:** The tissue is transferred through a series of increasingly concentrated alcohol solutions: from 65% to 80%, ending in 100%, which removes all water. 3. **Clearing:** Alcohol is removed in organic solvents in which both alcohol and paraffin are miscible. It makes the sample more transparent 4. **Infiltration:** The tissue is then placed in melted paraffin (a wax) until it becomes completely infiltrated with this substance. Paraffin derives from oil and it melts very easily; the tissue dipped into paraffin becomes solid, it is more easy to cut a piece of wax than a piece of tissue 5. **Embedding:** The paraffin- infiltrated tissue is placed in a small mold with melted paraffin and allowed to harden. 6. **Trimming:** The resulting paraffin block is trimmed to expose the tissue for sectioning (slicing) on a microtome. Then the cut pieces are put in water in order not to get them wrinkled. Instead of paraffin we can freeze the section with liquid nitrogen and then cut the section. It freezes quickly because the slower the water freezes the bigger the crystals are, they don't permit to see very well the tissue. ## Staining Sections must be stained in order to get details of the image, stains make the structures visible and highlight the presence of specific structures or substances. - Using a specific stain we stain a specific structure: - **H&E** is the most useful stain for the examination of biological material. Cell nuclei stain blue while most components of the cell cytoplasm stain pink/red. - The nucleus, a basophilic structure, is stained better with basic substances because they're made of DNA (an acid, deoxyribose, that binds with bases). - Cytoplasm is stained better with eosin because it is an acidophilic/eosinophilic structure. When there is a lot of RNA the cytoplasm can become basophilic too. Based on the stain we use we can get different informations. - **PAS cell staining** is used with carbohydrates, either alone (e.g. glycogen) or combined with other molecules, such as proteins (e.g. glycoproteins), which are stained magenta. It can be used to delineate basement membranes and some neutral mucins secreted by secretory epithelial cells - **May-Grünwald-Giemsa method** is confined to the examination of blood and bone marrow cells. ## Immunochemistry - The use of antibodies, proteins produced by B-lymphocytes that can bind with target molecule: - **Antibody** - Paratope - Epitope - **Antigen** - Paratope - Epitope - **Antibody** - Variable domain, it attaches the antigen in the epitope - Recognized by the antibody ## More techniques - Observing untainted sample, when we have subcultures we cannot stain cells because we would kill them, and to culture them we need to observe them without killing them: We use a different microscopy: - **Phase-contrast microscopy:** Two small waves of light can become a big one if they encounter and have the same phase (they must move at the same time). If two waves are out of phase they can still be summed up but they will cancel each other. In this microscopy the light goes through the sample, some of it will hit the sample causing the waves to slow down thus the waves will go out of phase. The filters of the microscopy will magnify this out of phase thing showing the shadow of the cells. - **Confocal microscope:** It focuses on one detail. It has a filter with a hole that allows only one specific light to cross it thus the microscope analyses only one precise slice of the sample. - **Stereomicroscope:** Perfect to observe a 3D non transparent object. The light source comes from the top and highlights the sample. It has limits on magnification: 0,2 microns. - **Super resolution microscopy: Storm:** It takes the sample and marks the target then excites briefly the fluorescent probe randomly. The PC will take all the dots highlighted and reconstruct the image. - **Super resolution microscopy: Sted:** It excites the probe with an excitation laser and a second laser will repress the fluorescent molecule so the excited area becomes smaller and we get a higher resolution, a very precise restricted signal. - **Electron microscopy:** It uses beam of electrons instead of visible light. - **TEM:** Electrons will cross the sample and create an image, we have magnetic lenses and to operate them we must have 2d dimensional sections. - **SEM:** It's the equivalent of the stereomicroscope, electrons hit the sample and are reflected off it. Since the surface of the sample must reflect electrons, it must be a metal sample so that it's metalized and electrons bounce off the sample. They are captured to create a 3D- image wihtout details of what's insde of the structre. Not used in histology. ## The Cell: The smallest unit that composes tissues. ### The plasma membrane: - An envelope that surrounds cells made of: - **Phospholipid:** Molecule composed by one polar head and two hydrophobic fatty acid (long chains of carbon). In water they assemble turning the tails of acid inwards and the head outwards forming the double layer of the membrane. One fatty acid is unsaturated: Has a double bond which creates a kink on the chain that prevents phospholipidics to bond together too tightly thus the membrane is less dense; the kink also effects the shape of the membrane. - **Cholesterol:** In between phospholipidics, it modulates the density of the membrane which is fluid and flexible. The membrane is a fluid mosaic. - **Proteins:** They have recognition and signaling fuctions. Some face the membrane outwards, some inwards, some cross it; the position of the protein is determined by its structure. - **Oligosaccharide chains:** Covalently linked to many of the phospholipids and proteins. Possible presence of sugar chains of a glycolipid - **Lipid rafts:** If there's many of them or many proteins the membrane will be stable - **Glycocalix:** It's a sugar layer that surrounds the membrane, made of sugar bonded with proteins and phospholipidics. - **Proteoglycon:** Made of glycosaminoglycans bonded with proteins, they have a similar structure to sugar but are negatively charged and this attracts a lot of water with which it interacts to form a gel. - **Hyaluronic acid:** It retains water and creates volume, it creates a jelly layer on top of the cells. The membrane is selectively permeable, it is a selective barrier regulating the passage of materials into and out of the cell and facilitate the transport of specific molecules. It keeps constant the ion content of cytoplasm. Gases ($CO_2$, $O_2$), hydrophobic molecules (Benzen) and small polar molecules ($H_2O$, Ethanol) can diffuse freely; while large polar molecules (Glucose) and charged molecules (Ions, amino acids) cannot pass. Cells communicate through molecules that binds a receptor that sends the message to the cell causing it generates a signal. If the molecule can't pass through the membrane the receptor will be outside the membrane, if the molecule pass through it the receptor will be in the membrane. Cells create an electric potential between inside and outside of the cell, neurons communicate through signals made possible by the potential. The potential is created by pumps that pump K inside and Na outside through energy. ### Membrane transport - **Simple diffusion:** Lipophilic and some small, uncharged molecules can cross membranes by simple diffusion - **Channel/ facilitate diffusion:** The membrane is selectively permeable: Some molecules need channels to go through it, channels made of proteins that allow the diffusion of certain chemical species. It is still a passive transport because it diffuses from a place where there's more of it to a place where there's less. - **Carrier/pump:** They are transmembrane proteins that bind small molecules, they use energy because they work against gradient (active transport). Molecules can endure some changes while passing through a pump. ex: Na/K pump: It maintains the gradient and it transports two species in opposite direction (antiport); simport when the species are pushed in the same direction. - **Transport by vesicles:** - **Endocytosis:** Brings something from outside to the inside. - **Phagocytosis:** Involves the extension from the cell of the membrane which engulf particles such as bacteria, and then internalize this material into a cytoplasmic vacuole or phagosome. - **Pinocytosis:** The cell membrane invaginates (dimples inward) to create a pit containing a drop of extracellular fluid. The pit pinches off inside the cell when the cell membrane fuses and forms a pinocytotic vesicle containing the fluid. - **Receptor-mediated endocytosis:** Includes membrane proteins called receptors that bind specific molecules (ligands). When many receptors are bound by their ligands, they aggregate in 1 membrane region, which then invaginates to create a vesicle containing the receptors and the bound ligands. - **Exocytosis:** Process in which the contents of a vesicle is released outside of the cell through fusion of the vesicle membrane with the cell membrane. It releases molecular species outside of the cell and adds a new piece of the membrane in place. If the cell needs to express new receptors, it will send the vesicle to the membrane, the vesicle will fuse to the membrane and there will be a piece of new membrane in place with the receptor in it. - There are special vesicles known as **coated vesicles**. They are vesicles that are coated by special proteins, one of the best examples is **clathrin** for its peculiar shape that forms a canister that surrounds the vesicles. Coated vesicles are activated when the ligand binds the receptor. Binding the ligand to its receptor activates the assembling of clathrin molecules into a canister; during this process, clathrin pulls down the membrane and creates the vesicle, which is completely coated by the clathrin. ### Cell signaling through vesicles - A cell expresses receptors on its surface. These receptors bind to their own ligands which activate the signaling inside of the cell. What often happens is that the binding of the receptor to the ligand activates the endocytosis of the ligand-receptor complex. So, a vesicle is formed, it is taken inside of the cell and keeps emitting signals, because the ligand inside of the vesicle keeps binding to the receptor which acts like a beacon of signals. This doesn't happen all the time, but it can happen that vesicles are also areas or sites of increased signaling. - **Exosomes or extracellular vesicles:** Are special vesicles that the cells can create and can be released outside of the cell. Normally, in exocytosis there is one vesicle which fuses with the membrane and releases molecules outside. Instead, what is released by the exosome, which is a big vesicle known as multivesicular body (MVB), is not molecules, but more vesicles each with their own content. As research is progressing, it turns out that these exosomes that release vesicles can have very rich contents; they can contain nucleic acids, ligands, amino acids and more. So the functions of exosomes are very rich, like angiogenesis and wound healing or apoptosis. ## The Cytoplasm - The substance that is contained within the membrane is called cytoplasm. It's a gel which contains water and different proteins. ## The Cytoskeleton - The membrane contains cytoplasm, a jelly substance containing water and proteins, and the cytoskeleton. - 3 main components: - **Microtubules:** Hollow tubes formed by tubulin, a protein. Cell can add or delete tubulin. They have a positive end, where the polarization occurs (they are assembled), and a negative one where depolarization occurs (disassembled). They are linear and denser to a certain area closer to the nucleus (they converge to the nucleus). They form mitotic spindle. - 3 segments of microtubules stuck together form a triplet, 9 triplets form a centriole. 2 centrioles, one perpendicular to the other, surrounded by microtubules attached in the center from a centrosome. - Quiescent cell have a structure called **Primary Cilium**, a finger like formation that stands out from the membrane. It has a cytoskeleton made of tubulin, formed by 9 doublets of microtubules based on top of a centriole, so the centriole acts as the anchoring system of the primary cilium. Area of intense signaling, it captures the signals of the cell. - **Microfilaments:** 2 strains of actin together make a filament with a - end and a + end. The filaments are more straight because they are under tension (myosin pulls on actin) and attached to the cytoskeleton. - Focal adhesion are formed by different proteins, they attach the microfilaments to the extracellular environment (can bind to myosine). When cells migrates into a wound to heal it, they can sense how stiffed the substrate is because they can contract thanks to microfilaments or myosine filaments; the cell is under tension. Integrins are transmembrane components of focal adhesion, they work in pairs and can bind to the substrate. - **Intermediate filaments:** Can be made of more than 1 protein. Provides structural support for the cell, a net web of filaments that keeps all the structures (nucleus, organelles) in place. - Vesicles are anchored to the cytoskeleton through a mobile protein that transport the vesicles along it. The proteins change conformation and move along the cytoskeleton. 3 types: **myosin**, **kinesin** (towards the positive end), **dynein** (towards the negative end). - **Geometry, stiffness of the substrate and shape of cells effect how they will differentiate:** - Cell shape affects their function, when cells attach to the substrate they stat to stretch out and acquire a specific shape (star, flower, round). Flower shape's cells more easily become adipocytes while star shape's cells become osteoblast. We can control cell differentiation by forcing cells to grow with a given shape. - The stiffness of the matrix also express how cells will differentiate. Cells use tension to read the surrounding environment, blebbistatin blocks the contraction of cells thus they can't understand if the matrix is soft or rigid. Cells are cultured on different stiff substrates (some are more rigid, some more soft); the stiffness of the substrate incit the formation of different types of cells: When growing on softer material they express genes typical of brain cells, while stiffer substrate is typical of bone cells. - Tissue engineering regenerates lost tissues (ex. hole in a tissue) when trauma or a tumor occurs. 3 approaches: 1. Using a matrix: Helps the tissue to heal and regenerate. Considered a medical device because it's not required to be tested as much as drugs do + less money involved. 2. Drugs: Helps the differentiation of tissues 3. Implant cells where you need it; It's considered a drug which can cost even 3-4 billions of dollars. ## Organelles ### Ribosome - First studied by ultracentrifugation, a method used to separate the different parts of a cell - The prokaryotic ribosome is made of 2 subunits: The 50S subunit and the 30S subunit. - The eukaryotic ribosome is made of 2 subunits: The 60S subunit and the 40S subunit. The subunits are made of proteins and rRNA, the most common RNA that has an enzymatic activity during translation. The subunits are separated in the cytoplasm and they come together for the translation of the mRNA into proteins. They're assembled in the nucleolus and then exported into the cytoplasm. The subunits hold the mRNA filament because the ribosome moves along the mRNA to read it. - Ribosomes have 3 sites: - A: aminoacidic - P: peptido - E: exit ### Endoplasmic Reticulum - Made of membranes made of phospholipids (not the same as the plasma membrane). It's a hollow organ thus it has a lumen and it has cisternae, recipient that collect liquids - **RER**: Rough because it has ribosomes and the shape of the cisternae is flat. Is located around the nucleus. If stained with H&E the cytoplasm is pink because it's eosinophilic and thus rich in ribosomes. The RER continues into the SER. - **SER**: Smooth because it doesn't have ribosomes and the shape of the cisternae is rounder. It has different functions, metabolic functions and cooperates with other organelles of the cell: - With mitochondria to synthesize phospholipid - With peroxisomes to synthesize cholesterol and generate peroxisome - Lipid droplet biogenesis - Some enzymes allow detoxification of harmful exogenous molecules such as alcohol and other drugs; ex. the P450 enzyme which takes one compound XH and attaches an OH group to it to make the compound more easily soluble in water so it can be removed (important role in liver cells). - SER vesicles are responsible for sequestration and controlled release of $Ca^{2+}$, part of the response of cells to stimuli. This function is well developed in striated muscle cells, where the SER has an important role in the contraction process and assumes a specialized form called the sarcoplasmic reticulum. - ER lumen hosts lipid droplets inside of the cell that are accumulated between the two layers of the membrane. - Cells that make few or no proteins for secretion have little RER, with all polyribosomes free in the cytoplasm. ex. erythroblast, precursors of red blood cells which are basofilic because they have many ribosomes; they produce hemoglobin and have mostly free ribosomes because hemoglobin stays inside of the cell. - Cells that synthesize, segregate, and store proteins in specific secretory granules or vesicles always have RER, a Golgi apparatus, and a supply of granules containing the proteins ready to be secreted. ex. eosinophilic leucocyte which produces antibodies, protein that will be released outside of the cell. It has a basophilic cytoplasm. - Epithelial cells specialized for secretion have distinct polarity, with RER abundant at their basal ends and mature secretory granules at the apical poles undergoing exocytosis into an enclosed extracellular compartment, the lumen of a gland. ex. pancreatic acinar cell, produces substances needed in the intestine thus substances that must be released outside of the pancreas. It's basophilic, made of ribosomes attached to RER ### Golgi Apparatus - Made of flat cisternae that can be concave, the concavity faces away the nucleus. It doesn't have ribosomes. - Each Golgi stack has three levels of cisternae: - The cis-face, the receiving end. Closest to RER, convex. - The medial face - The trans-face, the releasing end, concave - RER content is packed in vesicles and released and transported to the cis/face of Golgi. Cisternae of Golgi are separated but they communicate with a vesicle tracking, a circular traffic because empty vesicles go back to RER. Enzymes inside the Golgi are separated because they have different functions. Golgi does post translational modification to the proteins. - There are two additional compartments of interest: - Located between the RER and the cis-face of the Golgi apparatus is an intermediate vesicle, known as compartment of the vesicular-tubular clusters (VTCs). - The second compartment, known as the trans Golgi network (TGN), is at the distal side of the Golgi. - 3 different destinations of the vesicles: 1. Lysosomes 2. Cell membrane, proteins are expressed in the membrane 3. Exocytosis, the vesicle fuses with the membrane to release proteins outside ### Lysosomes - Vesicles surrounded by membrane and rich in receptors produced by Golgi, they contain hydrolytic and digestive enzymes that break down proteins. They need a low pH to be active. - RER synthesizes proteins - Golgi modifies and packs them in vesicles - Lysosomes are used to eat and digest bacteria or to remove part of the membrane of the cell (ex. receptors) - some cells can secrete the enzymes of lysosomes ### Mitochondria - Functions: - ATP production, molecule where energy is stored for biochemical reactions - Metabolism of amino acids and lipids, with ER - Act as a sensor for the health of the cell and able to trigger cell death through apoptosis - The outer membrane is smooth and contains many transmembrane proteins called porins that form channels through which small molecules such as pyruvate and other metabolites pass. The inner membrane has many sharp folds called crests that increase its surface area. The matrix is a gel with a high concentration of enzymes. - **Endosymbiotic theory:** Origins of mitochondria: - States that the bacteria, billions of years ago, joined the development of the cell and created a symbiosis. As a result, organelles such as mitochondria and chloroplast were ascendents of bacterial cells. Proof: Mitochondria have their own DNA, which is basically a round molecule of DNA, just like bacteria + they are capable of dividing, just like cells. Sperm cells have mitochondria but when they fuse to create a zygote they leave it out: We have the mitochondrial DNA of our mother - **Metabolites such as pyruvate and fatty acids enter mitochondria via membrane porins and are converted to acetyl CoA by matrix enzymes of the Krebs cycle, yielding some ATP and NADH, a major source of electrons for the electron-transport chain.** - **The movement of electrons through the protein complexes of the inner membrane's electron-transport system is accompanied by the movement of protons ($H^+$) from the matrix into the intermembranous space, producing an electrochemical gradient across the membrane.** - **Other membrane-associated proteins make up the ATP synthase systems, each of which forms a complex that projects from the matrix side of the inner membrane. ATP synthase work like an inverted pump, a gradient let $H^+$ flow back in and the conformation of the enzyme changes and the enzyme phosphorylate ADP back into ATP** - **A channel in this enzyme complex allows proton to flow down the electrochemical gradient and across the membrane back into the matrix. The flow of protons causes spinning of specific polypeptides in the globular ATP synthase complex, converting the energy of proton flow into mechanical energy, which other subunit proteins store in the new phosphate bond of ATP.** ### Peroxisomes - Small vesicles found in cytoplasm, inside there are crystal cord made of proteins and oxidation reactions occur - Peroxisomes have organelles that contain more than 40 oxidative enzymes, they are present in almost all animal cells and function in the catabolism of long-chained fatty acids (beta oxidation), forming acetyl coenzyme A (CoA), and form hydrogen peroxide ($H_2O_2$) by combining hydrogen from the fatty acid with molecular oxygen. ### Cellular Inclusion - When lipids are accumulated, the membrane swells up to the point where it creates its own vesicles, and these vesicles are called lipid droplets. They have only one layer of membrane and are abundant in cells of the adrenal cortex. They are not just ways to store energy for long periods, but they are important as they cooperate with mitochondria, lysosomes and peroxisomes. Lipids droplets are useful for every function of the cell, or reaction and they are indeed very active transport systems that carry lipids around where they are needed - **Glycogen granules:** Aggregates of the carbohydrate polymer in which glucose is stored (glycogen) and they lack membrane. Glycogen is a ready source of energy, and such granules are often abundant in cells with high metabolic activity. - **Pigment deposits (PD)** occur in many cell types and contain complex substances, such as melanin granules which serve to protect cell nuclei from damage to DNA caused by light. Many cells contain pigmented deposits of hemosiderin granules containing the protein ferritin, which forms a storage complex for iron. ## When a cell dies 1. **Necrosis:** The cell swells and the plasma membrane raptures causing the release of molecules that used to be inside of cells, called DAMP. The cellular and nucleus lysis causes inflammation 2. **Apoptosis:** Programmed cell death. A blebbing is formed and then the cell breaks down in small pieces (apoptotic bodies) that then are removed / phagocytosed without inflammation. One of the controlling mechanisms of apoptosis is the release of a molecule called cytochrome C that creates enzymes as Caspase that break down the cell, executing the process of apoptosis ## The Nucleus - Not all cells have nucleus. - Surrounded by nuclear envelope, a double membrane, each one has a double layer of phospholipids. It's supported by the cytoskeleton. The space in between the two membranes is connected to the RER and called the perinuclear cisterna, ribosomes are on top of the nuclear envelope. - There are pores in the envelope that allow the passage of materials across the nuclear membrane. The bidirectional traffic between the nucleus and the cytoplasm is mediated by a group of proteins containing nuclear localization signals (NLSs) known as **importins** and nuclear export signals (NESs), **exportin** - Exportins transport macromolecules (e.g., RNA) from the nucleus into the cytoplasm. - Importins transport cargo (e.g., protein subunits of ribosomes) from the cytoplasm into the nucleus. - Has a nucleolus in which ribosomes are assembled. - The nuclear structure is anchored to the cytoskeleton - Functions: - Houses the genetic material which regulates cellular structure and directs all cellular activities - Produces ribosomal subunits in nucleolus and exports them into the cytoplasm for assembly into ribosomes. - **Chromatin:** Is inside the nucleus, made of coiled DNA and proteins, called histones. There are several histones (H2A, H2B, H3, H4) and they form a spherical complex that coils the DNA tightly around them, known as nucleosomes. It can be classified as **euchromatin** and **heterochromatin**: - Euchromatin is lighter, it's the active form of chromatin thus it's transcribed - Heterochromatin is darker, is not transcribed and remains more highly condensed. Located mostly at the periphery of the nucleus - **Nucleosome:** Is a structure that produces the initial organization of free double stranded DNA into chromatin. Each nucleosome has an octomeric core complex made up of four types of histones, two copies each of H2A, H2B, H3, and H4. Around this core is wound DNA approximately 150 base pairs in length. One H1 histone is located outside the DNA on the surface of each nucleosome and it helps to hold the DNA wrapped around the histones. Nucleosomes are very dynamic structures, with H1 loosening and DNA unwrapping at least once every second to allow other proteins, including transcription factors and enzymes, access to the DNA. - There are regulatory sequences in the gene that control when the gene is transcribed, for instance by binding it to a transcription factor. And that is one way of controlling gene expression. Controlling the way that nucleosomes are packed together is also an epigenetic mechanism. **Histone acetylation** increases gene expression while **methylation** represses gene expression. ## Chromosomes - It is not possible to see chromosomes in cells when they are not condensed - Chromatin is segmented into chromosomes - During cellular division, chromosomes are condensed so they can split from the mother cell to the daughter cells - They have two "arms", known as chromatids and during mitosis, each chromosome is composed of two sister chromatids joined together at a central site called centromere (composed of proteins that form the kinetochore). - Chromosomes can be divided into **sex chromosomes** and **autosomes** - **Sex chromosomes:** XX or XY normally (1 pair of these in the karyotype) - **Autosomes:** There are 22 pairs alike - To indicate the amount of DNA in a cell, we use the letter "n" for the set of chromosomes. So, if we have one set of chromosomes, this condition is "n": **haploid**. If there are two sets of chromosomes, this condition is 2n: **diploid**. By stretching the nucleus we pull and rearrange the position of chromosomes and we possibly damage their expression because closer area of DNA can share the same machinery for translation ### The Karyotype - The way of displaying chromosomes uniquely - A normal and complete karyotype has 46 equal chromosomes, 23 pairs, one set that comes from the mother and the other one from the father - FISH (fluorescencein situ hybridisation) are special probes that binds with sequences of DNA so that you can recognize them. - The Barr body is one of the two X chromosomes, therefore, only present in female organisms. One of these two X chromosomes is silenced: very condensed and "put away". In some granulocytes, it can be seen, but not always - Telomeres are the terminal ends of the chromosomes, they are associated with the life spam of a cell. Normal cells do not divide forever. They can divide 10x and then their rate of division gets slower and rarer, as the cells become old. One of the signals that tell the cell that it is becoming old is that in every cell division, the telomere becomes shorter (like a candle melting). The exception are the tumor cells that are considered "immortal" ## Nucleolus - A spherical structure found inside the cell's nucleus. Its primary function is to produce and assemble the cell's ribosomes and export them to the cytoplasm - It consists of three major components: - A fibrillar center (corresponding to chromatin containing repeated rRNA genes and the presence of RNA polymerase I (Pol I) and signal recognition particle [SRP] RNA) - A dense fibrillar component (where nascent rRNA is present and undergoing some of its processing). - A granular component (where the assembly of ribosomal subunits, containing 18S rRNA [small subunit] and 28S rRNA [large subunit], is completed). Nucleostemin, a protein unrelated to ribosomal biogenesis, coexists with the granular components ## Cell Cycle - **Cell Cycle:** Divided into two major events: - **Mitosis:** The short period of time during which the cell divides its nucleus and cytoplasm, giving rise to two daughter cells - **Interphase:** A longer period during which the cell increases its size and content and replicates its genetic material - Cells that have let the cell cycle are said to be in a resting stage, the **G0 phase**, or the **stable phase** - Those cells that become highly differentiated after the last mitotic event may cease to undergo mitosis either for a very long time (e.g., neurons and skeletal muscle cells are said to be in a terminally differentiated G0 state) or for a short period of time (e.g., certain hematopoietic stem cells) and return to the cell cycle later - Not every cell divides at the same rate/speed. There are some cells that keep dividing. For example, epidermis is a tissue where cells continuously keep dividing. As cells get older, they lose this capacity of dividing. - **Somatic cells use mitosis** - **Germ cells use meiosis** - **Interphase, subdivided into three phases:** - **G1 (gap) phase:** When the synthesis of macromolecules essential for DNA duplication begins - **S (synthetic) phase:** When the DNA is duplicated, and it synthesizes the sister chromatids - **G2 phase:** When the cell undergoes preparations for mitosis ### Mitosis - **Prophase** - The nucleolus disappears and the replicated chromatin condenses into discrete threadlike chromosomes, each consisting of duplicate sister chromatids joined at the centromere - The two centrosomes with their now-duplicated centrioles separate and migrate to opposite poles of the cell and organize the microtubules of the mitotic spindle. - Late in prophase, laminas and inner nuclear membrane are phosphorylated, causing the nuclear lamina and nuclear pore complexes to disassemble and disperse in cytoplasmic membrane vesicles - **Metaphase** - Chromosomes condense further, and large protein complexes called kinetochores, located at the centromere DNA region of each chromosome, attach to the mitotic spindle - The cell is now more spherical, and microtubules move the chromosomes into alignment at the equatorial plate - **Anaphase** - Sister chromatids separate and move toward opposite spindle poles by a combination of microtubule motor proteins and dynamic changes in the lengths of the microtubules as the spindle poles move farther apart - **Telophase** - The two sets of chromosomes are at the spindle poles and begin to revert to their uncondensed state - The microtubules of the spindle depolymerize, and the nuclear envelope begins to reassemble around each set of daughter chromosomes - **Cytokinesis** - A belt -like contractile ring of actin filaments associated with myosin develops in the cytoplasm at the cell's equator. During cytokinesis the constriction of this ring produces a cleavage furrow and progresses until the cytoplasm and its organelles are divided into two daughter cells, each with one nucleus - Mitosis is usually symmetric: The DNA is divided equally between the two daughter cells. Sometimes, the cell division is asymmetrical, although symmetry is not in the way of DNA, but in the way the cell behaves after the cell division. Which means that stem cells usually divide in an asymmetrical way. One cell maintains stemness and the other one differentiates into another type of cell. What causes the asymmetrical division of stem cells is the extracellular environment. ## Stem Cells: - Undifferentiated or partially differentiated cells that can change into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type of cell in a cell lineage. They are found in both embryonic and adult organisms, but they have slightly different properties in each. They are usually distinguished from progenitor cells, which cannot divide indefinitely, and precursor or blast cells, which are usually committed to differentiating into one cell type. ## Tissues: - Are not just clusters of cells, they're organized structures made of cells and extracellular matrix. Very few tissues have no cell because they probably lost them, ex. enamel of teeth used to have cells during development. Each tissue is an assemblage of similarly specialized cells united in performing function. There are four tissue types: - **Epithelial:** Has aggregated polyhedral cells and a small amount of extracellular matrix. Functions: Lining on surface or body cavities, glandular secretion - **Connective:** Has several types of fixed and wandering cells inside of an abundant amount of matrix. Functions: Support and protection of tissues or organs. - **Muscular:** Has contractile cells and a moderate amount of matrix. Functions: Contraction, body movements. - **Nervous:** Has elongated cells and a small amount of matrix. Function: Transmission of nerve impulses. ## Epithelial Tissue: - Made of cells tightly connected and close to one another. - There are 2 types of epithelia that have different functions: 1. **Covering/lining epithelia:** Covers and protects a tissue 2. **Secretory/glandular epithelia:** Forms glands that secrete substances - Epithelia cells have three sides; they're oriented/polarized. - **The apical domain** is exposed to the lumen or external environment and displays apical differentiations. - **The lateral domain** faces neighboring epithelial cells linked to each other by cell adhesion molecules and junctional complexes. - **The basal domain** is in contact with a basement membrane that separates the epithelium from underlying connective tissue that provides mechanical stability and supplies nutrients and oxygen through blood vessels. Connective cells don't have sides and float in the extracellular matrix. - **The basement membrane** is a thin structure made of proteins that anchor the epithelia to connective tissue and keep the cells in place through adhesion structures, they're also a signaling structure because epithelium cells sitting on the basement membrane know if they're touched by it so they can change their activity. - **The basement membrane is composed of:** - A dense basal lamina. Within basal lamina there's laminin, a glycoprotein formed by IV collagen. Cells use it to connect to the extracellular environment. The lamina is also made of type IV collagen. - A thicker, more diffuse reticular lamina containing collagen III fibres. It is bound to the basal lamina by anchoring fibrils of type VII collagen, both of which are produced by cells of the connective tissue. - The spread of cancer cells from the place where they first formed to another part of the body is called **metastasis**, cancer cells cross the basement membrane of the epithelia of the primary tumor, travel through the blood or lymph system, and form a new tumor in other organs or tissues of the body. The new, metastatic tumor is the same type of cancer as the primary tumor. Carcinoma: Malignant