Histology Lab - Cell and Epithelia PDF

HISTO LAB 8283L / MLS 212L EXERCISE 2 – THE CELL NUCLEUS: Nut kernel. CELL: A large organelle that houses most of a cell’s DN...

HISTO LAB 8283L / MLS 212L EXERCISE 2 – THE CELL NUCLEUS: Nut kernel. CELL: A large organelle that houses most of a cell’s DNA. Basic functional units of all living organisms. Within the nucleus, each chromosome, a single Has two major parts: molecule of DNA associated with several proteins, ○ Nucleus contains thousands of genes that control most ○ Cytoplasm aspects of cellular structure. The different substances that make up the cell are collectively called protoplasm. Protoplasm is composed mainly of five basic substances namely: ○ Water (70-85%) ○ Electrolytes (include potassium, magnesium, phosphate, sulfate, bicarbonate, and smaller O quantities of sodium, chloride and calcium) ○ Proteins (10-20%, divided into structural proteins and functional proteins) ○ Lipids (2%) ○ Carbohydrates (ave. 1%) THREE BASIC PARTS OF A CELL: G PLASMA MEMBRANE (CELL MEMBRANE/PLASMALEMA): Forms the cell’s flexible outer surface and separates the cell’s internal environment from the external environment. A selective barrier that regulates the flow of materials into and out of a cell that helps establish and maintain AL the appropriate environment for normal cellular activities. Plays a key role in communication among cells and between cells and their external environment. CYTOPLASM: Plasm means formed or molded. Consists of all the cellular contents between the plasma membrane and the nucleus. This compartment has two components: cytosol and ID organelles. PLASMA MEMBRANE: CYTOSOL: A flexible yet sturdy barrier that surrounds and The fluid portion of cytoplasm, also called contains the cytoplasm of a cell. intracellular fluid, contains water, dissolved It is a thin, pliable, elastic structure about 7.5 to 10 nm solutes, and suspended particles. thick. Functions: ORGANELLES: H ○ Acts as a barrier separating inside and Within the cytosol. outside of the cell. Each type of organelle has a characteristic ○ Controls the flow of substances into and out shape and specific functions. Examples of the cell. include the cytoskeleton, ribosomes ○ Helps identify the cell to other cells (immune endoplasmic reticulum, Golgi complex, cells). lysosomes, peroxisomes, and mitochondria. ○ Participates in intercellular signaling. 1 HISTO LAB 8283L / MLS 212L STRUCTURE: MEMBRANE PROTEINS: LIPID BILAYER: Classified as integral or peripheral proteins The basic structural framework. according to whether they are firmly embedded in the Two back-to-back layers made up of three types of membranes. lipid molecules–phospholipids, cholesterol and Functions as: glycolipids. ○ Ion channels The bilayer arrangement occurs because the lipids ○ Carriers are amphipathic molecules, meaning the lipids have ○ Receptors both polar and nonpolar parts. ○ Enzymes ○ Linkers PHOSPHOLIPIDS (75%): ○ Cell-identity markers Polar “Head” ○ Support ○ Consists of glycerol conjugated to a nitrogenous compound via a INTEGRAL PROTEINS: O phosphate group. Embedded within the lipid bilayer of the cell ○ Hydrophilic. membrane. ○ Directed on both the inner and Some (called transmembrane proteins) outer surfaces of the cell span the entire thickness of the cell membrane. membrane. Nonpolar “Tail” Firmly anchored by their hydrophobic amino ○ Consists of two (2) long fatty acid acids to the fatty acid chains of the G chains covalently linked to the membrane lipids. glycerol of the polar head. Can be removed only by detergents that Straight-chain saturated disrupt the bilayer. fatty acid Act as ion channels, water channels “Kinked” unsaturated fatty (aquaporins), solute carriers and acid ATP-dependent transporters; also serve as AL ○ Prevents close packing of the tails. receptor proteins for water-soluble ○ Hydrophobic. chemicals, such as peptide hormones. ○ Directed inwards to the center of Are glycoproteins: proteins with the membrane. carbohydrate groups attached to the ends Gives fluidity and flexibility to the membrane: that protrude into the extracellular fluid. selective permeability, the property that permits some substances to pass more PERIPHERAL PROTEINS: readily than others forms an important Exposed on the outer (extracellular) and the barrier between the internal and external inner (intracellular) surface of the cell environments of the cell, which then membrane. ID maintains a constant intracellular Not as firmly embedded in the membrane. environment ○ Loosely attached to the polar heads of the membrane lipids or integral CHOLESTEROL (20%): proteins by weak (non-covalent) A steroid with an attached hydroxyl group. electrostatic forces. Weakly amphipathic. Easily removed by altering the pH or ionic ○ Polar: Hydroxyl groups. strength of their environments. ○ Nonpolar: Steroid rings with Function almost entirely as enzymes. H hydrocarbon tail. Interspersed at varying densities throughout the phospholipid bilayer. Stabilizes the membrane and regulates the fluidity of the phospholipid bilayer. GLYCOLIPIDS (5%): Lipids with attached carbohydrate groups Appear only in the membrane layer that faces the extracellular fluid, which is one reason the two sides of the bilayer are asymmetric, or different. 2 HISTO LAB 8283L / MLS 212L GLYCOCALYX: ○ Intermembranous Space An extensive sugary coat formed from the Space between the outer and inner carbohydrate portions of glycolipids and membranes. glycoproteins. Contains a variety of enzymes. The “fuzzy” coat on the outer surface of the ○ Mitochondrial Matrix membrane. Large space enclosed by the inner Acts like a molecular “signature” that enables membrane. cells to recognize one another (due to the Contains double-stranded circular different pattern of carbohydrates of cells). DNA: a mitochondrion is Enables cells to adhere to one another in self-replicating; ribosomes and also some tissues and protects cells from being a number of dense matrix granules. digested by enzymes in the extracellular fluid. Hydrophilic so it attracts a film of fluid to the O surface of many cells (makes red blood cells slippery as they flow through narrow vessels and protects cells that line the airways and GIT from drying out). CYTOPLASM AND ITS ORGANELLES: The substance extending from the plasma membrane G to the nuclear envelope. Has two components: (1) Cytosol; (2) Organelles. In addition to the organelles, there are protein RIBOSOMES: components of the cytoplasmic cytoskeleton and Are small (approximately 12 nm x 25nm), inclusions that are generally deposits of electron-dense, non membranous granules. carbohydrates, lipids or pigments. AL Composed of proteins and ribosomal ribonucleic acid (rRNA). CYTOSOL: Consists of two subunits: large 60S and small 40S. Fluid suspension, consisting mainly of water (75 to Often binds to mRNA molecules in small circular 90%), in which organic and inorganic chemicals (ions, aggregations called polyribosomes or polysomes. glucose, amino acids, fatty acids, proteins, lipids, ATP Maybe “free” in the cytoplasm of the cell or “attached” and waste products) are dissolved or suspended. or bound to endoplasmic reticulum. ○ Free ribosomes: synthesize proteins for use ORGANELLES: within the cell cytoplasm Metabolically active structures that execute specific ○ Attached ribosomes: synthesizes proteins metabolic functions in cellular growth, maintenance ID that are packaged and stored in the cell as and reproduction. lysosomes or released from the cell as May be membrane-bound (membrane similar to the secretory products cell membrane) or NOT membrane-bound. ENDOPLASMIC RETICULUM: MITOCHONDRIA Are the largest membranous system of the cell. Powerhouse of the cell because they generate most Consists of an interconnecting network of ATP through aerobic respiration. membranous tubules, vesicles and sacs called Often elongated, sausage-shaped. H cisternae. Has four (4) compartments: Extends from the surface of the nucleus to the cell ○ Outer Mitochondrial Membrane membrane. Smooth and contains a Two (2) types: pore-forming protein known as ○ Rough Endoplasmic Reticulum (RER) porin, which allows free passage of Also known as granular small molecules. endoplasmic reticulum. ○ Inner Mitochondrial Membrane Continuous with the nuclear Thrown into complex folds and membrane. tubules called cristae that project Folded into a series of flattened into the inner cavity to greatly sacs. increase the surface area of the The outer surface is studded with membrane. ribosomes. 3 HISTO LAB 8283L / MLS 212L The site for synthesis of proteins for ○ Forms transport vesicles that carry secretion, incorporation to the molecules to other organelles like membrane as enzymes within lysosomes. lysosomes. ○ Smooth Endoplasmic Reticulum (SER) Also known as agranular endoplasmic reticulum It is devoid of ribosomes so it does not synthesize proteins but fatty acids and steroids Activities associated with SER: Lipid (steroids, cholesterol, phospholipids) biosynthesis. O Detoxification of potentially harmful compounds TRANSPORT/SECRETORY VESICLES: (drugs, alcohol). Membrane-bound vesicles often with a protein coat. Sequestration of Ca++ Transport materials between different cell ions. compartments and to plasma membranes for export. G LYSOSOMES: Round (0.2-0.8um) or polymorphous membrane-limited vesicles formed from the Golgi apparatus. Contain as many as 60 kinds of powerful digestive and hydrolytic enzymes. AL Functions: ○ Digest substances that enter a cell via endocytosis and transport final products of digestion into cytosol. ○ Carry out autophagy, the digestion of GOLGI APPARATUS/GOLGI COMPLEX: worn-out organelles. Consists of a series of flattened slightly curved, ○ Implement autolysis, the digestion of an membrane bound cisternae and Golgi stack. entire cell. Three (3) levels of cisternae: ○ Accomplish extracellular digestion. ○ Cis/Entry face ID Convex-shaped and closest to the RER. Entry face: where newly formed proteins from the ER are received. ○ b. Medial face Intermediate face. ○ c. Trans/Exit face Concave-shaped and closest to the PEROXISOMES: Also known as microbodies. H plasma membrane. Exit face: exports proteins out of the Similar to ovoid membrane-bound, self-replicating Golgi apparatus. organelles. Functions: Contain oxidative enzymes (oxidase, catalase). ○ Modifies, sorts, packages and transports Important in the production of hydrogen peroxide for portions received from the RER. killing pathogens; detoxification of certain toxic ○ Forms secretory vesicles that discharge materials; and catabolism of long chain fatty acids. processed proteins via exocytosis into extracellular fluid. PROTEASOMES: ○ Forms membrane vesicles that carry new Small, barrel-shaped organelle. molecules to the plasma membrane. Composed of protein complexes in both the cytosol and nucleus. Degrade and digest damaged or unneeded proteins (cystolic proteolysis). 4 HISTO LAB 8283L / MLS 212L NUCLEUS: NUCLEOLI: Largest organelle of the cell; most obvious feature of Spherical bodies found inside the nucleus. the cell seen by light microscopy. Dense, very basophilic, non-membrane-bound Usually spherical, may be spindle-shaped to structure in the nucleus. oblong-shaped, twisted, lobulated or even Consists of an aggregate of ribosomal genes, newly disk-shaped. synthesized rRNA, ribosomal proteins and Contains chromatin, nucleolus, and nucleoplasm ribonucleoproteins. enclosed in a nuclear membrane. The site of rRNA synthesis and ribosome assembly “Control center” of the cell: houses the and transfer RNA (tRNA) is also processed. deoxyribonucleic acid (DNA) that serves as the genetic material of the cell and for directing protein CHROMATIN: synthesis A complex of double-stranded helical DNA and histone proteins that forms nucleosomes, producing a NUCLEAR MEMBRANE/ENVELOPE: structure resembling beads on a string. O Encloses the nucleus and is similar to the plasma Relaxed, uncoiled in the interphase nucleus. membrane. Composed of two (2) parallel unit membranes EUCHROMATIN: separated from each other by a 10 to 30 nm space Chromatin with DNA that is active in called perinuclear cisterna. transcription. Finely dispersed granular material seen in INNER NUCLEAR MEMBRANE: the electron microscope. G About 6nm thick. Lightly stained basophilic areas seen in the Faces the nuclear contents. light microscope. Supported internally high organized meshwork, the nuclear lamina, composed HETEROCHROMATIN: of intermediate proteins called lamina. Inactive chromatin. Coarse, electron-dense material seen in the AL OUTER NUCLEAR MEMBRANE: electron microscope. About 6nm thick. Darkly stained (basophilic) seen in the light Faces the cytoplasm. microscope. Studded with ribosomes in its cytosolic surface. CHROMOSOME: Continuous with the RER by thin, loose Condensed and tightly coiled chromatin meshwork of intermediate filaments, fibers during mitosis and meiosis. vimentin. Composed of two (2) chromatids joined by centromeres. NUCLEAR PORE: In human cells, there are 46 chromosomes, ID Interruptions in the nuclear membrane where comprising 22 homologous pairs of the inner and outer membranes fuse with autosomes, and 1 pair of sex chromosomes each other. either XX in female or XY in male. Contains a nuclear pore complex, an elaborate cylindrical structure of proteins CILIA, FLAGELLUM, MICROVILLI: known as nucleoporins forming a central Project from the apical surfaces of certain cells in the pore (9 nm in diameter). body. Permit and regulate the exchange of H metabolites, macromolecules and ribosomal CILIA: subunits between nucleus and cytoplasm. Short, numerous membrane extensions which occur Ions and small molecules diffuse freely on exposed membrane surfaces of some cells. through the nuclear pore. Supported with a core of uniformly arranged microtubules in longitudinal orientation. NUCLEOPLASM: The core of each cilium consists of nine (9) Contains macromolecules and nuclear particles. microtubule doublets located peripherally and two (2) Involved in the maintenance of the cell. single microtubules in the center. Each cilium is attached to and extends from the basal body in the apical region of the cell. Move substances (eg. mucus, and dissolved materials) over the cell surface. 5 HISTO LAB 8283L / MLS 212L FLAGELLUM: CENTROSOMES: Long, singular membrane extension supported by An amorphous area of the cytoplasm adjacent to the microtubules. nucleus and often surrounded by Golgi apparatus. Similar to the structure of cilia but is longer. Generates forward motion along its axis by rapidly CENTRIOLE: wiggling in a wave-like pattern. Also known as diplosome. Small cylindrical structures perpendicular to each MICROVILLI: other. Smaller, shorter, closely packed finger-like extensions Each consists of nine parallel microtubular triplets that project from the free cell surface. (clusters of three microtubules); the inner microtubule Non-motile. is connected to the outermost microtubule of the Exhibit a core of thin microfilaments. adjacent triplet by fine filaments, thus forming a Increase membrane surface area for greater circular pattern. absorption. O PERICENTRIOLAR MATERIAL: CYTOSKELETON AND CENTROSOME: Also known as centrosome matrix. CYTOSKELETON: Act as a microtubule organizing center of the A supporting framework of fibrillar proteins organized cytoskeleton. into minute filaments and tubules. Microtubules radiate outwards from the centrioles in a star-like arrangement, often called an aster. MICROFILAMENTS: G Are extremely fine strands (5 to 7 nm in diameter). CYTOPLASMIC INCLUSIONS: Composed of the protein actin. Represent the nonliving parts of the cell that do NOT G-actin: the globular subunit (monomer). possess metabolic activity. F-actin: the overall structure, filamentous protein Are temporary structures that accumulate in the consisting of G-actin monomers twisted to form a cytoplasm of certain cells. helix. Not bounded by membrane. AL Contract and move cells; change cell shape; Fat droplets - accumulations of lipid molecules cytokinesis; cytoplasmic transport and streaming. prominent in adipocytes (fat cells), adrenal cortex, liver and other cells. MICROTUBULES: Glycogen - storage form of glucose; ready source of Long, straight, hollow-like cylindrical structure. energy. 25 nm in diameter, with a lumen whose diameter is 15 Pigment deposits. nm. ○ Lipofuscin granules: yellowish-brown Consist of 13 parallel protofilaments (heterodimers of pigment; represent an accumulating by- αβ-tubulin). product of lysosomal digestion in long-lived Maintain cell’s shape and polarity; provide tracks for cells. ID organelle and chromosome movement; move cilia ○ Melanin granules: brown pigment; protect and flagella. nuclei from damage to DNA caused by light. Hemosiderin granules - contain the protein ferritin, INTERMEDIATE FILAMENTS: which forms a storage complex for iron; are very Rope-like, approximately 10 nm in diameter, electron dense, but with the light microscope they intermediate in size between microfilaments and appear brownish and resemble lipofuscin. microtubules. Constructed of a cable of four (4) intertwined CELL DIVISION: H protofibrils, each consisting of bundled tetramers in The process by which cells reproduce themselves. helical arrays. Two types of cell division: somatic cell division and Classes, with some classic characteristic of particular reproductive cell division. cell types: Each of this type of cell division accomplishes ○ Keratin (or cytokeratin): epithelial cell (hair different goals for the organism. and nails). A somatic cell (soma: body) is any cell of the body ○ Vimentin: cells of mesodermal origin. other than a germ cell. A germ cell is a gamete ○ Desmin: muscle cell. (sperm or oocyte) or any precursor cell destined to ○ Neurofilament: nerve cells. become a gamete. ○ Glial fibrillary acidic protein: glial cells. In somatic cell division, a cell undergoes a nuclear ○ Lamin: inner side of the nuclear membrane. division called mitosis and a cytoplasmic division Strengthen cell and tissue structure; maintain cell called cytokinesis (cyto: cell; -kinesis: movement) to shape; maintain nuclear shape (lamins). produce two genetically identical cells, each with the 6 HISTO LAB 8283L / MLS 212L same number and kind of chromosomes as the The events that occur during mitosis and cytokinesis original cell. Somatic cell division replaces dead or are visible under a microscope because chromatin injured cells and adds new ones during tissue growth. condenses into discrete chromosomes. Reproductive cell division is the mechanism that produces gametes, the cells needed to form the next PROPHASE generation of sexually reproducing organisms. This The nucleolus disappears and the replicated process consists of a special two-step division called chromatin condenses into discrete threadlike meiosis, in which the number of chromosomes in the chromosomes, each consisting of duplicate sister nucleus is reduced by half. chromatids joined by centromere. At the centromere region of each chromosome, a SOMATIC CELL DIVISION: large protein complex called the kinetochore serves The cell cycle is an orderly sequence of events as a site for attachment to microtubules. where a somatic cell duplicates its contents and The centrosomes with their now-duplicated centrioles divides in two. separate and migrate to opposite poles of the cell. O When a cell reproduces, it must replicate (duplicate) Tubulins in the pericentriolar material of the all its chromosomes to pass its genes to the next centrosomes start to form the mitotic spindle, a generation of cells. football-shaped assembly of microtubules that attach The cell cycle consists of two major periods: to the kinetochore. interphase, when a cell is not dividing, and the mitotic As the microtubules lengthen, they push the (M) phase, when a cell is dividing. centrosomes to the poles (ends) of the cell so that the spindle extends from pole to pole. G Late in prophase, lamins and inner nuclear membrane are phosphorylated, causing the nuclear lamina and nuclear pore complexes to disassemble and disperse in cytoplasmic membrane vesicles. METAPHASE: AL During metaphase, the microtubules of the mitotic spindle align the centromeres of the chromatid pairs at the exact center of the mitotic spindle. This midpoint region is called the metaphase plate. ANAPHASE: Sister chromatids (called chromosomes) separate and move toward opposite spindle poles by a combination of microtubule motor proteins and INTERPHASE: dynamic changes in the lengths of the microtubules ID During this time, the cell replicates its DNA. It also as the spindle poles move farther apart. produces additional organelles and cytosolic components in anticipation of cell division. TELOPHASE: A state of high metabolic activity. Two sets of chromosomes are at the spindle poles It is during this time that the cell does most of its and begin reverting to their decondensed state. growing. The spindle depolymerizes and the nuclear envelope There are three (3) phases: G1, S, and G2. G stands begins to reassemble around each set of daughter for gaps or interruptions in DNA duplication because chromosomes. H there is no activity related to DNA duplication. The S A belt-like contractile ring of actin filaments stands for synthesis of DNA. associated with myosins develops in the peripheral cytoplasm at the cell’s equator. During cytokinesis at MITOTIC PHASE (M PHASE): the end of telophase, constriction of this ring produces At this time, two identical cells are formed due to a cleavage furrow and progresses until the cytoplasm nuclear division (mitosis) and cytoplasmic division and its organelles are divided into two daughter cells, (cytokinesis). each with one nucleus. During mitosis, a parent cell divides and each of the two daughter cells receives a chromosomal set identical to that of the parent cell. The chromosomes replicated during the preceding S phase are distributed to the daughter cells. 7 HISTO LAB 8283L / MLS 212L CYTOKINESIS: Division of a cell’s cytoplasm and organelles into two identical cells is called cytokinesis. This process usually begins in late anaphase with the formation of a cleavage furrow, a slight indentation of the plasma membrane, and is completed after telophase. The cleavage furrow usually appears midway between the centrosomes and extends around the periphery of the cell. Actin microfilaments that lie just inside the plasma membrane form a contractile ring pulling the plasma membrane progressively inward. The ring constricts in the center of the cell and O ultimately pinches it into two. Because the plane of the cleavage furrow is always perpendicular to the mitotic spindle, the two sets of chromosomes end up in separate cells. Once complete, interphase begins. G AL ID H 8 HISTO LAB 8283L / MLS 212L EXERCISE 3 – THE EPITHELIAL TISSUES RETICULAR LAMINA: Found beneath the basal lamina. THREE MAJOR FUNCTIONS OF EPITHELIAL TISSUES: It is more diffuse and fibrous than BL. 1. Covering, lining, and protecting surfaces (eg. It contains type III collagen and is bound to the basal epidermis) laminae by anchoring fibrils of type VII collagen. 2. Absorption (eg. the intestinal lining) 3. Secretion (eg. parenchymal cells of glands) CHARACTERISTIC FEATURES OF EPITHELIAL CELLS: The shapes and dimensions of epithelial cells vary from tall columnar, to cuboidal, to low squamous cells. ○ Nuclear shape corresponds roughly to cell shape: tall cells have elongated nuclei, and O squamous cells have flattened nuclei. ○ Cuboidal or pyramidal cells usually have more spherical nuclei. Rest on connective tissue containing the CELL JUNCTIONS: microvasculature supplying nutrients and oxygen. Contact points between the plasma membranes of Lamina propria is the connective tissue that tissue cells. underlies the epithelial lining the organs of the G Provide adhesion and communication between cells. digestive, respiratory, and urinary systems. In many epithelia, these junctions are present in a The area of contact between the epithelium and definite order at the apical end of the cells: connective tissue may be increased by irregularities at the interface in the form of small evaginations TIGHT JUNCTION OR ZONULA OCCLUDENS: called papillae. Most apical of the junctions. ○ Occur most frequently in epithelial tissues AL “Zonula” indicates that the junction forms a band subject to friction, like covering of the skin or completely encircling each cell. tongue. Consist of web-like strands of transmembrane Epithelial cells generally show polarity. proteins that fuse together the outer surfaces of ○ Basal pole/surface is the region of the cell adjacent plasma membranes to seal off passageways contacting the connective tissue. between adjacent cells. ○ The opposite end, usually facing a space is Seen in cells of epithelial tissue that lines the the apical pole/surface. stomach, intestines and urinary bladder (inhibit Regions of cuboidal or columnar cells that adjoin the passage of substances between cells and prevent neighboring cells are the lateral surfaces. contents of these organs from leaking into blood or ID surrounding tissues). BASEMENT MEMBRANE: The seal between the membranes is due to A thin, noncellular region separating the epithelium interactions between the transmembrane proteins from underlying connective tissues. claudins and occludins of each cell. Functions: BASAL LAMINA (LAMINA: THIN): ○ Control paracellular diffusion. Electron-dense layer about 20 to 100 nm thick. ○ Prevents exchange of intrinsic proteins and It is nearest to the epithelial basal poles and consists lipids between apical and basolateral plasma of fine fibrils. H membranes. May appear with thin clear zones on each side and is anchored to a thicker and more diffuse RL. Macromolecules are secreted at the basal poles of the epithelial cells and form 3-dimensional arrays. The ECM components include laminin, type IV collagen. Laminin and type IV collagen networks are held together by the adhesive glycoprotein entactin/ nidogen, and a proteoglycan, perelman. 1 HISTO LAB 8283L / MLS 212L ADHERENT JUNCTION OR ZONULA ADHERENS: HEMIDESMOSOME (HEMI - HALF): Forms a single continuous band lying deep to the tight Modified desmosomes found at the basal surface of junction at the luminal end of the lateral membranes the cell. of columnar epithelium. The transmembrane proteins are integrins that bind Cell adhesion is mediated by cadherins, the extracellular macromolecules laminin and transmembrane glycoproteins of each cell that collagen type IV. interact in the presence of Ca2+. Function: Cadherins bind catenin linked via actin-binding ○ Anchors cytoskeleton to the basal lamina. proteins to actin filaments. Actin filaments form part of the terminal web that plays a role in cytoplasmic motility. Function: ○ Link cytoskeletons of adjacent cells to form strong cohesive epithelium. O G GAP JUNCTION OR NEXUS: Mediate communication between cells. Consists of proteins called connexions that form hexameric complexes called connexons with a central hydrophilic pore of 1.5nm in diameter. AL Important in the control of growth, development, cell DESMOSOME OR MACULA ADHERENS (DESMO - BAND): recognition and differentiation. Resembles a single “spot-weld” and does not form a Numerous in embryonic epithelia where they are belt around the cell. involved in exchange of chemical messengers and Disc-shaped structures at the surface of one cell that cell recognition and differentiation and control of cell are matched with identical structures at an adjacent position. cell surface. Functions: Employ different members of the cadherin superfamily ○ Allows direct transfer of small molecules and as transmembrane proteins. ions from one cell to another (