University:
Kentucky State University
Course:
BIO 301 | Human Anatomy
Academic year:

2022
Views:

275

Pages:

45
Author:

flugeldarmn7

Epithelium-2 Relationship between CTand Epithelium • All epithelial cells rest on connectivetissue • In case of epithelia that line internal organs, this connective tissue is called laminapropria • Area of contact between epithelium and lamina propria increased by irregularities called papillae, most frequent in areas of stress Basal Lamina and Basement Membrane • Most epithelial cells are separated from the connective tissue by a sheet of extracellular material called the basal lamina (only visible under EM), it barriers between connective & epithelial tissue . • The term basement membrane (fusion of 2 layers) is used to specify a PAS-positive layer visible under LM present beneath some epithelia.“PAS used for sugar “ Basal lamina and basement membrane Basement membrane Basement membrane is composed of two layers: 1- Basal lamina 2- Reticular lamina “Reticular means network “ *this is using EM . Note that base surface of epithelium is part of the cell but basal lamina isn’t Layers of basal lamina: Lamina Lucida (rara) Lamina Densa Remember : we here see the plasma membrane as a single layer whereas it’s trilaminar due to the low magnification . -lamina=layer -Lucida=transparent -dense= densed Layers of basal lamina: Lamina Lucida Lamina Densa Lamina lucida: The clear layer closer to the epithelium Lamina densa: the dense layer closer to the connective tissue Researchers kept studying specimens containing epithelial tissue, basement membrane & connective tissue using traditional michroteqeuneches that involve fixation processes trying to figure out the component of the lamina Lucida ; because of the several failing results they used a fresh tissue with no fixatives within another preparation processes , the results was there is no such a layer (lamina Lucida does not exist). The reason for this empty space to show up after preparation is that the fixatives pull the integrals ( discussed later on ) producing a space => lamina Lucida. •Composed of lucida and densa •Only visible with E.M •Found also in other tissues, muscle cells, adipocytes, peripheral neurons (external lamina=>between the connective tissue & the other tissue ). •Components are secreted by epithelium Basal lamina Molecular components are variable but include: - Type IV collagen is coated with - proteoglycans: e.g. Heparan sulfate proteoglycan called Perlecan=> it covers the collagen IV. - Glycoproteins (Laminin, entactin…) - Specific composition is depended on tissue and location Many structural types of laminan & entactin. In basal lamina you will find collagen 4 coated with proteoglycans “perlecan” In reticular lamina you will find collagen 3 reticular fiber and you will see collagen 7 as anchoring fibril > also you will find anchoring plaque “made of different types of protein but mainly collagen 4. Another structure which is not a part of the basal lamina : integrals. Integrals : proteins that integrates the cell into the basal lamina ; they protrude from the cell ( epithelial cell –eg.) & spans the plasma membrane (transmembrane proteins). Scientists discovered collagen proteins using many mechanisms one of them is : - Using antigens . Basement membrane you can find collagen lll in: - The Reticular fibers = a type of fibers in the connective tissue. • Used to specify a PAS positive layer "it means that Basement membrane is full of sugars which are glycoproteins and proteoglycans ", visible on light microscope • It is thicker and usually formed by fusion of two basal laminae or basal lamina and reticular lamina • the basement membrane is not actually a membrane; rather, it is a matrix #2 basal laminae together will form Basement membrane We can find this case in lungs specially in Alveoli . It is lined with simple squamous epithelium. Functions of basal lamina: 1- Support 2Selective barrier 3Influencing cell polarity = distinguish the basal & apical sides. 4Regulation of proliferation and differentiation 5Affect cell-cell interaction 6- Pathway for cell migration !! Every thing that enters or leaves the body must cross an epithelial sheet. Epithelium occurs in the body as a sheet of cells that covers a body surface, lines a cavity, or forms a gland. Coverings, linings, forming glands. Cell Junctions Basolateral domain Tight junction/ zonula occludens - Occluding junction forms a band (encircles epithelial cells) - Barrier to diffusion between cells (paracellular pathway) -Separates apical and basolateral plasma membranes, the outer layers of 2 adjacent plasmalemma fuse together. Occludens : Close. Zonula means belt, occludens means close So it means junctions that close the pathway “ Paracellular pathway “+ Near apex . The tight junction is like a guard ; the thing that determines what molecule inters or leaves the cell is the cell itself . Some molecules inter through transcellular transport then complete in the paracellar transport. Figure 4-28 Copyright © -Hill Differ in function. - TEM: is the most apical junction - Freeze fracture of TJ reveals ridges in membranes that correspond to sites of contact between cells - Ridges are linear arrays of occludin and claudin proteins Is connected to the microfilaments ( actin filaments ) of the cytoskeleton of the cell through proteins that we call Zonula occludens . Tight Junction (Zonula occludens) Occludins and claudins Each strand is a row of transmembrane proteins in both PMs with ECDs joining together -Occludins and claudins are transmembrane proteins that interact across the intercellular space to form TJs -ZO (zonula occludens) proteins 1-3 link occludin and claudin to each other, to JAMs, and to actin filaments Transmembrane protein that resist slipping or sliding of cells from each other Zonula adherens -Anchoring junction (encircles the cell) belt junction, or belt desmosome - Located "under" tight junction in epithelial cells -Connected to actin microfilaments that join terminal web -Cadherin proteins attach to cross-linked actin filaments -Mechanical support - ZA and actin filaments transmit and distribute stress throughout cell and to neighboring cells Zonula adherens Catenins join cadherins to actin filaments in adherence junctions This junction keeps epithelial cells from slipping/sliding out of position Desmosome/ macula adherens -macula= spots -Anchoring junctions -Provides firm adhesion between cells -Function as "spot welds" to join cells -Located along lateral plasma membranes of columnar epithelial cells or on processes of squamous cells - Intermediate filaments associate with plaque proteins in cytoplasm Of the cytoskeleton - Non-classical cadherins interact across intercellular space - Adaptor proteins form a dense plaque that interconnects cadherins and binds them to intermediate filaments Desmosomes Desmosome (Macula adherens) - Desmoglein and desmocollin are non-classical cadherins - Adaptor proteins such as -catenin (plakoglobin) and desmoplakin link cadherins to intermediate filaments Gap junction •Channel-forming junction •Named for gap of regular width between cells visualized by TEM •Water-filled junctions transport molecules <1 kDal such as ions, nucleotides (including cAMP), and metabolites •Rapid propagation of action potential from one cell to another cell Gap (Communicating) Junction The gap junction is seen as an area of close plasma membrane apposition Connexin - protein subunit, six form a hexameric connexon Connexons - two align to form the gap junction channel Each connexon has a hydrophilic pore of 1.5nm in diameter Regulation - elevated calcium concentrations close channel Hemidesmosomes • Hemidesmosome - "half-desmosome" in appearance only • Mediates attachment to basal lamina (extracellular matrix) • Cytoplasmic plaque is attached to intermediate filaments Integrins - membrane protein that "integrates" cell into matrix Integrins bind to ECM (laminin and collagen 4) Blistering Disease -Many mechanisms underlie blistering disorders of the skin -Pemphigus group autoimmune disease in which autoantibodies target desmogleins present in desmosomes These diseases results from a disorder in the basal lamina components or in the integrals (any thing participate in the natural construction ) ; thus basal lamina won’t be functional & tissues separate . It might be : -autoimmune diseases : the cure is giving cortisone to decrease the body immunity . - Genetic mutations : it could be life threatening , it causes disfiguration ; in such cases a baby is born with lack of collagen VII or an infected one , it results in a hopeless auto healing carried by the body in the form of scares ( scare tissue= fibrous tisse). This reaction results in shrinkage of the body organs ; thus might lead to death . The only therapy is gene therapy . Pemphigus Histology Keratin Epithelial tissue (gastric ulcers) Helicobacter pylori (bechteria) targets ZO-1 and disrupts this junction Summary of cell junctions found between epithelial cells (basolateral domain)

Epithelium-2

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